Proposed Rule2026-04668
Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to U.S. Navy Operations of Surveillance Towed Array Sensor System Low Frequency Active Sonar in the Western and Central North Pacific Ocean and Eastern Indian Ocean
Primary source
Metadata and text below are from the Federal Register, a public-domain U.S. government work. Always verify the official published version before relying on it for any legal matter.
Published
March 10, 2026
Issuing agencies
Commerce DepartmentNational Oceanic and Atmospheric Administration
Abstract
NMFS has received a request from the U.S. Department of the Navy (Navy) for Incidental Take Regulations (ITR) and an associated Letter of Authorization (LOA) pursuant to the Marine Mammal Protection Act (MMPA). The requested regulations would govern the authorization of take of marine mammals incidental to training and testing activities using Surveillance Towed Array Sensor System (SURTASS) Low Frequency Active (LFA) sonar systems in the western and central North Pacific and eastern Indian oceans over the course of 7 years from August 2026 through August 2033. NMFS requests comments on this proposed rule. NMFS will consider public comments prior to making any final decision on the promulgation of the requested ITR and issuance of the LOA; agency responses to public comments will be summarized in the final rule, if issued. The Navy's activities are considered military readiness activities pursuant to the MMPA, as amended by the National Defense Authorization Act for Fiscal Year 2004 (2004 NDAA) and the NDAA for Fiscal Year 2019 (2019 NDAA).
Full Text
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[Federal Register Volume 91, Number 46 (Tuesday, March 10, 2026)]
[Proposed Rules]
[Pages 11618-11747]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-04668]
[[Page 11617]]
Vol. 91
Tuesday,
No. 46
March 10, 2026
Part II
Department of Commerce
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National Oceanic and Atmospheric Administration
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50 CFR Part 218
Takes of Marine Mammals Incidental to Specified Activities; Taking
Marine Mammals Incidental to U.S. Navy Operations of Surveillance Towed
Array Sensor System Low Frequency Active Sonar in the Western and
Central North Pacific Ocean and Eastern Indian Ocean; Proposed Rule
Federal Register / Vol. 91 , No. 46 / Tuesday, March 10, 2026 /
Proposed Rules
[[Page 11618]]
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DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
50 CFR Part 218
[Docket No. 260304-0065]
RIN 0648-BN61
Takes of Marine Mammals Incidental to Specified Activities;
Taking Marine Mammals Incidental to U.S. Navy Operations of
Surveillance Towed Array Sensor System Low Frequency Active Sonar in
the Western and Central North Pacific Ocean and Eastern Indian Ocean
AGENCY: National Marine Fisheries Service (NMFS), National Oceanic and
Atmospheric Administration (NOAA), Commerce.
ACTION: Proposed rule; proposed letter of authorization; request for
comments.
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SUMMARY: NMFS has received a request from the U.S. Department of the
Navy (Navy) for Incidental Take Regulations (ITR) and an associated
Letter of Authorization (LOA) pursuant to the Marine Mammal Protection
Act (MMPA). The requested regulations would govern the authorization of
take of marine mammals incidental to training and testing activities
using Surveillance Towed Array Sensor System (SURTASS) Low Frequency
Active (LFA) sonar systems in the western and central North Pacific and
eastern Indian oceans over the course of 7 years from August 2026
through August 2033. NMFS requests comments on this proposed rule. NMFS
will consider public comments prior to making any final decision on the
promulgation of the requested ITR and issuance of the LOA; agency
responses to public comments will be summarized in the final rule, if
issued. The Navy's activities are considered military readiness
activities pursuant to the MMPA, as amended by the National Defense
Authorization Act for Fiscal Year 2004 (2004 NDAA) and the NDAA for
Fiscal Year 2019 (2019 NDAA).
DATES: Comments and information must be received no later than April 9,
2026.
ADDRESSES: A plain language summary of this proposed rule is available
at: <a href="https://www.regulations.gov/docket/NOAA-NMFS-2025-0999">https://www.regulations.gov/docket/NOAA-NMFS-2025-0999</a>. You may
submit comments on this document, identified by NOAA-NMFS-2025-0999, by
any of the following methods:
<bullet> Electronic Submission: Submit all electronic public
comments via the Federal e-Rulemaking Portal. Visit <a href="https://www.regulations.gov">https://www.regulations.gov</a> and type NOAA-NMFS-2025-0999 in the Search box.
Click on the ``Comment'' icon, complete the required fields, and enter
or attach your comments.
<bullet> Mail: Submit written comments to Ben Laws, Incidental Take
Program Supervisor, Permits and Conservation Division, Office of
Protected Resources, National Marine Fisheries Service, 1315 East-West
Highway, Silver Spring, MD 20910-3225.
<bullet> Fax: (301) 713-0376; Attn: Ben Laws.
Instructions: Comments sent by any other method, to any other
address or individual, or received after the end of the comment period,
may not be considered by NMFS. All comments received are a part of the
public record and will generally be posted for public viewing at:
<a href="https://www.regulations.gov">https://www.regulations.gov</a> without change. All personal identifying
information (e.g., name, address, etc.), confidential business
information, or otherwise sensitive information submitted voluntarily
by the sender will be publicly accessible. NMFS will accept anonymous
comments (enter ``N/A'' in the required fields if you wish to remain
anonymous). Attachments to electronic comments will be accepted in
Microsoft Word, Excel, or Adobe PDF file formats only.
A copy of the Navy's Incidental Take Authorization (ITA)
application and supporting documents, as well as a list of the
references cited in this document, may be obtained online at: <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-military-readiness-activities">https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-military-readiness-activities</a>. In case of problems
accessing these documents, please call the contact listed below (see
FOR FURTHER INFORMATION CONTACT).
FOR FURTHER INFORMATION CONTACT: Alyssa Clevenstine, Office of
Protected Resources, NMFS, (301) 427-8401.
SUPPLEMENTARY INFORMATION:
Purpose and Need for Regulatory Action
This proposed rule, if promulgated, would provide a framework under
the authority of the MMPA (16 U.S.C. 1361 et seq.) to allow for the
authorization of take of marine mammals incidental to the Navy's
training and testing activities (which qualify as military readiness
activities) using SURTASS LFA sonar in the western and central North
Pacific Ocean and eastern Indian Ocean (see figure 2-1 of the
rulemaking and LOA application (hereafter referred to as the
application)). Please see the Legal Authority for the Proposed Action
section for relevant definitions.
Legal Authority for the Proposed Action
The MMPA prohibits the ``take'' of marine mammals, with certain
exceptions. Section 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361 et
seq.) directs the Secretary of Commerce (as delegated to NMFS) to
allow, upon request, the incidental, but not intentional, taking of
small numbers of marine mammals by U.S. citizens who engage in a
specified activity (other than commercial fishing) within a specified
geographical region if certain findings are made and either regulations
are proposed or, if the taking is limited to harassment, a notice of a
proposed authorization is provided to the public for review and the
opportunity to submit comment.
Authorization for incidental takings shall be granted if NMFS finds
that the taking will have a negligible impact on the species or
stock(s) and will not have an unmitigable adverse impact on the
availability of the species or stock(s) for taking for subsistence uses
(where relevant). Further, NMFS must prescribe the permissible methods
of taking; other ``means of effecting the least practicable adverse
impact'' on the affected species or stocks and their habitat, paying
particular attention to rookeries, mating grounds, and areas of similar
significance, and on the availability of the species or stocks for
taking for certain subsistence uses (collectively referred to as
``mitigation''); and requirements pertaining to the monitoring and
reporting of the takings. The MMPA defines ``take'' to mean to harass,
hunt, capture, or kill, or attempt to harass, hunt, capture, or kill
any marine mammal (16 U.S.C. 1362). The Preliminary Analysis and
Negligible Impact Determination section discusses the definition of
``negligible impact.''
The 2004 NDAA (Pub. L. 108-136) amended section 101(a)(5) of the
MMPA to remove the ``small numbers'' and ``specified geographical
region'' provisions (16 U.S.C. 1371(a)(5)(F)) and amended the
definition of ``harassment'' in section 3(18)(B) of the MMPA as applied
to a ``military readiness activity'' to read as follows: (1) Any act
that injures or has the significant potential to injure a marine mammal
or marine mammal stock in the wild (Level A Harassment); or (2) Any act
that disturbs or is likely to disturb a marine mammal or marine mammal
stock in the wild by causing disruption of natural behavioral patterns,
including, but not limited to, migration, surfacing, nursing, breeding,
feeding, or sheltering, to a point where such behavioral patterns are
abandoned or significantly altered (Level B Harassment) (16 U.S.C.
[[Page 11619]]
1362(18)(B)). The 2004 NDAA also amended the MMPA to establish in
section 101(a)(5)(A)(iii) that ``[f]or a military readiness activity .
. . , a determination of `least practicable adverse impact' . . . shall
include consideration of personnel safety, practicality of
implementation, and impact on the effectiveness of the military
readiness activity'' (16 U.S.C. 1371(a)(5)(A)(iii)). On August 13,
2018, the 2019 NDAA (Pub. L. 115-232) amended the MMPA to allow ITRs
for military readiness activities to be issued for up to 7 years (16
U.S.C. 1371(a)(5)(A)(ii)).
Summary of Major Provisions Within the Proposed Rule
The major provisions of this proposed rule are as follows:
<bullet> The proposed authorization of take of marine mammals by
Level A harassment and Level B harassment;
<bullet> The proposed use of visual, passive acoustic, and active
acoustic monitoring mitigation;
<bullet> The proposed implementation of geographic activity
limitations including within 22 kilometers (km) (12 nautical miles
(nmi)) of any emergent land and in certain offshore areas and times
that are biologically important (i.e., for foraging, migration,
reproduction) for marine mammals;
<bullet> The proposed implementation of a Notification and
Reporting Plan (for dead, live stranded, or marine mammals struck by
any vessel engaged in military readiness activities); and
<bullet> The proposed implementation of a robust monitoring plan to
improve our understanding of the environmental effects resulting from
the Navy's training and testing activities.
This proposed rule includes an adaptive management component that
allows for timely modification of mitigation, monitoring, and/or
reporting measures based on new information, when appropriate.
Summary of Request
On April 6, 2025, NMFS received an application from the Navy
requesting authorization to take marine mammals, by Level A and Level B
harassment, incidental to training and testing activities
(characterized as military readiness activities) using SURTASS LFA
sonar in the western and central North Pacific Ocean and eastern Indian
Ocean. The Navy is requesting one 7-year LOA for training and testing
activities. In response to our comments and following an information
exchange, the Navy submitted a revised application, deemed adequate and
complete on July 1, 2025. The Navy's request is for take of 44 species
of marine mammals by Level B harassment and a subset of those species
by Level A harassment (9 species). On July 11, 2025, we published a
notice of receipt (NOR) of application in the Federal Register (90 FR
30877), requesting comments and information related to the Navy's
request for 30 days. During the 30-day public comment period on the
NOR, we received one public comment from Turtle Island Restoration
Network requesting that NMFS deny the Navy's ITA request and consider
alternatives that prioritize avoiding critical habitats, reducing sonar
intensity, or limiting operational time frames. NMFS reviewed and
considered all submitted material during the drafting of this proposed
rule.
NMFS has previously promulgated ITRs pursuant to the MMPA relating
to similar military readiness activities using SURTASS LFA sonar. NMFS
published the first rule effective August 15, 2002 through August 15,
2007 (67 FR 46712, July 16, 2002), the second rule effective from
August 16, 2007 through August 15, 2012 (72 FR 46846, August 21, 2007),
the third rule effective from August 15, 2012 through August 15, 2017
(77 FR 50290, August 20, 2012), and the fourth rule effective from
August 12, 2019 through August 11, 2026 (84 FR 40132, August 13, 2019).
Of note, on August 10, 2017, the Secretary of Defense,\1\ after
conferring with the Secretary of Commerce, determined that it was
necessary for the national defense to exempt all military readiness
activities that use SURTASS LFA sonar from compliance with the
requirements of the MMPA for 2 years from August 13, 2017, through
August 12, 2019, or until such time when NMFS issues regulations and a
LOA under title 16, section 1371 for military readiness activities
associated with the use of SURTASS LFA sonar, whichever is earlier. For
this proposed rulemaking, the Navy proposes to conduct substantially
similar training and testing activities using SURTASS LFA sonar that
were conducted under previous rules.
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\1\ Pursuant to Executive Order 14347, ``Restoring the United
States Department of War,'' (90 FR 43893), as of September 5, 2025,
the ``Secretary of Defense'' is authorized to use the additional
secondary title of ``Secretary of War.''
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The Navy's application reflects the most up-to-date compilation of
training and testing activities deemed necessary to accomplish military
readiness requirements. The types and numbers of activities included in
the proposed rule account for interannual variability in training and
testing to meet evolving or emergent military readiness requirements.
In this proposed rule, we have undertaken a comprehensive assessment of
the impacts of all SURTASS LFA sonar training and testing activities on
marine mammals likely to be present within the western and central
North Pacific Ocean and eastern Indian Ocean in the area described
below.
Description of Proposed Activity
Overview
The Navy requests authorization to take marine mammals incidental
to conducting military readiness activities. The Navy has determined
that acoustic stressors are likely to result in take of marine mammals
in the form of Level A and Level B harassment. Descriptions of these
activities are provided in the Navy's application (<a href="https://www.fisheries.noaa.gov/action/incidental-take-authorization-united-states-navys-surveillance-towed-array-sensor-system-low">https://www.fisheries.noaa.gov/action/incidental-take-authorization-united-states-navys-surveillance-towed-array-sensor-system-low</a>), with
additional detail provided in chapter 2 and appendix F of the 2025
SURTASS Draft Supplemental Environmental Impact Statement/Overseas
Environmental Impact Statement (2025 SURTASS Draft SEIS/OEIS) (<a href="https://www.nepa.navy.mil/surtass-lfa/">https://www.nepa.navy.mil/surtass-lfa/</a>) which are summarized here.
The Navy's statutory mission is to organize, train, equip, and
maintain combat-ready naval forces for the peacetime promotion of the
national security interests and prosperity of the United States,
including deterring maritime aggression and maintaining freedom of
navigation in ocean areas. This mission is mandated by Federal law (10
U.S.C. 8062), which requires the readiness of the naval forces of the
United States. Due to the advancements and use of quieting technologies
in diesel-electric and nuclear submarines, undersea submarine threats
have become increasingly difficult to locate solely using passive
acoustic technologies. At the same time, the distance at which
submarine threats can be detected has been decreasing due to these
quieting technologies, and improvements in torpedo and missile design
have extended the effective range of these weapons. To meet the
requirement for improved capability to detect quieter and harder-to-
find foreign submarines at greater distances, the Navy developed and
uses SURTASS LFA sonar.
Dates and Duration
The specified activities would occur at any time during the 7-year
period of validity of the regulations, from August 12, 2026 through
August 11, 2033. The
[[Page 11620]]
proposed number of military readiness activities are described in the
Detailed Description of the Specified Activity section.
Geographic Region
The Pacific SURTASS LFA Sonar Study Area includes the western and
central North Pacific Ocean and Eastern Indian Ocean, not including the
Western Indian Ocean or Sea of Okhotsk (figure 1). Please refer to
figure 2-1 of the application for a color map of the Study Area. The
Study Area remains unchanged from the previous rulemaking (84 FR 40132,
August 13, 2019) (see also the 2019 SURTASS SEIS/OEIS (U.S. Department
of the Navy, 2019)).
Importantly and as described in greater detail in the Proposed
Mitigation section, in areas within 22 km from any emergent land
(coastal standoff range (CSR)) and in areas outside of the CSR
identified as offshore biologically important areas (OBIAs), SURTASS
LFA sonar training and testing would be conducted such that received
levels of LFA sonar are below 180 decibels referenced to 1 microPascal
(dB re 1 [mu]Pa) root-mean-square (RMS) sound pressure level (SPL).
This restriction would be observed year-round for the CSR and during
known periods of biological importance for OBIAs.
BILLING CODE 3510-22-P
[[Page 11621]]
[GRAPHIC] [TIFF OMITTED] TP10MR26.000
BILLING CODE 3510-22-C
Detailed Description of the Specified Activity
The Navy proposes to use 1,100 hours of SURTASS LFA sonar per year.
The analysis for the current SURTASS LFA incidental take regulations
(84 FR 40132, August 13, 2019) analyzed the use of 592 hours. The
change from 592 to 1,100 hours does not reflect new or additional
training requirements. Instead, it is the result of a change in how the
Navy counts an ``hour'' of transmission. Previously, SURTASS LFA sonar
hours were calculated by adding the portions of time a sonar emits
sound during its duty cycle (ratio of time the signal is on compared to
off), whereas other Navy sonar systems, such as mid-frequency and high-
frequency active sonar (MFAS and HFAS, respectively), report hours
based on ``duration'' time (total time the source is active, including
silent periods between pings). To bring SURTASS LFA sonar in line with
these other systems, the Navy
[[Page 11622]]
developed a conversion method that considers various factors including
LFA sonar pings, wave trains, and other classified considerations. As a
result, the 1,100 hours of annual SURTASS LFA training proposed are
equivalent to the 592 hours under the previous counting method. The
SURTASS LFA sonar transmission hours, which are classified as military
readiness activities pursuant to the section 315(f) of Public Law 101-
314 (16 U.S.C. 703), represent a distribution across three activities
that include:
<bullet> Training (i.e., contractor crew proficiency training,
military crew proficiency training, active training);
<bullet> Maintenance and upgrade (i.e., equipment maintenance
checks and performance evaluations); and
<bullet> Exercises (e.g., Valiant Shield, Rim of the Pacific
(RIMPAC)).
Compared to the 2019 final rule (84 FR 40132, August 13, 2019), the
geographic bounds of the Study Area and geographic mitigations remain
the same, as do the operating characteristics of SURTASS LFA sonar
system, which remains the only stressor with the potential to cause
take during SURTASS training and testing activities. Although the sonar
hours are calculated differently between the 2019 final rule and
current proposed rulemaking (592 hours and 1,100 hours, respectively),
the active sonar duration remains the same as analyzed for the 2019
rule. All of the acoustic thresholds and take calculation methods used
here are referred to as ``Phase IV'' and described in the technical
report ``Criteria and Thresholds for U.S. Navy Acoustic and Explosive
Effects Analysis (Phase 4)'' (U.S. Department of the Navy, 2025)
(hereafter referred to as the Criteria and Thresholds Technical Report)
mirroring those used in analyses supporting the Phase IV AFTT (90 FR
50504, November 7, 2025) and HCTT (90 FR 58810, December 17, 2025)
training and testing regulations. Alternatively, in the previous
SURTASS LFA sonar rule (84 FR 40132, August 13, 2019), Phase III
thresholds were used for acoustic injury prediction, a SURTASS-specific
threshold was used to predict behavioral disturbance, and different
SURTASS-specific methods and modeling were used in the calculation of
take. This proposed rulemaking proposes to authorize take of marine
mammals by Level A harassment that was not previously requested or
authorized in the 2019 final rule. This change is due to updated marine
mammal hearing thresholds, the criteria for estimating impacts to
marine mammals, and the Navy's reliance on their Navy Acoustics Effects
Model (NAEMO) rather than the Acoustic Integration Model (AIM), which
was used to model and quantify estimated take for the 2019 rulemaking.
These changes are described in the Marine Mammal Hearing Groups
section, the Estimated Take of Marine Mammals section, and the Navy
Acoustic Effects Model section. This proposed rule also includes five
new marine mammal OBIAs for SURTASS LFA sonar, as described in the
Geographic Mitigation section of this proposed rule.
The Navy describes and analyzes the effects of their activities
within the application and provides additional details in the 2025
SURTASS Draft SEIS/OEIS. In their assessment, the Navy concluded that
the transmission of acoustic signals was the only stressor likely to
result in impacts on marine mammals that qualify as harassment as
defined under the MMPA. Therefore, the Navy's application provides
their assessment of potential effects from this stressor.
SURTASS LFA Sonar System
Sonar is an acronym for ``sound navigation and ranging'' and its
definition includes any system that uses underwater sound or acoustics
for observations and communications. The two basic types of sonar used
in the SURTASS LFA sonar system are passive sonar and active sonar.
Passive sonar detects sound created by a source. This is a one-way
transmission of sound waves through water from the source to the
receiver. Very simply, passive sonar ``listens'' without transmitting
any sound signals. Active sonar is the transmission of sound energy for
the purpose of sensing the environment by interpreting features of
received signals. Active sonar detects objects by creating a sound
pulse or ``ping'' that is transmitted from the sonar system through the
water, reflects off a target object, and returns in the form of an echo
to be detected by a receiver. Active sonar is a two-way transmission of
sound waves through water (sound source to reflector to receiver).
SURTASS LFA sonar is a system with three components: low-frequency
(less than 1,000 hertz (Hz)) active sonar system, passive sonar system,
and active high-frequency/marine mammal monitoring (HF/M3) sonar (see
figure 1-1 of the application). The passive component is the SURTASS
receiver array while the active component includes the LFA sonar source
array and HF/M3 sonar.
Although SURTASS LFA sonar vessels usually operate independently
from one another, SURTASS LFA sonar vessels may operate in conjunction
with other naval air, surface, or submarine assets as part of naval
exercises. SURTASS LFA sonar vessels generally travel in straight lines
or racetrack patterns depending on the scenario. When the SURTASS or
LFA sonar arrays are deployed, a SURTASS LFA sonar vessel must maintain
a speed of at least 5.6 kilometers per hour (km/hr) (3 knots), with a
typical speed of 7.4 km/hr. When not towing the SURTASS or LFA sonar
arrays, Tactical-Auxiliary General Ocean Surveillance (T-AGOS) vessels
travel at maximum speeds of approximately 22.2 km/hr. Movements of
SURTASS LFA sonar vessels are not unusual or extraordinary and are in
line with routine operations of seagoing vessels.
Low-Frequency Active (LFA) Sonar
LFA sonar is employed when active sound signals are needed to
detect and track underwater targets of interest. LFA sonar complements
SURTASS passive activities by actively acquiring and tracking
submarines when they are in quiet operating modes, measuring accurate
target range, and re-acquiring lost contacts. LFA sonar consists of a
vertical source array of sound-producing elements that are suspended by
cable under one of the T-AGOS vessels. These elements, called
projectors, are devices that produce the active sonar sound pulses or
pings. To produce a ping, the projectors transform electrical energy
into mechanical energy (i.e., vibrations), which travel as pressure
disturbances in water.
The LFA sonar source is a vertical line array consisting of up to
18 projectors. Each LFA projector transmits sonar beams that are
omnidirectional (360 degrees) in the horizontal, with a narrow vertical
beamwidth that can be steered above or below the horizontal. The
operating features of the LFA sonar are as follows:
<bullet> The source level (SL) of an individual projector on the
LFA sonar array is approximately 215 dB re 1 [mu]Pa RMS SPL or less;
<bullet> For the array SL, the effective SL of the system design
was used; effective SL is a theoretical value, hypothetically measured
at 1 meter (m) from the array on its horizontal axis, calculated from
the formula: SEL + 20 Log10(N), where sound exposure level (SEL) = SL
of an individual projector and N = number of projectors;
<bullet> The source frequency ranges from 100 to 500 Hz;
<bullet> The typical LFA sonar signal is not a constant tone but
consists of various waveforms that vary in frequency and duration. A
complete sequence of sound transmissions (waveforms) is referred to
[[Page 11623]]
as a wavetrain (also known as a ping). These wavetrains last between 6
and 100 seconds, with an average length of 60 seconds. Within each
wavetrain, a variety of signal types can be used, including continuous
wave and frequency-modulated signals. The duration of each continuous
frequency sound transmission within the wavetrain is no longer than 10
seconds;
<bullet> The maximum duty cycle (ratio of sound ``on'' time to
total time) is 20 percent. The typical duty cycle, based on historical
SURTASS LFA sonar operational parameters (from 2003 to 2017), is 7.5-10
percent; and
<bullet> The time between wavetrain transmissions typically ranges
from 6 to 15 minutes.
Compact LFA Active Component
In addition to the LFA sonar system currently deployed on the T-
AGOS vessel United States Naval Ship (USNS) IMPECCABLE, the Navy
developed a compact LFA (CLFA) sonar system, which is now deployed on
its three smaller T-AGOS vessels (USNS ABLE, USNS EFFECTIVE, and USNS
VICTORIOUS). The operational characteristics of the active component
for the CLFA sonar system are comparable to the LFA sonar system and
the potential impacts from the CLFA sonar system will be similar to the
effects from the LFA sonar system. The CLFA sonar system consists of
smaller projectors that weigh 64,410 kilograms (kg), which is 82,554 kg
less than the weight of the LFA projectors on the USNS IMPECCABLE. The
CLFA sonar system also consists of up to 18 projectors suspended
beneath the surveillance vessel in a vertical line array, and the CLFA
sonar system projectors transmit in the low-frequency band (also
between 100 and 500 Hz) with the same duty cycle as described for LFA
sonar. Similar to the active component of the LFA sonar system, the
source level of an individual projector in the CLFA sonar array is
approximately 215 dB re 1 [mu]Pa or less.
For the analysis in this rulemaking, NMFS will use the term LFA to
refer to both the LFA sonar system and/or the CLFA sonar system, unless
otherwise specified.
Passive Acoustic System
SURTASS is the passive, or listening, component of the system that
detects returning sounds from submerged objects, such as threat
submarines, through the use of hydrophones. Hydrophones transform
mechanical energy (i.e., received acoustic sound waves) into an
electrical signal that can be analyzed by the sonar processing system.
The return (received) signals, which are usually below background or
ambient noise level, are processed and evaluated to identify and
classify potential underwater threats. SURTASS consists of a twin-line
(TL-29A), ``Y'' shaped horizontal line array of hydrophones with two
apertures that is approximately 305 m (1,000 feet) long. The SURTASS
horizontal line array can be towed in shallow, littoral environments;
can provide significant directional noise rejection; and can resolve
bearing ambiguities without the vessel's course having to be changed.
High-Frequency/Marine Mammal Monitoring Active (HF/M3) Sonar
The HF/M3 sonar is a Navy-developed, enhanced high-frequency (HF)
commercial sonar used as a mitigation and monitoring asset to detect,
locate, and track marine mammals that may pass close enough to the
SURTASS LFA sonar's transmit array to enter the LFA mitigation zone
(1.8 km (2,000 yards)). This intermittent source has a low-duty cycle
and would operate at a SL of 220 dB re 1 [micro]Pa at 1 m RMS SPL and
source frequencies between 30 and 40 kHz, with maximum pulse length of
40 milliseconds and a variable duty cycle that is nominally 3-4
percent.
Vessel Movement
The Navy currently deploys SURTASS LFA sonar on four T-AGOS vessels
that are 72-86 m in length, with twin-shafted diesel electric engines
capable of providing 3,200-5,000 horsepower. T-AGOS vessels have a
catamaran-type split-hull shape and an enclosed propeller system, and
the bridge of T-AGOS vessels are positioned forward of the centerline,
offering good visibility ahead of the bow and good visibility aft to
visually monitor for marine mammals. Each vessel has an observation
area on the bridge that is more than 9.1 m above sea level from where
Lookouts will monitor for marine mammals whenever SURTASS LFA sonar is
transmitting.
NMFS considered the likelihood that vessel movement during military
readiness activities could result in an incidental, but not
intentional, strike of a marine mammal in the Study Area, which has the
potential to result in serious injury or mortality. Vessel strikes are
not specific to any specific military readiness activity but rather, a
limited, sporadic, and incidental result of the Navy's vessel movement
during military readiness activities within the Study Area. Vessel
strikes from commercial, recreational, and military vessels are known
to seriously injure and occasionally kill cetaceans (Abramson et al.,
2011; Berman-Kowalewski et al., 2010; Calambokidis, 2012; Crum et al.,
2019; Douglas et al., 2008; Laggner 2009; Van der Hoop et al., 2012;
Van der Hoop et al., 2013), although reviews of the literature on
vessel strikes mainly involve collisions between commercial vessels and
whales (Jensen and Silber, 2003, Laist et al., 2001). Vessel speed,
size, and mass are all important factors in determining both the
potential likelihood and impacts of a vessel strike to marine mammals
(Blondin et al. 2025; Conn and Silber, 2013; Garrison et al. 2025;
Gende et al., 2011; Redfern et al., 2019; Silber et al., 2010;
Szesciorka et al., 2019; Vanderlaan and Taggart, 2007; Wiley et al.,
2016). For large vessels, speed and angle of approach can influence the
severity of a strike.
Here, the limited number of Navy vessels operating within the Study
Area, the design and operation of the T-AGOS vessels, the monitoring
and mitigation utilized, and the fact that no strikes of marine mammals
from vessels conducting SURTASS training and testing have occurred in
the past support the conclusion that Navy vessel strikes are not
expected to result from the activities covered by this proposed rule.
The number of vessels used in the Navy's SURTASS testing and training
activities is minimal (especially as compared to the number of
commercial ships transiting in the same areas on an annual basis),
rendering the base probability of a vessel strike very low. The design
of the T-AGOS vessels further reduces the risk of a vessel strike, with
the catamaran-type split hull shape and enclosed propeller system, a
propeller system design that has been suggested as effectively reducing
sharp force injuries to marine species, particularly when combined with
reduced speeds (i.e., 5.6-7.4 km/hr) (Schoeman et al., 2020). Both
detection and avoidance of marine mammals are more effective at slower
speeds and the T-AGOS vessel's slow operational speed (averaging 7.4
km/hr), as well as the relatively slow vessel cruising speed when
SURTASS LFA sonar is not in use (a maximum of approximately 22.2 km/
hr), are generally below the speed at which, records suggest, that
marine mammal injury or death is more common (Laist et al., 2001).
Surface ships operated by the Navy have multiple personnel assigned
to stand watch at all times when moving through the water (underway). A
primary duty of personnel standing watch on surface ships is to detect
and report all objects and disturbances
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sighted in the water that may indicate a threat to the vessel and its
crew, such as debris, a periscope, surfaced submarine, or surface
disturbance. Per vessel safety requirements, personnel standing watch
also report any marine mammals sighted in the path of the vessel as a
standard collision avoidance procedure. All vessels proceed at a safe
speed so they can take proper and effective action to avoid a collision
with any sighted object or disturbance and can stop within a distance
appropriate to the prevailing circumstances and conditions. The Navy
utilizes Lookouts to avoid collisions, and Lookouts are trained to spot
marine mammals so that vessels may change course or take other
appropriate action to avoid collisions (for more information, see
section 2.1.2 of the application).
Further, the use of HF/M3 sonar to monitor for marine mammals in
tandem with Lookouts would support detection of cetaceans well in
advance of any potential vessel strike during SURTASS LFA sonar
training and testing activities. Proposed mitigation, monitoring, and
reporting measures are described in detail later in this document
(please see Proposed Mitigation Measures section, Proposed Monitoring
section, and Proposed Reporting section).
Due to the reasons described above (i.e., low probability of Navy
vessel and marine mammal interactions, vessel design, relatively slow
vessel speeds, high probability of detection and avoidance due to
applied monitoring and mitigation measures), and the fact that there
have been no known Navy vessel strikes in the 22-year history of
SURTASS LFA sonar activities, the Navy has determined, and NMFS
preliminarily concurs, that take of marine mammals by vessel strike is
highly unlikely. Therefore, the Navy has not requested any take of
marine mammals by vessel strike, and NMFS is not proposing to authorize
take by serious injury or mortality by vessel strike.
Standard Operating Procedures
For training and testing to be effective, Navy personnel must be
able to safely use their sensors, platforms, weapons, and other devices
to their optimum capabilities and as intended for use in missions and
combat operations. The Navy has developed standard operating procedures
through decades of experience to provide for safety and mission
success. Because they are essential to safety and mission success,
standard operating procedures are part of the proposed activities and
are considered in the environmental analysis for applicable resources
(see chapter 3 (Affected Environment and Environmental Consequences) of
the 2025 SURTASS Draft SEIS/OEIS). While standard operating procedures
are designed for the safety of personnel and equipment and to ensure
the success of training and testing activities, their implementation
often yields additional benefits for environmental (e.g., marine
mammals), socioeconomic, public health and safety, and cultural
resources.
Because standard operating procedures are essential to safety and
mission success, the Navy considers them to be part of the proposed
activities and has included them in the environmental analysis.
Standard operating procedures that are recognized as providing a
potential secondary benefit on marine mammals that apply to SURTASS LFA
sonar training and testing activities include those related to the
following, described in more detail in section 2.1.2 of the
application:
<bullet> Vessel safety; and
<bullet> Towed in-water device safety.
Standard operating procedures (which are implemented regardless of
their secondary benefits) are different from mitigation measures (which
are designed entirely for the purpose of avoiding or reducing impacts).
Information on mitigation measures is provided in the Proposed
Mitigation Measures section.
Description of Marine Mammals and Their Habitat in the Area of
Specified Activities
Marine mammal species and their associated stocks that have the
potential to occur in the Study Area are presented in table 1 along
with each stock's Endangered Species Act (ESA) and MMPA statuses,
abundance estimate and associated coefficient of variation (CV) value,
minimum abundance estimate, potential biological removal (PBR), annual
M/SI, as applicable, and potential occurrence in the Study Area. The
Navy requests authorization to take individuals of 44 species by Level
B harassment and, for a subset of those species, Level A harassment (9
species), incidental to military readiness activities from the use of
SURTASS LFA sonar in the Study Area. Of note, based on improvements to
the Navy's density research since the 2019 SURTASS LFA Final Rule (84
FR 40132, August 13, 2019), seven more species were modeled for this
proposed rulemaking than the 2019 rulemaking. Of those seven, the
Navy's application includes estimated take of four species from the
proposed activity that were not included in the 2019 final rule: (1)
bearded seal; (2) ringed seal; (3) harbor seal; and (4) Steller sea
lion. Multiple stocks of some species are affected, and independent
assessments are conducted to make the necessary findings and
determinations for each of these.
There are 34 stocks under NMFS' jurisdiction with confirmed or
possible occurrence in the Study Area, of which 11 are listed as
endangered or threatened under the ESA (16 U.S.C. 1531 et seq.).
Currently, the false killer whale (Main Hawaiian Islands Insular DPS)
and Hawaiian monk seal have critical habitat designated under the ESA
in the Study Area (see Critical Habitat section below).
The remaining species in the Central and Western Pacific (CWP) and
Eastern Indian Oceans (EIO) have no stock designation (NSD) under the
MMPA.
Sections 3 and 4 and appendix A (Marine Mammal Species Supplemental
Information) of the application summarize available information
regarding status and trends, distribution and habitat preferences, and
behavior and life history of the potentially affected species. NMFS
fully considered all of this information, and we refer the reader to
these descriptions, instead of reprinting the information. Additional
information regarding population trends and threats may be found in
NMFS' Stock Assessment Reports (SARs) (<a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments</a>) and
more general information about these species (e.g., physical and
behavioral descriptions) may be found on NMFS' website at: <a href="https://www.fisheries.noaa.gov/find-species">https://www.fisheries.noaa.gov/find-species</a>. Additional information on the
general biology and ecology of marine mammals is included in the 2025
SURTASS Draft SEIS/OEIS. Table 1 incorporates the best available
science, including data from the 2023 Pacific and Alaska Marine Mammal
Stock Assessment Reports (Carretta et al., 2024; Young et al., 2024)
(see <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments</a>), and 2024 draft SARs, as well as
monitoring data from the Navy's marine mammal research efforts.
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Species Not Included in the Analysis
The species carried forward for analysis (and described in table 1)
are those likely to be found in the Study Area based on the most recent
data available, and do not include species that may have once inhabited
or transited the area but have not been sighted in recent years (e.g.,
species which were extirpated from factors such as 19th and 20th
century commercial exploitation). The following species were not
included in the analysis.
The North Pacific right whale (NPRW) is one of the rarest marine
mammals worldwide. Their historical range spanned the entire North
Pacific Ocean from approximately 35 degrees north, with feeding grounds
in the Bering Sea, Gulf of Alaska, Okhotsk Sea, and northwestern North
Pacific, the latter of which overlaps a portion of the Study Area. Two
transboundary stocks of NPRW are currently recognized: a Western North
Pacific stock feeding primarily in the Sea of Okhotsk and an Eastern
North Pacific (ENP) stock feeding primarily in the southeastern Bering
Sea (Rosenbaum et al., 2000; Brownell et al., 2001; LeDuc et al., 2012;
Pastene et al., 2022). The summer range of the ENP stock includes the
Gulf of Alaska and Bering Sea but the winter calving grounds for both
stocks are unknown, as they likely migrate out of the Bering Sea during
winter months (Wright, 2017). Recent sightings of NPRW have occurred in
the eastern Bering Sea, southeastern Bering Sea, northern Bering Sea,
and British Columbia (Little, 2021). Acoustic detections of NPRW have
also occurred in the southeastern Bering Sea, Gulf of Alaska, and the
eastern Aleutian Islands. Young et al. (2024) indicate the
N<INF>min</INF> of the ENP stock is 26 individuals based on the 20th
percentile of the photo-identification estimate of 31 whales (CV =
0.226) (Wade et al., 2011). Despite the uncertainty in the current
extent of the range of the ENP stock, and because the abundance of the
ENP stock is so low and recent sightings data does not overlap the
Study Area, it is considered unlikely the ENP stock would be impacted
by the proposed activities.
Bowhead whales (Balaena mysticetus) are limited to the Arctic and
sub-Arctic regions in the Northern Hemisphere, with shorter migrations
than most baleen whales; the Okhotsk Sea and Bering Chukchi-Beaufort
Seas stocks are confined to the Okhotsk and Bering Seas, respectively
(Citta et al., 2023; Citta et al., 2015; Ivashchenko and Clapham,
2010). There has been one record of a calf in lower latitudes in
Canadian waters; however, it is the only sighting of a bowhead within
the eastern North Pacific (Towers et al., 2022). There are no known or
documented sightings of this species within the Study Area.
Beluga whales (Delphinapterus leucas) of the Beaufort Sea, Eastern
Chukchi Sea, and Cook Inlet stocks of beluga whales do not overlap the
Study Area. The Sakhalin-Amur, Ulbansky, Tugursky, Udskaya, and
Shelikhov populations are all found in the northern part of the Okhotsk
Sea, whereas the Anadyr population and the Bristol Bay and Eastern
Bering Sea stocks are all found within the northern parts of the Bering
Sea (Hobbs et al., 2019). While both of these seas border the Study
Area, both seas occur outside the Study Area, and there have been no
documented sightings of belugas within the Study Area.
The distribution of shallow-water porpoise species, such as the
Indo-Pacific finless porpoise (Neophocaena phocaenoides) and narrow-
ridged finless porpoise (N. asiaeorientalis) are in shallow nearshore
waters, where SURTASS LFA sonar is highly unlikely to be detectable.
Freshwater dolphin species, such as the Ganges River dolphin
(Platanista gangetica gangetica), the Indus River dolphin (P. gangetica
minor), and the baiji/Chinese river dolphin (Lipotes vexillifer), are
restricted to riverine waters of the Ganges, Indus, and Yangtze Rivers,
respectively. These river dolphins occur only in the main channels of
these rivers, well inshore of where SURTASS LFA sonar would be
detectable.
Inshore and coastal delphinid species, such as the Irrawaddy
dolphin (Orcaella brevirostris), Australian snubfin dolphin (O.
heinsohni), Indian Ocean humpback dolphin (Sousa plumbea), Indo-Pacific
humpbacked dolphin (S. chinensis), Australian humpback dolphin (S.
sahulensis), and Taiwanese humpbacked dolphin (S. chinensis
taiwanensis), all occur in shallow, coastal waters near shore, where
SURTASS LFA sonar is unlikely to be detectable.
Two species of marine mammal, sea otters (Enhydra lutris) and
dugongs (Dugong dugon), occur in the Study Area but are managed by the
U.S. Fish and Wildlife Service (U.S. FWS) and thus are not considered
further in this analysis.
NMFS standardly considers additional information about the marine
mammals in the area of the specified activities that informs our
analysis, such as identifying known areas of important habitat or
behaviors, or where unusual mortality events have been designated. For
SURTASS LFA sonar training and testing activities, the Navy coordinated
with NMFS to develop a comprehensive method for consideration of these
types of important areas globally and specifically within the Study
Area (for the purposes of identifying OBIAs, discussed further in the
Offshore Biologically Important Areas for SURTASS LFA Sonar section).
This method and these areas are summarized in the Proposed Mitigation
Measures section and described in detail in appendix F of the 2025
SURTASS Draft SEIS/OEIS. Further, we note here that the OBIA
identification criteria include consideration of the hearing
sensitivity of the affected species, given the low frequency (100-500
Hz) of the LFA sonar signal, which most odontocetes and pinnipeds hear
with significantly reduced sensitivity (9-65 dB lower, or more).
Additional details regarding marine mammal hearing sensitivity are
included in the Marine Mammal Hearing Groups section.
Below, we describe the critical habitat for the two species in the
Study Area for which it has been designated under the ESA, as well two
National Marine Sanctuaries that include marine mammal resources,
although both critical habitat and Sanctuaries are considered through
the referenced OBIA process. Further, we briefly describe biologically
important areas (BIAs) for cetaceans identified and scored in Kratofil
et al. (2023), which are not explicitly addressed through the OBIA
process, but which NMFS addresses in the context of mitigation in the
Geographic Mitigation section.
Critical Habitat
Currently, the false killer whale (Main Hawaiian Islands Insular
DPS) and Hawaiian monk seal have ESA-designated critical habitat in the
Study Area.
False Killer Whale (Main Hawaiian Island Insular DPS)
Critical habitat for the ESA-listed Main Hawaiian Islands insular
false killer whale DPS was finalized in July 2018 (83 FR 35062, July
24, 2018) designating waters from the 45 m depth contour to the 3,200 m
depth contour around the main Hawaiian Islands from Niihau east to
Hawaii. This designation does not include most bays, harbors, or
coastal in-water structures. NMFS excluded 14 areas. The total area
designated was approximately 45,504 square kilometers (km\2\) (13,267
nmi\2\) of marine habitat. Critical habitat for the main Hawaiian
Islands insular DPS of
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false killer whale overlaps the Study Area.
Main Hawaiian Islands insular false killer whales are island-
associated whales that rely entirely on the productive submerged
habitat of the main Hawaiian Islands to support all of their life-
history stages. Island-associated marine habitat for Main Hawaiian
Islands insular false killer whale is the only essential feature of the
critical habitat. The following characteristics of this habitat support
insular false killer whales' ability to travel, forage, communicate,
and move freely around and among the waters surrounding the main
Hawaiian Islands: (1) adequate space for movement and use within shelf
and slope habitat; (2) prey species of sufficient quantity, quality,
and availability to support individual growth, reproduction, and
development, as well as overall population growth; (3) waters free of
pollutants of a type and amount harmful to Main Hawaiian Islands
insular false killer whales; and (4) sound levels that would not
significantly impair false killer whales' use or occupancy.
Hawaiian Monk Seal
Critical habitat for Hawaiian monk seals was designated in 1986 (51
FR 16047, April 30, 1986) and later revised in 1988 (53 FR 18988, May
26, 1988) and in 2015 (80 FR 50925, August 21, 2015). In the
Northwestern Hawaiian Islands Hawaiian monk seal critical habitat
includes all beach areas, sand spits and islets, including all beach
crest vegetation to its deepest extent inland as well as the seafloor
and marine habitat 10 m in height above the seafloor from the shoreline
out to the 200 m depth contour around Kure Atoll
(H[omacr]lanik[umacr]), Midway Atoll (Kuaihelani), Pearl and Hermes
Reef (Manawai), Lisianski Island (Kapou), Laysan Island (Kamole), Maro
Reef (Kamokuokamohoali[revaps]i), Gardner Pinnacles
([revaps][Omacr]n[umacr]nui), French Frigate Shoals (Lalo), Necker
Island (Mokumanamana) and Nihoa Island. In the main Hawaiian Islands,
Hawaiian monk seal critical habitat includes the seafloor and marine
habitat to 10 m above the seafloor from the 200 m depth contour through
the shoreline and extending into terrestrial habitat 5 m inland from
the shoreline between identified boundary points around Kaula Island
(includes marine habitat only), Ni[revaps]ihau (includes marine habitat
from 10 to 200 m in depth), Kaua[revaps]i, [Omacr]ahu, Maui Nui
(including Kaho[revaps]olawe, L[amacr]na[revaps]i, Maui, and
Moloka[revaps]i), and Hawaii Island. Critical habitat for the Hawaiian
monk seal overlaps the Study Area.
The essential features of Hawaiian monk seal critical habitat are:
(1) terrestrial areas and adjacent shallow, sheltered aquatic areas
with characteristics preferred by monk seals for pupping and nursing;
(2) marine areas from 0 to 200 m in depth that support adequate prey
quality and quantity for juvenile and adult monk seal foraging; and (3)
significant areas used by monk seals for hauling out, resting or
molting.
Biologically Important Areas
Ferguson et al. (2015) identified BIAs within U.S. waters, which
represent areas and times in which cetaceans are known to concentrate
for reproduction, feeding, and migration, or areas where small and
resident populations are known to occur. Harrison et al. (2023)
identified a new scoring system, described below, and the BIAs in
Hawaiian waters were updated (Kratofil et al., 2023). Unlike ESA
critical habitat, these areas are not formally designated pursuant to
any statute or law but are a compilation of the best available science
intended to inform impact and mitigation analyses. An interactive map
of the BIAs is available at: <a href="https://oceannoise.noaa.gov/biologically-important-areas">https://oceannoise.noaa.gov/biologically-important-areas</a>. A summary of all of the BIAs in the Study Area is
included below.
Kratofil et al. (2023) delineates and scores BIAs for cetaceans in
the Hawaii region following standardized protocols. Experts identified
an overall Importance Score for each BIA that considers: (1)
``Intensity,'' meaning the intensity and characteristics underlying an
area's identification as a BIA; and (2) ``Data Support,'' meaning the
quantity, quality, and type of information, and associated
uncertainties, upon which the BIA delineation and scoring depend.
Importance Scores range from 1 to 3, with a higher score representing
an area of higher intensity and data support. Each BIA is also scored
for boundary uncertainty and spatiotemporal variability (dynamic,
ephemeral, or static). Additionally, hierarchical BIAs are identified
for some species and stocks where a higher intensity score is
appropriate for a smaller core area(s) (child BIA) within a larger BIA
unit (parent BIA).
The Study Area overlaps BIAs in Hawaii for small and resident
populations of the following species: spinner dolphin, short-finned
pilot whale, rough-toothed dolphin, pygmy killer whale, pantropical
spotted dolphin, melon-headed whale, false killer whale, dwarf sperm
whale, goose-beaked whale, common bottlenose dolphin, and Blainville's
beaked whale, and the updated BIAs for humpback whale reproduction
(Kratofil et al., 2023). Table 2 describes each BIA that overlaps the
Study Area and the scores for the above criteria. We note that the BIAs
for small and resident populations of spinner dolphin, melon-headed
whale, and dwarf sperm whale are all fully contained within OBIAs. The
BIAs for small and resident populations of short-finned pilot whale,
rough-toothed dolphin, pygmy killer whale, goose-beaked whale, and
common bottlenose dolphin, and the reproductive BIA for humpback whale,
are mostly contained within the OBIAs. The BIAs for small and resident
populations of pantropical spotted dolphin, false killer whale, and
Blainville's beaked whale are partially contained within the OBIAs
described in the Geographic Mitigation section and proposed for
implementation in this rule.
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National Marine Sanctuaries
Under Title III of the Marine Protection, Research, and Sanctuaries
Act of 1972 (also known as the National Marine Sanctuaries Act (NMSA)),
NOAA has authority to establish as national marine sanctuaries (NMS)
areas of the marine environment with special conservation,
recreational, ecological, historical, cultural, archaeological,
scientific, educational, or aesthetic qualities. Sanctuary regulations
prohibit destroying, causing the loss of, or injuring any sanctuary
resource managed under the law or regulations for that sanctuary (15
CFR part 922).
[[Page 11636]]
NMS are managed on a site-specific basis, and each sanctuary has site-
specific regulations. Most, but not all sanctuaries have site-specific
regulatory exemptions from the prohibitions for certain military
activities. Separately, section 304(d) of the NMSA requires Federal
agencies to consult with the Office of National Marine Sanctuaries
(ONMS) whenever their Proposed Activities are likely to destroy, cause
the loss of, or injure a sanctuary resource. There are two designated
NMSs within the Study Area that contain areas or resources important to
marine mammals (see chapter 3 of the 2025 SURTASS Draft SEIS/OEIS):
<bullet> Hawaiian Islands Humpback Whale NMS; and
<bullet> Papah[amacr]naumoku[amacr]kea NMS.
Hawaiian Islands Humpback Whale NMS is a single-species managed
sanctuary, composed of 3,540 km\2\ of the submerged lands and waters
off the coast of Maui, L[amacr]na[revaps]i, and Moloka[revaps]i; and
smaller areas off the north shore of Kaua[revaps]i, off Hawaii's west
coast, and off the north and southeast coasts of Oahu. Hawaiian Islands
Humpback Whale NMS is entirely within the Study Area and constitutes
one of the world's most important Hawaii humpback whale DPS habitats
(81 FR 62259, September 8, 2016) and is a primary region for humpback
reproduction in the U.S. (National Marine Sanctuaries Program, 2002).
Scientists estimate that more than 50 percent of the entire North
Pacific humpback whale population migrates to Hawaiian waters each
winter to mate, calve, and nurse their young. The North Pacific
humpback whale population has been split into two DPSs. The Hawaii
humpback whale DPS migrates to Hawaiian waters each winter and is not
listed under the ESA. In addition to protection under the MMPA, the
Hawaii humpback whale DPS is protected in sanctuary waters by the
Hawaiian Islands Humpback Whale NMS. The sanctuary was created to
protect humpback whales and shallow, protected waters important for
calving and nursing (Office of National Marine Sanctuaries, 2010).
Papah[amacr]naumoku[amacr]kea NMS, the largest NMS, consists of
approximately 1,508,849 km\2\ of Pacific Ocean waters surrounding the
Northwestern Hawaiian Islands and the submerged lands thereunder. The
sanctuary comprises several interconnected ecosystems, such as coral
islands surrounded by shallow reefs, low-light mesophotic reefs with
extensive algal beds, open ocean waters connected to the greater North
Pacific Ocean, deep-water habitats such as abyssal plains 4,999 m below
sea level, and deep reef habitat characterized by seamounts, banks, and
shoals. The 2,172 km stretch of coral islands, seamounts, banks, and
shoals supports a diversity of coral, fish, birds, and marine mammals,
many of which are unique to the Hawaiian Island chain. Many of the
islands and shallow water environments are important habitats for rare
species such as the endangered Hawaiian monk seal, while the waters are
also important for humpback whale breeding and calving.
Unusual Mortality Events
An unusual mortality event (UME) is defined under section 410(9) of
the MMPA as a stranding that is unexpected; involves a significant die-
off of any marine mammal population; and demands immediate response (16
U.S.C. 1421h(9)). There are no active UMEs in the Study Area.
Marine Mammal Hearing Groups
Hearing is the most important sensory modality for marine mammals
underwater, and exposure to anthropogenic sound can have deleterious
effects. To appropriately assess the potential effects of exposure to
sound, it is necessary to understand the frequency ranges marine
mammals are able to hear. Not all marine mammal species have equal
hearing capabilities (e.g., Richardson et al., 1995, Wartzok and
Ketten, 1999, Au and Hastings, 2008; Erbe et al., 2025). To reflect
this, Southall et al. (2007) and Southall et al. (2019c) recommended
that marine mammals be divided into hearing groups based on directly
measured (behavioral or auditory evoked potential techniques) or
estimated hearing ranges (e.g., behavioral response data, anatomical
modeling). NMFS (2024) generalized hearing ranges were chosen based on
the approximately 65-dB threshold from the composite audiograms,
previous analysis in NMFS (2018), and/or data from Southall et al.
(2007) and Southall et al. (2019c). We note that the names of two
hearing groups and the generalized hearing ranges of all marine mammal
hearing groups have been recently updated (NMFS, 2024) as reflected
below in table 3.
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Of particular relevance to the assessment of the impacts of the
Navy's SURTASS LFA sonar training and testing are the auditory
weighting functions shown in figure 2 and figure 3 (NMFS, 2024), which
illustrate the significantly reduced sensitivity of most marine mammal
taxa to frequencies in the 100-500 Hz range (i.e., 0.1-0.5 kHz as
indicated on the x-axis of these figures in NMFS (2024)), such as
SURTASS LFA sonar. Specifically, the HF cetacean weighting function
curve shows approximately 17-40 dB reduced sensitivity in that
frequency range (i.e., the sound would be perceived as that much lower
level than a sound in the most noise susceptible portion of their
hearing range) (figure 2), the underwater pinniped weighting function
curves (PW, OW) show 9-30 dB reductions (figure 3), and the VHF
cetacean weighting function curve shows a 47-65 dB reduction at
frequencies from 200 to 500 Hz (i.e., generalized hearing range for
this hearing group starts at 200 Hz) and suggest even further reduced
sensitivity (figure 2). Even the LF cetacean species have somewhat
reduced sensitivity in the 100 to 500 Hz range (0.5-6 dB) (figure 2).
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For more detail concerning these hearing groups and associated
frequency ranges and weighting functions, please see NMFS (2024) for a
review of available information.
Of note, the Navy adjusted the LF cetacean hearing group using data
from recent hearing measurements in minke whales (Houser et al., 2024).
These data support separating mysticetes (the LF cetaceans marine
mammal hearing group in table 3) into two hearing groups, which the
Navy designates as ``very low-frequency (VLF) cetaceans'' and ``low-
frequency (LF) cetaceans,''
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which follows the recommendations of Southall et al., (2019c). Within
the Navy's adjusted hearing groups, the VLF cetacean group contains the
larger mysticetes (i.e., blue, pygmy blue, fin, right, and bowhead
whales) and the LF cetacean group contains the mysticete species not
included in the VLF group (e.g., minke, humpback, gray, pygmy right
whales). Although there have been no direct measurements of hearing
sensitivity in the larger mysticetes included in Navy's VLF hearing
group, an audible frequency range of approximately 10 Hz to 30 kHz has
been estimated from measured vocalization frequencies, observed
responses to playback of sounds, and anatomical analyses of the
auditory system. The upper frequency limit of hearing in Navy's LF
hearing group has been estimated as 64 kHz, based on direct
measurements of auditory evoked potentials in minke whales (Houser et
al., 2024).
Potential Effects of Specified Activities on Marine Mammals and Their
Habitat
This section provides a discussion of the ways in which components
of the specified activity may impact marine mammals and their habitat.
The Estimated Take of Marine Mammals section later in this document
includes a quantitative analysis of the number of individuals that are
expected to be taken by this activity. The Preliminary Analysis and
Negligible Impact Determination section considers the content of this
section, the Estimated Take of Marine Mammals section, and the Proposed
Mitigation Measures section to draw conclusions regarding the likely
impacts of these activities on the reproductive success or survivorship
of individuals and whether those impacts on individuals are likely to
adversely affect the species or stock through effects on annual rates
of recruitment or survival.
The Navy has requested authorization for the take of marine mammals
that may occur incidental to training and testing activities in the
Study Area. The Navy analyzed potential impacts to marine mammals from
acoustic sources in the application. NMFS carefully reviewed the
information provided by the Navy and concurs with their synthesis of
science, along with independently reviewing applicable scientific
research and literature and other information to evaluate the potential
effects of the Navy's activities on marine mammals, which are presented
in this section (see appendix D in the 2025 SURTASS Draft SEIS/OEIS for
additional information).
Potential impacts to marine mammals from training and testing
activities in the Study Area were analyzed in the 2025 SURTASS Draft
SEIS/OEIS, in consultation with NMFS as a cooperating agency, and
stressors other than acoustic sources were determined to be unlikely to
result in marine mammal take. Therefore, the Navy has not requested
authorization for take of marine mammals incidental to other components
of their proposed Specified Activities, and we agree that incidental
take is unlikely to occur from those components. In this proposed rule,
NMFS analyzes the potential effects on marine mammals from the activity
components that may result in take of marine mammals: exposure to
acoustic stressors (i.e., sonar).
For the purpose of MMPA ITAs, NMFS' effects assessments serve four
primary purposes: (1) to determine whether the specified activities
would have a negligible impact on the affected species or stocks of
marine mammals (based on whether it is likely that the activities would
adversely affect the species or stocks through effects on annual rates
of recruitment or survival); (2) to determine whether the specified
activities would have an unmitigable adverse impact on the availability
of the species or stocks for subsistence uses; (3) to prescribe the
permissible methods of taking (i.e., Level B harassment (behavioral
harassment and temporary threshold shift (TTS)), Level A harassment
(auditory injury (AUD INJ), non-auditory injury), serious injury, or
mortality), including identification of the number and types of take
that could occur by harassment, serious injury, or mortality, and to
prescribe other means of effecting the least practicable adverse impact
on the species or stocks and their habitat (i.e., mitigation measures);
and (4) to prescribe requirements pertaining to monitoring and
reporting.
In this section, NMFS provides a description of the ways marine
mammals may generally be affected by acoustic stressors in the form of
mortality, physical injury, sensory impairment (permanent and temporary
threshold shifts and acoustic masking), physiological responses
(particular stress responses), behavioral disturbance, or habitat
effects. The Estimated Take of Marine Mammals section discusses how the
potential effects on marine mammals from non-impulsive sources relate
to the MMPA definitions of Level A Harassment and Level B Harassment
and quantifies those effects that do not qualify as a take under the
MMPA. The Preliminary Analysis and Negligible Impact Determination
section assesses whether the proposed authorized take would have a
negligible impact on the affected species and stocks.
Potential Effects of Underwater Sound on Marine Mammals
The marine soundscape is composed of both ambient and anthropogenic
sounds. Ambient sound is defined as the all-encompassing sound in a
given place and is usually a composite of sound from many sources both
near and far (American National Standards Institute, 1995). The sound
level of an area is defined by the total acoustical energy being
generated by known and unknown sources, which may include physical
(e.g., waves, wind, precipitation, earthquakes, ice, atmospheric
sound), biological (e.g., sounds produced by marine mammals, fish, and
invertebrates), and anthropogenic sound (e.g., vessels, dredging,
aircraft, construction).
The sum of the various natural and anthropogenic sound sources at
any given location and time--which comprise ``ambient'' or
``background'' sound--depends not only on the source levels (as
determined by current weather conditions and levels of biological and
shipping activity) but also on the ability of sound to propagate
through the environment. In turn, sound propagation is dependent on the
spatially and temporally varying properties of the water column and sea
floor and is frequency-dependent. As a result of the dependence on a
large number of varying factors, ambient sound levels can be expected
to vary widely over both coarse and fine spatial and temporal scales.
Sound levels at a given frequency and location can vary by 10-20 dB
from day to day (Richardson et al., 1995). The result is that,
depending on the source type and its intensity, sound from the
specified activities may be a negligible addition to the local
environment or could form a distinctive signal that may affect marine
mammals.
Anthropogenic sounds cover a broad range of frequencies and sound
levels and can have a range of highly variable impacts on marine life,
from none or minor to potentially severe responses, depending on
received levels, duration of exposure, behavioral context, and various
other factors. The potential effects of underwater sound from active
acoustic sources can possibly result in one or more of the following:
temporary or permanent hearing impairment, other auditory injury, non-
auditory physical or physiological effects, behavioral disturbance,
stress, and masking (Richardson et al., 1995; Gordon et al., 2003;
Nowacek et al., 2007; Southall et
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al., 2007; G[ouml]tz et al., 2009, Southall et al., 2019a; Erbe et al.,
2025). The degree of effect is intrinsically related to the signal
characteristics, received level, distance from the source, and duration
of the sound exposure. In general, sudden, high-level sounds can cause
auditory injury, as can longer exposures to lower level sounds.
Temporary or permanent loss of hearing can occur after exposure to
noise and occur almost exclusively for noise within an animal's hearing
range.
Richardson et al. (1995) described zones of increasing intensity of
effect that might be expected to occur, in relation to distance from a
source and assuming that the signal is within an animal's hearing
range. First is the area within which the acoustic signal would be
audible (potentially perceived) to the animal, but not strong enough to
elicit any overt behavioral or physiological response. The next zone
corresponds with the area where the signal is audible to the animal and
of sufficient intensity to elicit behavioral or physiological
responsiveness. Third is a zone within which, for signals of high
intensity, the received level is sufficient to potentially cause
discomfort or tissue damage to auditory systems. Overlaying these zones
to a certain extent is the area within which masking (i.e., when a
sound interferes with or masks the ability of an animal to detect a
signal of interest that is above the absolute hearing threshold) may
occur. The masking zone may be highly variable in size.
We also describe more severe potential effects (i.e., certain non-
auditory physical or physiological effects). Potential effects from
high-level sound sources can range in severity from effects such as
behavioral disturbance or tactile perception to physical discomfort,
physiological damage, and injury of the auditory system (Yelverton et
al., 1973). Non-auditory physiological effects or injuries that
theoretically might occur in marine mammals exposed to high levels of
underwater sound or as a secondary effect of extreme behavioral
responses (e.g., change in dive profile as a result of an avoidance
response) caused by exposure to sound include neurological effects,
bubble formation, resonance effects, and other types of organ or tissue
damage (Cox et al., 2006; Southall et al., 2007; Zimmer and Tyack,
2007; Tal et al., 2015).
Hearing
Marine mammals have adapted hearing based on their biology and
habitat. Amphibious marine mammals (e.g., pinnipeds that spend time on
land and underwater) have modified ears that allow them to hear both
in-air and in-water, while fully aquatic marine mammals (e.g.,
cetaceans that are always underwater) have specialized ear adaptations
for in-water hearing (Wartzok and Ketten, 1999). These adaptations
explain the variation in hearing ability and sensitivity among marine
mammals and have led to the characterization of marine mammal
functional hearing groups based on those sensitivities (see Marine
Mammal Hearing Groups section).
The hearing sensitivity of marine mammals is also directional,
meaning the angle between an animal's position and the location of a
sound source impacts the animal's hearing threshold, thereby impacting
an animal's ability to perceive the sound emanating from that source.
This directionality is likely useful for determining the general
location of a sound, whether for detection of prey, predators, or
members of the same species, and can be dependent upon the frequency of
the sound (Accomando et al., 2020; Au and Moore, 1984; Byl et al.,
2016; Byl et al. 2019; Kastelein et al., 2005; Kastelein et al., 2019;
Popov and Supin, 2009).
Acoustic Signaling
An acoustic signal refers to the sound waves used to communicate
underwater, and marine mammals use a variety of acoustic signals for
socially important functions, such as communicating, as well as
biologically important functions, such as echolocating (Richardson et
al., 1995; Wartzok and Ketten, 1999; Erbe et al., 2025). Acoustic
signals used for communication are lower frequency (i.e., 20 Hz to 30
kHz) than those signals used for echolocation, which are high-frequency
(approximately 10-200 kHz peak frequency) signals used by odontocetes
to sense their underwater environment. Lower frequency vocalizations
used for communication may have a specific, prominent fundamental
frequency (Brady et al., 2021) or have a wide frequency range,
depending on the functional hearing group and whether the marine mammal
is vocalizing in-water or in-air. Acoustic signals used for
echolocation are high-frequency, high-energy sounds with patterns and
peak frequencies that are often species-specific (Baumann-Pickering et
al., 2013).
Marine mammal species typically produce sounds at frequencies
within their own hearing range, though auditory and vocal ranges do not
perfectly align (e.g., odontocetes may hear only a portion of the
frequencies of an echolocation click). Because determining a species
vocal range is easier than determining a species' hearing range, vocal
ranges are often used to infer a species' hearing range when species-
specific hearing data are not available (e.g., large whale species).
Table 3, figure 2, and figure 3 in the Marine Mammal Hearing Groups
section summarize the available data on marine mammal hearing groups,
which is relevant given the significantly reduced sensitivity of most
marine mammal taxa (9-65 dB and above for all but LF species) to the
SURTASS LFA sonar source.
Hearing Loss and Auditory Injury
Marine mammals, like all mammals, lose their ability to hear over
time due to age-related degeneration of auditory pathways and sensory
cells of the inner ear. This natural, age-related hearing loss is
distinct from acute noise-induced hearing loss (M[oslash]ller, 2013).
Noise-induced hearing loss can be temporary (i.e., TTS) or result in a
permanent (permanent threshold shift (PTS)), with higher-level sound
exposures more likely to cause PTS or other AUD INJ. For marine
mammals, AUD INJ is considered to be possible when sound exposures are
sufficient to produce 40 dB of TTS measured approximately 4 minutes
after exposure (NMFS, 2024; U.S. Department of the Navy, 2025).
Numerous studies have directly examined noise-induced hearing loss in
marine mammals by measuring an animal's hearing threshold before and
after exposure to intense or long duration sounds. The difference
between the post-exposure and pre-exposure hearing thresholds is then
used to determine the amount of TTS (in dB) that was produced as a
result of the sound exposure (see appendix D of the 2025 SURTASS Draft
SEIS/OEIS for additional details). The Navy used these studies to
generate exposure functions, which are predictions of the onset of TTS
or PTS or other AUD INJ based on sound frequency, level, and type (non-
impulsive or impulsive), for each marine mammal hearing group (NMFS,
2024; U.S. Department of the Navy, 2025).
TTS can last from minutes or hours to days (i.e., however, there is
complete recovery back to baseline/pre-exposure hearing threshold), can
occur within a specific frequency range (i.e., an animal might have a
temporary loss of hearing sensitivity within only a limited frequency
band of its auditory range), and can be of varying amounts (e.g., an
animal's hearing sensitivity might be reduced by only 6 dB or reduced
by 30 dB). While there is no simple functional relationship between TTS
and PTS or
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other AUD INJ (e.g., neural degeneration), as TTS increases, the
likelihood that additional exposure to increased SPL or duration will
result in PTS or other injury also increases (see appendix D of the
2025 SURTASS Draft SEIS/OEIS for additional discussion). Exposure
thresholds for the occurrence of AUD INJ, which include the potential
for PTS, as well as situations when AUD INJ occurs without PTS, can
therefore be defined based on a specific amount of TTS; that is,
although an exposure has been shown to produce only TTS, we assume that
any additional TTS exposure may result in some AUD INJ. The specific
upper limit of TTS is based on experimental data showing amounts of TTS
that have not resulted in AUD INJ. In other words, we do not need to
know the exact functional relationship between TTS and AUD INJ, we need
to know only the upper limit for TTS to determine when some AUD INJ is
possible. In most cases of AUD INJ, the animal has an impaired ability
to hear sounds in specific frequency ranges (Kryter, 1985; Finneran,
2015).
The following physiological mechanisms are thought to play a role
in inducing auditory threshold shift: (1) effects to sensory hair cells
in the inner ear that reduce their sensitivity; (2) modification of the
chemical environment within the sensory cells; (3) displacement of
certain inner ear membranes; (4) increased blood flow; and (5) post-
stimulatory reduction in both efferent and sensory neural output
(Southall et al., 2007). The amplitude, duration, frequency, temporal
pattern, and energy distribution of sound exposure all can affect the
amount of associated threshold shift and the frequency range in which
it occurs. Generally, the amount of threshold shift, and the time
needed to recover from the effect, increase as amplitude and/or
duration of sound exposure increases. Human non-impulsive noise
exposure guidelines are based on the assumption that exposures of equal
energy (the same cumulative SEL) produce equal amounts of hearing
impairment regardless of how the sound energy is distributed in time
(NIOSH, 1998). Previous marine mammal TTS studies have also generally
supported this equal energy relationship (Southall et al., 2007;
Finneran, 2015; Southall et al., 2019). Cumulative SEL is used to
predict TTS in marine mammals and is considered a good predictor of TTS
for shorter duration exposures than longer duration exposures. The
amount of TTS increases with exposure SPL and duration, and is
correlated with cumulative SEL, but duration of the exposure has a more
significant effect on TTS than would be predicted based on cumulative
SEL alone (e.g., Finneran et al., 2010b; Kastak et al., 2007; Kastak et
al., 2005; Kastelein et al., 2014a; Mooney et al., 2009a; Popov et al.,
2014; Gransier and Kastelein, 2024). These studies highlight the
inherent complexity of predicting TTS onset in marine mammals, as well
as the importance of considering exposure duration when assessing
potential impacts.
Generally, TTS increases with cumulative SEL in a non-linear
fashion, where lower SEL exposures will elicit a steady rate of TTS
increase while higher SEL exposures will either increase TTS more
rapidly or plateau (Finneran, 2015; U.S. Department of the Navy, 2025).
Additionally, with sound exposures of equal energy, those that had
lower SPL with longer duration were found to induce TTS onset at lower
levels than those of higher SPL and shorter duration. Less threshold
shift will occur from intermittent sounds than from a continuous
exposure with the same energy (some recovery can occur between
intermittent exposures) (Kryter et al., 1966; Ward, 1997; Mooney et
al., 2009a, 2009b; Finneran et al., 2010; Kastelein et al., 2014;
Kastelein et al., 2015). For example, one short, higher SPL sound
exposure may induce the same impairment as one longer lower SPL sound,
which in turn may cause more impairment than a series of several
intermittent softer sounds with the same total energy (Ward, 1997).
Additionally, though TTS is temporary, very prolonged or repeated
exposure to sound loud enough to elicit TTS, or shorter-term exposure
to sound levels well above the TTS threshold, can cause AUD INJ, at
least in terrestrial mammals (Kryter, 1985; Lonsbury-Martin et al.,
1987).
Although TTS increases non-linearly in marine mammals, recovery
from TTS typically occurs in a linear fashion with the logarithm of
time (Finneran, 2015; Finneran et al., 2010a; Finneran et al., 2010b;
Finneran and Schlundt, 2013; Kastelein et al., 2012a; Kastelein et al.,
2012b; Kastelein et al., 2013a; Kastelein et al., 2014a; Kastelein et
al., 2014b; Kastelein et al., 2014c; Popov et al., 2014; Popov et al.,
2013; Popov et al., 2011; Muslow et al., 2023; Finneran et al., 2023).
Considerable variation has been measured in individuals of the same
species in both the amount of TTS incurred from similar cumulative SELs
(Kastelein et al., 2012a; Popov et al., 2013) and the time-to-recovery
from TTS (Finneran, 2015; Kastelein et al., 2019e). Many of these
studies relied on continuous sound exposures, but intermittent,
impulsive sound exposures have also been tested. Few studies (Finneran
et al., 2002; Lucke et al., 2009; Sills et al., 2020; Muslow et al.,
2023) using impulsive sounds have produced enough TTS to make
predictions about hearing loss due to this source type (see U.S.
Department of the Navy, 2025). In general, predictions of TTS based on
cumulative SEL for this type of sound exposure are likely to
overestimate TTS because some recovery from TTS may occur in the quiet
periods between impulsive or intermittent sounds, especially when the
duty cycle is low. Peak SPL (unweighted) is also used to predict TTS
due to impulsive sounds (Southall et al., 2007; Southall et al., 2019c;
U.S. Department of the Navy, 2025).
In some cases, associated with terrestrial mammal noise studies,
intense noise exposures have caused AUD INJ (e.g., loss of cochlear
neuron synapses), despite thresholds eventually returning to normal
(i.e., it is possible to have AUD INJ without a resulting PTS (e.g.,
Kujawa and Liberman, 2006, 2009; Fernandez et al., 2015; Ryan et al.,
2016; Houser, 2021). In these situations, however, threshold shifts
were 30-50 dB measured 24 hours after the exposure (i.e., there is no
evidence that an exposure resulting in less than 40 dB TTS measured a
few minutes after exposure can produce AUD INJ). Therefore, an exposure
producing 40 dB of TTS, measured a few minutes after exposure, can also
be used as an upper limit to prevent AUD INJ (i.e., it is assumed that
exposures beyond those capable of causing 40 dB of TTS have the
potential to result in INJ (which may or may not result in PTS)).
Irreparable damage to the inner or outer cochlear hair cells may
cause PTS; however, other mechanisms are also involved, such as
exceeding the elastic limits of certain tissues and membranes in the
inner ears and resultant changes in the chemical composition of the
inner ear fluids (Southall et al., 2007). When AUD INJ occurs, there is
physical damage (i.e., typically mechanical) to the sound receptors in
the ear, whereas TTS represents primarily tissue fatigue (i.e.,
typically metabolic) and is fully reversible (Southall et al., 2007).
AUD INJ is permanent (i.e., there is incomplete recovery back to
baseline/pre-exposure levels) but also can occur in a specific
frequency range and amount as mentioned above for TTS. In addition,
other investigators have suggested that TTS is within the normal bounds
of physiological variability and tolerance and does not represent
physical injury (e.g., Ward, 1997). Therefore, NMFS does not consider
less than 40 dB of TTS to constitute AUD
[[Page 11643]]
INJ. The NMFS Acoustic Updated Technical Guidance (NMFS, 2024), which
was used in the assessment of effects for this proposed rule, compiled,
interpreted, and synthesized the best available scientific information
for noise-induced hearing effects for marine mammals to derive updated
thresholds for assessing the impacts of noise on marine mammal hearing.
While many studies have examined noise-induced hearing loss in
marine mammals (see Finneran (2015) and Southall et al. (2019a) for
summaries), published data on the onset of TTS for cetaceans are
limited to the captive bottlenose dolphin, beluga, harbor porpoise, and
Yangtze finless porpoise, and for pinnipeds in water, measurements of
TTS are limited to harbor seals, elephant seals (Mirounga species),
California sea lions (Zalophus californianus), and bearded seals. These
studies examine hearing thresholds measured in marine mammals before
and after exposure to intense sounds, which can then be used to
determine the amount of threshold shift at various post-exposure times.
NMFS has reviewed the available studies, which are summarized below
(see also the 2025 SURTASS Draft SEIS/OEIS which includes additional
discussion on TTS studies related to sonar and other transducers).
<bullet> The method used to test hearing may affect the resulting
amount of measured TTS, with neurophysiological measures producing
larger amounts of TTS compared to psychophysical measures (Finneran et
al., 2007; Finneran, 2015).
<bullet> The amount of TTS varies with the hearing test frequency.
As the exposure SPL increases, the frequency at which the maximum TTS
occurs also increases (Kastelein et al., 2014b). For high-level
exposures, the maximum TTS typically occurs one-half to one octave
above the exposure frequency (Finneran et al., 2007; Mooney et al.,
2009a; Nachtigall et al., 2004; Popov et al., 2011; Popov et al., 2013;
Schlundt et al., 2000). The overall spread of TTS from tonal exposures
can therefore extend over a large frequency range (i.e., narrowband
exposures can produce broadband (greater than one octave) TTS).
<bullet> The amount of TTS increases with exposure SPL and duration
and is correlated with cumulative SEL, especially if the range of
exposure durations is relatively small (Kastak et al., 2007; Kastelein
et al., 2014b; Popov et al., 2014). As the exposure duration increases,
however, the relationship between TTS and cumulative SEL begins to
break down. Specifically, duration has a more significant effect on TTS
than would be predicted on the basis of cumulative SEL alone (Finneran
et al., 2010a; Kastak et al., 2005; Mooney et al., 2009a). This means
if two exposures have the same cumulative SEL but different durations,
the exposure with the longer duration (thus lower SPL) will tend to
produce more TTS than the exposure with the higher SPL and shorter
duration. In most acoustic impact assessments, the scenarios of
interest involve shorter duration exposures than the marine mammal
experimental data from which impact thresholds are derived; therefore,
use of cumulative SEL tends to over-estimate the amount of TTS. Despite
this, cumulative SEL continues to be used in many situations because it
is relatively simple, more accurate than SPL alone, and lends itself
easily to scenarios involving multiple exposures with different SPL
(Finneran, 2015).
<bullet> Gradual increases of TTS over multiple exposures may not
be directly observable with increasing exposure levels, before the
onset of PTS (Reichmuth et al., 2019). Similarly, PTS can occur without
measurable behavioral modifications (Reichmuth et al., 2019).
<bullet> The amount of TTS depends on the exposure frequency.
Sounds at low frequencies, well below the region of highest
susceptibility, are less hazardous than those at higher frequencies,
near the region of highest susceptibility (Finneran and Schlundt,
2013). The onset of TTS--defined as the exposure level necessary to
produce 6 dB of TTS (i.e., clearly above the typical variation in
threshold measurements)--also varies with exposure frequency. At the
low frequency end of a species' hearing curve (i.e., audiogram), onset-
TTS exposure levels are higher compared to those in the region of best
sensitivity.
<bullet> TTS can accumulate across multiple intermittent exposures,
but the resulting TTS will be less than the TTS from a single,
continuous exposure with the same cumulative SEL (Finneran et al.,
2010a; Kastelein et al., 2014b; Kastelein et al., 2015b; Mooney et al.,
2009b). This means that TTS predictions based on the total, cumulative
SEL will overestimate the amount of TTS from intermittent exposures
such as sonars and impulsive sources.
<bullet> The amount of observed TTS tends to decrease with
increasing time following the exposure; however, the relationship is
not monotonic (i.e., increasing exposure does not always increase TTS).
The time required for complete recovery of hearing depends on the
magnitude of the initial shift; for relatively small shifts recovery
may be complete in a few minutes, while large shifts (e.g.,
approximately 40 dB or greater) may require several days for recovery.
Under many circumstances TTS recovers linearly with the logarithm of
time (Finneran et al., 2010a, 2010b; Finneran and Schlundt, 2013;
Kastelein et al., 2012a; Kastelein et al., 2012b; Kastelein et al.,
2013a; Kastelein et al., 2014b; Kastelein et al., 2014c; Popov et al.,
2011; Popov et al., 2013; Popov et al., 2014). This means that for each
doubling of recovery time, the amount of TTS will decrease by the same
amount (e.g., 6 dB recovery per doubling of time).
Nachtigall et al. (2018) and Finneran (2018) describe the
measurements of hearing sensitivity of multiple odontocete species
(i.e., bottlenose dolphin, harbor porpoise, beluga, and false killer
whale) when a relatively loud sound was preceded by a warning sound.
These captive animals were shown to reduce hearing sensitivity when
warned of an impending intense sound. Based on these experimental
observations of captive animals, the authors suggest that wild animals
may dampen their hearing during prolonged exposures or if conditioned
to anticipate intense sounds. Finneran (2018) recommends further
investigation of the mechanisms of hearing sensitivity reduction in
order to understand the implications for interpretation of existing TTS
data obtained from captive animals, notably for considering TTS due to
short duration, unpredictable exposures.
Marine mammal hearing plays a critical role in communication with
conspecifics and in interpretation of environmental cues for purposes
such as predator avoidance and prey capture. Depending on the degree
(elevation of threshold in dB), duration (i.e., recovery time), and
frequency range of TTS, and the context in which it is experienced, TTS
can have effects on marine mammals, ranging from discountable to
serious, similar to those discussed in auditory masking below. For
example, a marine mammal may be able to readily compensate for a brief,
relatively small amount of TTS in a non-critical frequency range that
takes place during a time where ambient noise is lower and there are
not as many competing sounds present. Alternatively, a larger amount
and longer duration of TTS sustained during a time when communication
is critical for successful mother/calf interactions could have more
serious impacts if it were in the same frequency band as the necessary
vocalizations and of a severity that impeded communication. The fact
that animals
[[Page 11644]]
exposed to high levels of sound that would be expected to result in
this physiological response would also be expected to have behavioral
responses of a comparatively more severe or sustained nature is
potentially more significant than the simple existence of a TTS.
However, it is important to note that TTS could occur due to longer
exposures to sound at lower levels so that a behavioral response may
not be elicited.
Depending on the degree and frequency range, the effects of AUD INJ
on an animal could also range in severity, although it is considered
generally more serious than TTS because it is a permanent condition
(Reichmuth et al., 2019). Of note, reduced hearing sensitivity as a
simple function of aging has been observed in marine mammals as well as
in humans and other taxa (Southall et al., 2007). We can infer that
strategies exist for coping with this condition to some degree, though
likely not without some cost to the animal.
As the amount of research on hearing sensitivity has grown, so,
too, has the understanding that marine mammals may be able to self-
mitigate or protect against noise-induced hearing loss. An animal may
learn to reduce or suppress their hearing sensitivity when warned of an
impending intense sound exposure, or if the duty cycle of the sound
source is predictable, as has been demonstrated in some odontocete
species (Finneran, 2018; Finneran et al., 2024; Nachtigall and Supin,
2013, 2014, 2015; Nachtigall et al., 2016a, 2016b, 2016c, 2018). This
has been shown with several species, including the false killer whale
(Nachtigall and Supin, 2013), bottlenose dolphin (Finneran, 2018;
Nachtigall and Supin, 2014, 2015; Nachtigall et al., 2016c), beluga
whale (Nachtigall et al., 2016a), and harbor porpoise (Nachtigall et
al., 2016b; Kastelein et al., 2020). Additionally, Finneran et al.
(2023) and Finneran et al. (2024) found that odontocetes that had
participated in TTS experiments in the past could have learned from
that experience and subsequently protected their hearing during new
sound exposure experiments.
Behavioral Responses
Behavioral responses to sound are highly variable and context-
specific (Nowacek et al., 2007; Southall et al., 2007; Southall et al.,
2019). Many different variables can influence an animal's perception of
and response to (nature and magnitude) an acoustic event. An animal's
prior experience with a sound or sound source affects whether it is
less likely (habituation, self-mitigation) or more likely
(sensitization) to respond to certain sounds in the future (animals can
also be innately predisposed to respond to certain sounds in certain
ways) (Southall et al., 2007; Southall et al., 2016; Finneran, 2018;
Finneran et al., 2024; Nachtigall and Supin, 2013, 2014, 2015;
Nachtigall et al., 2015, 2016a, 2016b, 2018). Related to the sound
itself, the perceived proximity of the sound, bearing of the sound
(approaching vs. retreating), the similarity of a sound to biologically
relevant sounds in the animal's environment (i.e., calls of predators,
prey, or conspecifics), familiarity of the sound, and navigational
constraints may affect the way an animal responds to the sound (Ellison
et al., 2012; Southall et al., 2007; DeRuiter et al., 2013a; Southall
et al., 2021; Wartzok et al., 2003). Individuals (of different age,
sex, reproductive status, etc.) among most populations will have
variable hearing capabilities, and differing behavioral sensitivities
to sounds that will be affected by prior conditioning, experience, and
current activities of those individuals. Southall et al., (2007) and
Southall et al. (2021) have developed and subsequently refined methods
developed to categorize and assess the severity of acute behavioral
responses, considering impacts to individuals that may consequently
impact populations. Often, specific acoustic features of the sound and
contextual variables (i.e., proximity, duration, or recurrence of the
sound or the current behavior that the marine mammal is engaged in or
its prior experience), as well as entirely separate factors such as the
physical presence of a nearby vessel, may be more relevant to the
animal's response than the received level alone.
Studies by DeRuiter et al., (2013a) indicate that variability of
responses to acoustic stimuli depends not only on the species receiving
the sound and the sound source, but also on the social, behavioral, or
environmental contexts of exposure. Another study by DeRuiter et al.,
(2013b) examined behavioral responses of goose-beaked whales to MF
sonar and found that whales responded strongly at low received levels
(89-127 dB re 1 [micro]Pa) by ceasing normal fluking and echolocation,
swimming rapidly away, and extending both dive duration and subsequent
non-foraging intervals when the sound source was 3.4-9.5 km away.
Importantly, this study also showed that whales exposed to a similar
range of received levels (78-106 dB re 1 [micro]Pa) from distant sonar
exercises 118 km away did not elicit such responses, suggesting that
context may moderate responses.
Ellison et al. (2012) outlined an approach to assessing the effects
of sound on marine mammals that incorporates contextual-based factors.
The authors recommend considering not just the received level of sound,
but also the activity the animal is engaged in at the time the sound is
received, the nature and novelty of the sound (i.e., whether this a new
sound from the animal's perspective), and the distance between the
sound source and the animal. They submit that this ``exposure
context,'' as described, greatly influences the type of behavioral
response exhibited by the animal. Forney et al., (2017) also point out
that an apparent lack of response (e.g., no displacement or avoidance
of a sound source) may not necessarily mean there is no cost to the
individual or population, as some resources or habitats may be of such
high value that animals may choose to stay, even when experiencing
stress or hearing loss. Forney et al., (2017) recommend considering
both the costs of remaining in an area of noise exposure such as TTS,
PTS, or masking, which could lead to an increased risk of predation or
other threats or a decreased capability to forage, and the costs of
displacement, including potential increased risk of vessel strike,
increased risks of predation or competition for resources, or decreased
habitat suitable for foraging, resting, or socializing. This sort of
contextual information is challenging to predict with accuracy for
ongoing activities that occur over large spatial and temporal expanses.
However, distance is one contextual factor for which data exist to
quantitatively inform a take estimate, and the method for predicting
Level B harassment in this proposed rule does consider distance to the
source. Other factors are often considered qualitatively in the
analysis of the likely consequences of sound exposure, where supporting
information is available.
Friedlaender et al., (2016) provided the first integration of
direct measures of prey distribution and density variables incorporated
into across-individual analyses of behavior responses of blue whales to
sonar and demonstrated a five-fold increase in the ability to quantify
variability in blue whale diving behavior. These results illustrate
that responses evaluated without such measurements for foraging animals
may be misleading, which again illustrates the context-dependent nature
of the probability of response.
Exposure of marine mammals to sound sources can result in, but is
not
[[Page 11645]]
limited to, no response or any of the following observable responses:
(1) increased alertness; (2) orientation or attraction to a sound
source; (3) vocal modifications; (4) cessation of feeding; (5)
cessation of social interaction; (6) alteration of movement or diving
behavior; habitat abandonment (temporary or permanent); and (in severe
cases) (7) panic, flight, stampede, or stranding, potentially resulting
in death (Southall et al., 2007). A review of marine mammal responses
to anthropogenic sound was first conducted by Richardson (1995). More
recent reviews (Nowacek et al., 2007; DeRuiter et al., 2013a and 2013b;
Ellison et al., 2012; Gomez et al., 2016; Erbe et al., 2025) address
studies conducted since 1995 and focused on observations where the
received sound level of the exposed marine mammal(s) was known or could
be estimated. Gomez et al., (2016) conducted a review of the literature
considering the contextual information of exposure in addition to
received level and found that higher received levels were not always
associated with more severe behavioral responses and vice versa.
Southall et al., (2016) states that results demonstrate that some
individuals of different species display clear yet varied responses,
some of which have negative implications, while others appear to
tolerate high levels, and that responses may not be fully predictable
with simple acoustic exposure metrics (e.g., received sound level).
Rather, the authors state that differences among species and
individuals along with contextual aspects of exposure (e.g., behavioral
state) appear to affect response probability (Southall et al., 2019).
The following parts provide examples of behavioral responses to
stressors that provide an idea of the variability in responses that
would be expected given the differential sensitivities of marine mammal
species to sound and the wide range of potential acoustic sources to
which a marine mammal may be exposed. Behavioral responses that could
occur for a given sound exposure should be determined from the
literature that is available for each species (see appendix B of the
2025 SURTASS Draft SEIS/OEIS for a comprehensive list of behavioral
studies and species-specific findings) or extrapolated from closely
related species when no information exists, along with contextual
factors.
Responses Due to Sonar and Other Transducers
Baleen whales are hypothesized to react more strongly to LF sounds
that overlap with their vocalization range. One series of behavioral
response studies (BRSs) was undertaken in 1997-1998 pursuant to the
Navy's Low-Frequency Sound Scientific Research Program (LFS SRP). The
frequency bands of LF sonars used were between 100 and 500 Hz, with
received levels between 115 and 150 dB re 1 [mu]Pa. Exposures occurred
on fin whale and blue whale foraging grounds, on humpback whale
breeding grounds, and along gray whale (Eschrichtius robustus)
migratory routes. These studies found short-term responses to LF sound
by some individual fin and humpback whales, including changes in vocal
activity and avoidance of the source vessel, while other fin, humpback,
and blue whale individuals did not respond at all. When the source was
in the path of migrating gray whales, they changed course up to 2 km to
avoid the sound, but when the source was outside their path, little
response was observed (Clark and Fristrup, 2001; Croll et al., 2001;
Fristrup et al., 2003; Miller et al., 2000; Nowacek et al., 2007).
These responses were short-lived across all individuals, and animals
returned to their normal activities within tens of minutes after
initial exposure (Clark and Fristrup, 2001). The context of an exposure
scenario is important for determining the probability, magnitude, and
duration of a response (Ellison et al., 2012; Southall et al., 2021).
At this time, no other BRSs have used an LFA (less than 1 kHz)
sound source, so the applicability of all BRSs discussed herein to
determining potential behavioral responses to the specified activities
is limited. Specifically, while there are several studies illustrating
the responses of LF species to MF sources (which are still in or near
the most sensitive part of their predicted hearing range), many of the
studies discussed below relate to the responses of HF hearing
specialists to MF sources, which are essentially sources with
frequencies in or near the most sensitive area of the species hearing,
whereas (as noted above), all but LF species have significantly reduced
sensitivity (9-65 dB and above) in the range of SURTASS LFA sonar.
However, these data can generally inform the analysis of marine mammal
response to sonar.
Mysticetes responses to sonar and other duty-cycled tonal sounds
are dependent upon the characteristics of the signal, behavioral state
of the animal, sensitivity and previous experience of an individual,
and other contextual factors including distance of the source, movement
of the source, physical presence of vessels, time of year, and
geographic location (Goldbogen et al., 2013; Harris et al., 2019a;
Harris et al., 2015; Martin et al., 2015; Sivle et al., 2015b). For
example, a BRS in Southern California demonstrated that individual
behavioral state was critically important in determining response of
blue whales to Navy sonar. In this BRS, some blue whales engaged in
deep (greater than 50 m) feeding behavior had greater dive responses
than those in shallow feeding or non-feeding conditions, while some
blue whales that were engaged in shallow feeding behavior demonstrated
no clear changes in diving or movement even when received levels were
high (approximately 160 dB re 1 [micro]Pa) from exposures to 3-4 kHz
sonar signals, while others showed a clear response at exposures at
lower received level of sonar and pseudorandom noise (Goldbogen et al.,
2013). Generally, behavioral responses were brief and of low to
moderate severity, and the whales returned to baseline behavior shortly
after the end of the acoustic exposure (DeRuiter et al., 2017;
Goldbogen et al., 2013; Southall et al., 2019c). To better understand
the context of these behavioral responses, Friedlaender et al., (2016)
mapped the prey field of the deep-diving blue whales and found that the
response to sound was more apparent for individuals engaged in feeding
than those that were not. The probability of a moderate behavioral
response increased when the source was closer for these foraging blue
whales, although there was a high degree of uncertainty in that
relationship (Southall et al., 2019b). In the same BRS, none of the
tagged fin whales demonstrated more than a brief or minor response
regardless of their behavioral state (Harris et al., 2019a). The fin
whales were exposed to both mid-frequency simulated sonar and
pseudorandom noise of similar frequency, duration, and source level.
They were less sensitive to disturbance than blue whales, with no
significant differences in response between behavioral states or signal
types. The authors rated responses as low-to-moderate severity with no
negative impact to foraging success (Southall et al., 2023).
Similarly, while the rates of foraging lunges decrease in humpback
whales due to sonar exposure, there was variability in the response
across individuals, with one animal ceasing to forage completely and
another animal starting to forage during the exposure (Sivle et al.,
2016). In addition, almost half of the animals that exhibited avoidance
behavior were foraging before the exposure, but the others were not;
[[Page 11646]]
the animals that exhibited avoidance behavior while not feeding
responded at a slightly lower received level and greater distance than
those that were feeding (Wensveen et al., 2017). These findings
indicate that the behavioral state of the animal plays a role in the
type and severity of a behavioral response. Henderson et al. (2019)
examined tagged humpback whale dive and movement behavior, including
individuals incidentally exposed to Navy sonar during training
activities, at the Pacific Missile Range Facility (PMRF) off
Kaua[revaps]i, Hawaii. Tracking data showed that, regardless of
exposure to sonar, individual humpbacks spent limited time, no more
than a few days, in the vicinity of Kaua[revaps]i. Potential behavioral
responses due to sonar exposure were limited and may have been
influenced by breeding and social behaviors. Martin et al., (2015)
found that the density of calling minke whales was reduced during
periods of Navy training involving sonar relative to the periods before
training began and increased again in the days following the completion
of training activities. The responses of individual whales could not be
assessed, so in this case it is unknown whether the decrease in calling
animals indicated that the animals left the range or simply ceased
calling. Harris et al., (2019b) utilized acoustically generated minke
whale tracks to statistically demonstrate changes in the spatial
distribution of minke whale acoustic presence before, during, and after
surface ship MFAS training. The spatial distribution of probability of
acoustic presence was different in the ``during'' phase compared to the
``before'' phase, and the probability of presence at the center of ship
activity during MFAS training was close to zero for both years. The
``after'' phases for both years retained lower probabilities of
presence suggesting the return to baseline conditions may take more
than 5 days. The results show a clear spatial redistribution of calling
minke whales during surface ship MFAS training; however, a limitation
of passive acoustic monitoring is that one cannot conclude if the
whales moved away, went silent, or a combination of the two.
Building on this work, Durbach et al., (2021) used the same data
and determined that individual minke whales tended to be in either a
fast or slow movement behavioral state while on the missile range,
whereas the whales tended to be in the slow state in baseline or before
periods but transitioned into the fast state with more directed
movement during sonar exposures. They also moved away from the area of
sonar activity on the range, either to the north or east depending on
where the activity was located; this explains the spatial
redistribution found by Harris et al., (2019b). Minke whales were also
more likely to stop calling when in the fast movement behavioral state
regardless of whether there was sonar activity and stop calling when in
the slow movement behavioral state during sonar activity (Durbach et
al., 2021). Similarly, minke whale detections were reduced or ceased
altogether during periods of sonar use off Jacksonville, Florida,
(Norris et al., 2012; Simeone et al., 2015; U.S. Department of the
Navy, 2013), especially with an increased ping rate (Charif et al.,
2015).
Odontocetes have varied, context-dependent behavioral responses to
sonar and other transducers. Much of the research on odontocetes has
been focused on understanding the impacts of sonar and other
transducers on beaked whales because they were hypothesized to be more
susceptible to behavioral disturbance after several strandings of
beaked whales in which military MFAS was identified as a contributing
factor (see Stranding and Mortality section). Subsequent BRSs have
shown that beaked whales are likely more sensitive to disturbance than
most other cetaceans. Many species of odontocetes have been studied
during BRSs (though not for low frequency sources), including
Blainville's beaked whale, goose-beaked whale, Baird's beaked whale,
northern bottlenose whale, harbor porpoise, pilot whale, killer whale,
sperm whale, false killer whale, melon-headed whale, bottlenose
dolphin, rough-toothed dolphin, Risso's dolphin, Pacific white-sided
dolphin, and Commerson's dolphin. Observed responses by Blainville's
beaked whales, goose-beaked whales, Baird's beaked whales, and northern
bottlenose whales (the largest of the beaked whales), to mid-frequency
sonar sounds include cessation of clicking, decline in group vocal
periods, termination of foraging dives, changes in direction to avoid
the sound source, slower ascent rates to the surface, longer deep and
shallow dive durations, and other unusual dive behaviors (DeRuiter et
al., 2013b; Hewitt et al., 2022; Jacobson et al., 2022; McCarthy et
al., 2011; Miller et al., 2015; Moretti et al., 2014; Southall et al.,
2011; Stimpert et al., 2014; Tyack et al., 2011).
During a BRS in Southern California, a tagged Baird's beaked whale
exposed to simulated MFA sonar within 3 km increased swim speed and
modified its dive behavior (Stimpert et al., 2014). One goose-beaked
whale was also incidentally exposed to real Navy sonar located over 100
km away in addition to the source used in the controlled exposure
study, and the authors did not detect similar responses at comparable
received levels. Received levels from the MFA sonar signals from the
controlled (3.4 to 9.5 km) exposures were calculated as 84-144 dB re 1
[mu]Pa, and incidental (118 km) exposures were calculated as 78-106 dB
re 1 [mu]Pa, indicating that context of the exposures (e.g., source
proximity, controlled source ramp-up) may have been a significant
factor in the responses to the simulated sonars (DeRuiter et al.,
2013b).
Long-term tagging work during the same BRS demonstrated that the
longer duration dives considered a behavioral response by DeRuiter et
al. (2013b) fell within the normal range of dive durations found for
eight tagged goose-beaked whales on the Southern California Offshore
Range (Schorr et al., 2014). However, the longer inter-deep dive
intervals found by DeRuiter et al., (2013b), which were among the
longest found by Schorr et al., (2014) and Falcone et al., (2017), may
indicate a response to sonar. Williams et al., (2017) note that during
normal deep dives or during fast swim speeds, beaked whales and other
marine mammals use strategies to reduce their stroke rates (e.g.,
leaping, wave surfing when swimming, interspersing glides between bouts
of stroking when diving). The authors determined that in the post-
exposure dives by the tagged goose-beaked whales described in DeRuiter
et al., (2013b), the whales ceased gliding and swam with almost
continuous strokes. This change in swim behavior was calculated to
increase metabolic costs by about 30.5 percent and increase the amount
of energy expending on fast swim speeds from 27 to 59 percent of their
overall energy budget. This repartitioning of energy was detected in
the model up to 1.7 hours after the single sonar exposure. Therefore,
while the overall post-exposure dive durations were similar, the
metabolic energy calculated by Williams et al., (2017) was higher.
However, Southall et al., (2019a) found that prey availability was
higher in the western area of the Southern California Offshore Range
where goose-beaked whales preferentially occurred, while prey resources
were lower in the eastern area and moderate in the area just north of
the Range. This high prey availability may indicate that goose-beaked
whales need fewer foraging dives to meet energy requirements than
[[Page 11647]]
would be needed in another area with fewer resources.
During a BRS in Norway, northern bottlenose whales avoided a sonar
sound source over a wide range of distances (0.8 to 28 km) and
estimated avoidance thresholds ranging from received SPLs of 117 to 126
dB re 1 [mu]Pa. The behavioral response characteristics and avoidance
thresholds were comparable to those previously observed in beaked whale
studies; however, researchers did not observe an effect of distance on
behavioral response and found that onset and intensity of behavioral
response were better predicted by received SPL. There was one instance
where an individual northern bottlenose whale approached the vessel,
circled the sound source (source level was only 122 dB re 1 [mu]Pa),
and resumed foraging after the exposure. Conversely, one northern
bottlenose whale exposed to a sonar source was documented performing
the longest and deepest dive on record for the species, and continued
swimming away from the source for more than 7 hours (Miller et al.,
2015; Siegal et al., 2022; Wensveen et al., 2019).
Research on Blainville's beaked whales at the Atlantic Undersea
Test and Evaluation Center (AUTEC) range has shown that individuals
move off-range during sonar use, only returning after the cessation of
sonar transmission (Boyd et al., 2009; Henderson et al., 2015;
Jones[hyphen]Todd et al., 2021; Manzano-Roth et al., 2022; Manzano-Roth
et al., 2016; McCarthy et al., 2011; Tyack et al., 2011). Five
Blainville's beaked whales estimated to be within 2 to 29 km of the
AUTEC range at the onset of active sonar were displaced a maximum of 28
to 68 km after moving away from the range, although one individual did
approach the range during active sonar use. Researchers found a decline
in deep dives at the onset of the training and an increase in time
spent on foraging dives as whales moved away from the range. Predicted
received levels at which presumed responses were observed were
comparable to those previously observed in beaked whale studies.
Acoustic data indicated that vocal periods were detected on the range
within 72 hours after training ended (Joyce et al., 2019). However,
Blainville's beaked whales have been documented to remain on-range to
forage throughout the year (Henderson et al., 2016), indicating the
AUTEC range may be a preferred foraging habitat regardless of the
effects of active sonar noise, or it could be that there are no long-
term consequences of the sonar activity. In the SOCAL Range Complex,
researchers conducting photo-identification studies have identified
approximately 100 individual goose-beaked whales, with 40 percent
having been seen in one or more prior years, with re-sightings up to 7
years apart, indicating a possible on-range resident population
(Falcone and Schorr, 2014; Falcone et al., 2009).
The probability of Blainville's beaked whale group vocal periods on
the PMRF were modeled during periods of: (1) no naval activity; (2)
naval activity without hull-mounted MFA sonar; and (3) naval activity
with hull-mounted MFA sonar (Jacobson et al., 2022). At a received
level of 150 dB re 1 [mu]Pa RMS SPL, the probability of detecting a
group vocal period during MFA sonar use decreased by 77 percent
compared to periods when general training activity was ongoing, and by
87 percent compared to baseline (no naval activity) conditions.
Jacobsen et al., (2022) found a greater reduction in probability of a
group vocal period with MFA sonar than observed in a prior study of the
same species at the AUTEC range (Moretti et al., 2014), which may be
due to the baseline period in the AUTEC study including naval activity
without MFA sonar, potentially lowering the baseline group vocal period
activity in that study, or due to differences in the residency of the
populations at each range.
Stanistreet et al. (2022) used passive acoustic recordings during a
multinational naval activity to assess marine mammal acoustic presence
and behavioral response to especially long bouts of sonar lasting up to
13 consecutive hours, occurring repeatedly over 8 days (median and
maximum SPL = 120 dB and 164 dB). Goose-beaked whales and sperm whales
substantially reduced how often they produced clicks during sonar
activity, indicating a decrease or cessation in foraging behavior. Few
previous studies have shown sustained changes in foraging or
displacement of sperm whales, but there was an absence of sperm whale
clicks for 6 consecutive days of sonar activity. Sperm whales returned
to baseline levels of clicks within days after the activity, but beaked
whale click detection rates remained low even 7 days after the
exercise. In addition, there were no click detections from a Mesoplodon
beaked whale species within the area during, and at least 7 days after,
the sonar activity. Clicks from northern bottlenose whales and
Sowerby's beaked whales were also detected but were not frequent enough
at the recording site used to compare clicks between baseline and sonar
conditions.
Goose-beaked whale behavioral responses (i.e., deep and shallow
dive durations, surface interval durations, inter-deep dive intervals)
on the Southern California Anti-Submarine Warfare Range were modeled
against predictor values that included helicopter dipping sonar, mid-
power MFA sonar and hull-mounted, high-power MFA sonar along with other
non-MFA sonar predictors (Falcone et al., 2017). Falcone et al. (2017)
found both shallow and deep dive durations increased as the proximity
to both mid- and high-powered sources decreased and found that surface
intervals and inter-deep dive intervals increased in the presence of
both types of sonars (helicopter dipping and hull-mounted), although
surface intervals shortened during periods without MFA sonar. Proximity
of source and receiver were important considerations, as the responses
to the mid-power MFA sonar at closer ranges were comparable to the
responses to the higher source level vessel sonar, as was the context
of the exposure. Helicopter dipping sonars are shorter duration and
randomly located, therefore more difficult to predict or track by
beaked whales and potentially more likely to elicit a response,
especially at closer distances (6 to 25 km) (Falcone et al., 2017).
Sea floor depths and quantity of light (i.e., lunar cycle) are also
important variables to consider in BRSs, as goose-beaked whale foraging
dive depth increased with sea floor depth (maximum 2,000 m) and the
amount of time spent at foraging depths (and likely foraging) was
greater at night (likely avoiding predation by staying deeper during
periods of bright lunar illumination), although they spent more time
near the surface during the night, as well, particularly on dark nights
with little moonlight (Barlow et al., 2020). Sonar occurred during 10
percent of the dives studied and had little effect on the resulting
dive metrics. Watwood et al., (2017) found that the longer the duration
of a sonar event, the greater reduction in detected goose-beaked whale
group dives and, as helicopter dipping events occurred more frequently
but with shorter durations than periods of hull-mounted sonar, when
looking at the number of detected group dives there was a greater
reduction during periods of hull-mounted sonar than during helicopter
dipping sonar. DiMarzio et al. (2019) also found that group vocal
periods (i.e., clusters of foraging pulses), on average, decreased
during sonar events on the Southern California Anti-Submarine Warfare
Range, though the decline from before the event to during the event was
significantly less for helicopter dipping
[[Page 11648]]
events than hull-mounted events, and there was no difference in the
magnitude of the decline between vessel-only events and events with
both vessels and helicopters. Manzano-Roth et al. (2022) analyzed long-
term passive acoustic monitoring data from the PMRF in Kaua[revaps]i,
Hawaii, and found beaked whales reduced group vocal periods during
submarine command course events and remained low for a minimum of 3
days after the MFA sonar activity.
Harbor porpoise behavioral responses have been researched
extensively using acoustic deterrent and acoustic harassment devices;
however, BRSs using sonar are limited. Kastelein et al. (2018b) found
harbor porpoises did not respond to low-duty cycle mid-frequency sonar
tones (3.5-4.1 kHz at 2.7 percent duty cycle; e.g., one tone per
minute) at any received level, but one individual did respond (i.e.,
increased jumping, increased respiration rates) to high-duty cycle
sonar tones (3.5-4.1 kHz at 96 percent duty cycle; e.g., continuous
tone for almost a minute).
Behavioral responses by odontocetes (other than beaked whales and
harbor porpoises) to sonar and other transducers include horizontal
avoidance, reduced breathing rates, changes in behavioral state,
changes in dive behavior (Antunes et al., 2014; Isojunno et al., 2018;
Isojunno et al., 2017; Isojunno et al., 2020; Miller, 2012; Miller et
al., 2011; Miller et al., 2014; Southall et al., 2024), and, in one
study, separation of a killer whale calf from its group (Miller et al.,
2011). Some species of dolphin (e.g., bottlenose, spotted, spinner,
Clymene, Pacific white-sided, rough-toothed) are frequently documented
bowriding with vessels and the drive to engage in bowriding, whether
for pleasure or energetic savings (Fiori et al., 2024) may supersede
the impact of associated sonar noise (W[uuml]rsig et al., 1998).
In controlled exposure experiments on captive odontocetes, Houser
et al. (2013a) recorded behavioral responses from bottlenose dolphins
with 3 kHz sonar-like tones between 115 and 185 dB re 1 [mu]Pa, and
individuals across 10 trials demonstrated a 50 percent probability of
response at 172 dB re 1 [mu]Pa. Multiple studies have been conducted on
bottlenose dolphins and beluga whales to measure TTS (Finneran et al.,
2003a; Finneran et al., 2001; Finneran et al., 2005; Finneran and
Schlundt, 2004; Schlundt et al., 2000). During these studies, when
individuals were presented with 1-second tones up to 203 dB re 1
[mu]Pa, responses included changes in respiration rate, fluke slaps,
and a refusal to participate or return to the location of the sound
stimulus, including what appeared to be deliberate attempts by animals
to avoid a sound exposure (Finneran et al., 2002; Schlundt et al.,
2000). Bottlenose dolphins exposed to more intense 1-second tones
exhibited short-term changes in behavior above received levels of 178-
193 dB re 1 [mu]Pa, and beluga whales did so at received levels of 180-
196 dB re 1 [mu]Pa and above.
While several opportunistic observations of odontocete (other than
beaked whales and harbor porpoises) responses have been recorded during
previous Navy activities and BRSs that employed sonar and sonar-like
sources, it is difficult to definitively attribute responses of non-
focal species to sonar exposure. Responses range from no response to
potential highlight-impactful responses, such as the separation of a
killer whale calf from its group (Miller et al., 2011). This may be
due, in part, to the variety of species and sensitivities of the
odontocete taxonomic group, as well as the breadth of study types
conducted and field observations, leading to the assessment of both
contextually driven and dose-based responses. The available data
indicate exposures to sonar in close proximity and with multiple
vessels approaching an animal likely lead to higher-level responses by
most odontocete species, regardless of received level or behavioral
state. However, when sources are further away and moving in variable
directions, behavioral responses are likely driven by behavioral state,
individual experience, or species-level sensitivities, as well as
exposure duration and received level, with the likelihood of response
increasing with increased received levels. As such, it is expected
odontocete behavioral responses to sonar and other transducers will
vary by species, populations, and individuals, and long-term
consequences or population-level effects are likely dependent upon the
frequency and duration of the exposure and resulting behavioral
response.
Pinniped behavioral response to sonar and other transducers is
context-dependent (e.g., Hastie et al., 2014; Southall et al., 2019).
All studies on pinniped response to sonar thus far have been limited to
captive animals, though, based on exposures of wild pinnipeds to vessel
noise and impulsive sounds (see Responses Due to Vessel Noise section),
pinnipeds may only respond strongly to military sonar that is in close
proximity or approaching an animal. Kvadsheim et al. (2010b) found that
captive hooded seals exhibited avoidance response to sonar signals
between 1 and 7 kHz (160-170 dB re 1 [micro]Pa RMS SPL) by reducing
diving activity, rapid surface swimming away from the source, and
eventually moving to areas of least SPL. However, the authors noted a
rapid adaptation in behavior (passive surface floating) during the
second and subsequent exposures, indicating a level of habituation
within a short amount of time. Kastelein et al. (2015c) exposed captive
harbor seals to three different sonar signals at 25 kHz with variable
waveform characteristics and duty cycles and found individuals
responded to a frequency modulated signal at received levels over 137
dB re 1 [micro]Pa by hauling out more, swimming faster, and raising
their heads or jumping out of the water. However, seals did not respond
to a continuous wave or combination signals at any received level (up
to 156 dB re 1 [micro]Pa). Houser et al. (2013a) conducted a study to
determine behavioral responses of captive California sea lions to MFA
sonar at various received levels (125-185 dB re 1 [micro]Pa). They
found younger animals (less than 2 years old) were more likely to
respond than older animals and responses included increased respiration
rate, increased time spent submerged, refusal to participate in a
repetitive task, and hauling out. Most responses below 155 dB re 1
[micro]Pa were changes in respiration, while more severe responses
(i.e., refusing to participate, hauling out) began to occur over 170 dB
re 1 [micro]Pa, and many of the most severe responses came from the
young sea lions.
Responses Due to Vessel Noise
Mysticetes have varied responses to vessel noise and presence, from
having no response to approaching vessels to exhibiting an avoidance
response by both horizontal (swimming away) and vertical (increased
diving) movement (Baker et al., 1983; Fiori et al., 2019; Gende et al.,
2011; Watkins, 1981). Avoidance responses include changing swim
patterns, speed, or direction (Jahoda et al., 2003), remaining
submerged for longer periods of time (Au and Green, 2000), and
performing shallower dives with more frequent surfacing. Behavioral
responses to vessels range from smaller-scale changes, such as altered
breathing patterns (e.g., Baker et al., 1983; Jahoda et al., 2003), to
larger-scale changes such as a decrease in apparent presence (Anderwald
et al., 2013). Other common behavioral responses include changes in
vocalizations, surface time, feeding and social behaviors (Au and
Green, 2000; Dunlop, 2019; Fournet et al., 2018;
[[Page 11649]]
Machernis et al., 2018; Richter et al., 2003; Williams et al., 2002a).
For example, North Atlantic right whales (NARWs) have been reported to
increase the amplitude or frequency of their vocalizations or call at a
lower rate in the presence of increased vessel noise (Parks et al.,
2007; Parks et al., 2011) but generally demonstrate little to no
response to vessels or sounds from approaching vessels and often
continue to use habitats in high vessel traffic areas (Nowacek et al.
2004a). This lack of response may be due to habituation to the presence
and associated noise of vessels in NARW habitat or may be due to
propagation effects that may attenuate vessel noise near the surface
(Nowacek et al., 2004a; Terhune and Verboom, 1999).
Mysticete behavioral responses to vessels may also be affected by
vessel behavior (Di Clemente et al., 2018; Fiori et al., 2019).
Avoidance responses occurred most often after ``J'' type vessel
approaches (i.e., traveling parallel to the whales' direction of
travel, then overtaking the whales by turning in front of the group)
compared to parallel or direct approaches. Mother humpbacks were
particularly sensitive to direct and J type approaches and spent
significantly more time diving in response (Fiori et al., 2019). The
presence of a passing vessel did not change the behavior of resting
humpback whale mother-calf pairs, but fast vessels with louder low-
frequency weighted source levels (173 dB re 1 [mu]Pa, equating to
weighted received levels of 133 dB re 1 [mu]Pa) at an average distance
of 100 m resulted in a decreased resting behavior and increases in
dives, swim speeds, and respiration rates (Sprogis et al., 2020).
Overall, mysticete responses to vessel noise and traffic are
varied, and habituation or changes to vocalization are predominant
long-term responses. When baleen whales do avoid vessels, they seem to
do so by altering their swim and dive patterns to move away from the
vessel. Although a lack of response in the presence of a vessel may
minimize potential disturbance from passing vessels, it does increase
the whales' vulnerability to vessel strike, which may be of greater
concern for mysticetes than vessel noise.
Odontocete responses due to vessel noise are varied and context-
dependent, and it is difficult to separate the impacts of vessel noise
from the impacts of vessel presence. Vessel presence has been shown to
interrupt feeding behavior in delphinids in some studies (Meissner et
al., 2015; Pirotta et al., 2015b) while a recent study by Mills et al.
(2023) found that, in an important foraging area, bottlenose dolphins
may continue to forage and socialize even while constantly exposed to
high vessel traffic. Ng and Leung (2003) found that the type of vessel,
approach, and speed of approach can all affect the probability of a
negative behavioral response and, similarly, Guerra et al. (2014)
documented varied responses in group structure and vocal behavior.
While most odontocetes have documented neutral responses to
vessels, avoidance (Bejder et al., 2006a; W[uuml]rsig et al., 1998) and
attraction (Norris and Prescott, 1961; Ritter, 2002; Shane et al.,
1986; Westdal et al., 2023; W[uuml]rsig et al., 1998) behaviors have
also been observed (Hewitt, 1985).
Information is limited on beaked whale responses to vessel noise,
but W[uuml]rsig et al. (1998) noted that most beaked whales seem to
exhibit avoidance behaviors when exposed to vessels and beaked whales
may respond to all anthropogenic noise (i.e., sonar, vessel) at similar
sound levels (Aguilar de Soto et al., 2006; Tyack et al., 2011; Tyack,
2009). The information available includes a disruption of foraging by a
vocalizing goose-beaked whale in the presence of a passing vessel
(Aguilar de Soto et al., 2006) and restriction of group movement, or
possibly reduction in the number of individuals clicking within the
group, after exposure to broadband (received level of 135 dB re 1
[mu]Pa) vessel noise up to at least 5.2 km away from the source, though
no change in duration of Blainville's beaked whale foraging dives was
observed (Pirotta et al., 2012).
Porpoises and small delphinids are known to be sensitive to vessel
noise, as well. It should be noted that fewer responses in populations
of odontocetes regularly subjected to high levels of vessel traffic
could be a sign of habituation, or a sign that the more sensitive
individuals in the population have abandoned that area of higher human
activity.
Lusseau and Bejder (2007) have reported some long-term consequences
of vessel noise on odontocetes but, overall, there is little
information on the long-term and cumulative impacts of vessel noise
(National Academies of Sciences Engineering and Medicine, 2017; NMFS,
2007). Many researchers speculate that long-term impacts may occur on
odontocete populations that experience repeated interruption of
foraging behaviors (Stockin et al., 2008), and Southall et al. (2021)
indicates that, in many contexts, the localized and coastal home ranges
typical of many species make them less resilient to sustained or
repeated vessel noise than mysticetes.
Context and experience likely play a role in pinnipeds response to
vessel noise, which vary from negative responses including increased
vigilance and alerting to avoidance to reduced time spent doing
biologically important activities (e.g., resting, feeding, and nursing)
(Martin et al., 2023a; Martin et al., 2022; Mikkelsen et al., 2019;
Richardson et al., 1995) to attraction or lack of observable response
(Richardson et al., 1995). More severe responses, like flushing, could
be more detrimental to individuals during biologically important
activities and times, such as during pupping season. Blundell and
Pendleton (2015) found that vessel presence reduces haul out time of
Alaskan harbor seals during pupping season and larger vessels elicit
stronger responses. Cates and Acevedo-Guti[eacute]rrez (2017) modeled
harbor seal responses to passing vessels at haul out sites in less
trafficked areas and found the model best predicting flushing behavior
included the number of boats, type of boats, and distance of seals to
boats. The authors noted flushing occurred more in response to non-
motorized vessels (e.g., kayaks), likely because they tended to pass
closer (25 to 184 m) to haul out sites than motorized vessels (55 to
591 m) and tended to occur in groups rather than as a single vessel.
Cape fur seals were also more responsive to vessel noise at sites
with a large breeding colony than at sites with lower abundances of
conspecifics (Martin et al., 2023a). A field study of harbor and gray
seals showed that seal responses to vessels included interruption of
resting and foraging during times when vessel noise was increasing or
at its peak (Mikkelsen et al., 2019). And, although no behavioral
differences were observed in hauled out wild cape fur seals exposed to
low (60-64 dB re 20 [mu]Pa RMS SPL), medium (64-70 dB) and high-level
(70-80 dB) vessel noise playbacks, mother-pup pairs spent less time
nursing (15-31 percent) and more time awake (13-26 percent), vigilant
(7-31 percent), and mobile (2-4 percent) during vessel noise conditions
compared to control conditions (Martin et al., 2022). Of note, the T-
AGOS vessels engaged in SURTASS LFA sonar activities would remain at
least 22 km from emergent land and islands, thereby avoiding pinniped
colonies and haulouts.
Masking
Sound can disrupt behavior through masking, or interfering with, an
animal's ability to produce, detect, recognize, interpret, or
discriminate between acoustic signals of interest (e.g., those used for
intraspecific communication
[[Page 11650]]
and social interactions, prey detection, predator avoidance, or
navigation) (Clark et al., 2009; Richardson et al., 1995; Erbe and
Farmer, 2000; Tyack, 2000; Erbe et al., 2016; Branstetter and Sills,
2022; Erbe et al., 2025). Masking occurs when the production or receipt
of a sound is interfered with by another coincident sound at similar
frequencies and at similar or higher intensity and may occur whether
the coincident sound is natural (e.g., snapping shrimp, wind, waves,
precipitation) or anthropogenic (e.g., shipping, sonar, seismic
exploration) in origin.
The ability of a noise source to mask biologically important sounds
depends on the characteristics of both the noise source and the signal
of interest (e.g., signal-to-noise ratio, temporal variability,
direction), in relation to each other and to an animal's hearing
abilities (e.g., sensitivity, frequency range, critical ratios,
frequency discrimination, directional discrimination, age, or TTS
hearing loss) and/or ability to produce a signal (communication
masking), and existing ambient noise and propagation conditions.
Masking these acoustic signals can disturb the behavior of individual
animals, groups of animals, or entire populations. Masking can lead to
behavioral changes including vocal changes (e.g., Lombard effect,
increasing amplitude, or changing frequency or timing of
vocalizations), cessation of foraging, and leaving an area, to both
signalers and receivers, in an attempt to compensate for noise levels
(Erbe et al., 2016).
Most research on auditory masking is focused on energetic masking,
or the ability of the receiver (i.e., listener) to detect a signal in
noise. However, from a fitness perspective, both signal detection and
signal interpretation are necessary for success. This type of masking
is called informational masking and occurs when a signal is detected by
an animal but the meaning of that signal has been lost. Few data exist
on informational masking in marine mammals, but studies have shown that
some recognition of predator cues might be missed by species that are
preyed upon by killer whales if killer whale vocalizations are masked
(Cur[eacute] et al., 2016; Cur[eacute] et al., 2015; Deecke et al.,
2002; Isojunno et al., 2016; Visser et al., 2016).
Under certain circumstances, marine mammals experiencing
significant masking could also be impaired from maximizing their
performance fitness in survival and reproduction. Therefore, when the
coincident (i.e., masking) sound is man-made, it may be considered
harassment when, in the case of military readiness activities,
disrupting natural behavioral patterns to the point where the behavior
is abandoned or significantly altered. It is important to distinguish
TTS and PTS, which persist after the sound exposure, from masking,
which occurs only during the sound exposure. Because masking (without
resulting in threshold shift) is not associated with abnormal
physiological function, it is not considered a physiological effect,
but rather a potential behavioral effect.
Richardson et al. (1995) argued that the maximum radius of
influence of anthropogenic noise (including broadband low-frequency
sound transmission) on a marine mammal is the distance from the source
to the point at which the noise can barely be heard. This range is
determined by either the hearing sensitivity (including critical
ratios, or the lowest signal-to-noise ratio in which animals can detect
a signal) of the animal (Finneran and Branstetter, 2013; Johnson et
al., 1989; Southall et al., 2000) or the background noise level present
(Hatch et al., 2016). Masking is most likely to affect some species'
ability to detect communication calls and natural sounds (i.e., surf
noise, prey noise, etc.) (Clark et al., 2009; Erbe et al., 2025;
Richardson et al., 1995).
The frequency range of the potentially masking sound is important
in determining any potential behavioral impacts. For example, low-
frequency signals, like SURTASS LFA sonar, may have less effect on
high-frequency echolocation sounds produced by odontocetes but are more
likely to affect detection of mysticete communication calls and other
potentially important natural sounds such as those produced by surf and
some prey species. The masking of communication signals by
anthropogenic noise may be considered as a reduction in the
communication space of animals (e.g., Clark et al., 2009; Matthews et
al., 2016; Erbe et al., 2016) and may result in energetic or other
costs as animals change their vocalization behavior (e.g., Miller et
al., 2000; Foote et al., 2004; Parks et al., 2007; Di Iorio and Clark,
2010; Holt et al., 2009; Tennessen et al., 2024). Masking can be
reduced in situations where the signal and noise come from different
directions (Richardson et al., 1995; Erbe et al., 2025), through
amplitude modulation of the signal, or through other compensatory
behaviors (Houser and Moore, 2014; Erbe et al., 2016; Branstetter and
Sills, 2022). Masking can be tested directly in captive species, but in
wild populations it must be either modeled or inferred from evidence of
masking compensation. There are few studies addressing real-world
masking sounds likely to be experienced by marine mammals in the wild
(e.g., Cholewiak et al., 2018; Branstetter and Sills, 2022; Branstetter
et al., 2024; Tennessen et al., 2024).
Impacts on signal detection, measured by masked detection
thresholds, are not the only important factors to address when
considering the potential effects of masking. As marine mammals use
sound to recognize conspecifics, prey, predators, or other biologically
significant sources (Branstetter et al., 2016), it is also important to
understand the impacts of masked recognition thresholds (i.e.,
informational masking). Branstetter et al. (2016) measured masked
recognition thresholds for whistle-like sounds of bottlenose dolphins
and observed that they are approximately 4 dB above detection
thresholds (energetic masking) for the same signals. Reduced ability to
recognize a conspecific call or the acoustic signature of a predator
could have severe negative impacts. Branstetter et al. (2016) observed
that if ``quality communication'' is set at 90 percent recognition the
output of communication space models (which are based on 50 percent
detection) would likely result in a significant decrease in
communication range.
As marine mammals use sound to recognize predators (Allen et al.,
2014; Cummings and Thompson, 1971; Cur[eacute] et al., 2015; Fish and
Vania, 1971), the presence of masking noise may also prevent marine
mammals from responding to acoustic cues produced by their predators,
particularly if it occurs in the same frequency band. For example,
harbor seals that reside in the coastal waters of British Columbia are
frequently targeted by mammal-eating killer whales. The seals
acoustically discriminate between the calls of mammal-eating and fish-
eating killer whales (Deecke et al., 2002), a capability that should
increase survivorship while reducing the energy required to identify
all killer whale calls. Similarly, sperm whales (Cur[eacute] et al,
2016; Isojunno et al., 2016), long-finned pilot whales (Visser et al.,
2016), and humpback whales (Cure[eacute] et al., 2015) changed their
behavior in response to killer whale vocalization playbacks. The
potential effects of masked predator acoustic cues depend on the
duration of the masking noise and the likelihood of a marine mammal
encountering a predator during the time that detection and recognition
of predator cues are impeded. Given its low frequency, the SURTASS LFA
sonar signal would be expected to interfere little, if at all, with
marine mammal predator vocalization.
[[Page 11651]]
Redundancy and context can also facilitate detection of weak
signals. These phenomena may help marine mammals detect weak sounds in
the presence of natural or anthropogenic noise. Most masking studies in
marine mammals present the test signal and the masking noise from the
same direction. The dominant background noise may be highly directional
if it comes from a particular anthropogenic source such as a vessel or
industrial site. Directional hearing may significantly reduce the
masking effects of these sounds by improving the effective signal-to-
noise ratio (Erbe et al., 2016).
Masking affects both senders and receivers of acoustic signals and,
when present at large scales (e.g., spatial and/or temporal), can
potentially have long-term chronic effects on marine mammals at the
population level as well as at the individual level. Low-frequency
ambient sound levels have increased by as much as 20 dB (more than
three times in terms of SPL) in some of the world's ocean from pre-
industrial periods, with most of the increase from distant commercial
shipping (Hildebrand, 2009; Cholewiak et al., 2018). All anthropogenic
sound sources, but especially chronic, continuous, and lower-frequency
signals (e.g., from commercial vessel traffic), contribute to elevated
ambient sound levels, thus intensifying masking for marine mammals.
Masking Due to Sonar and Other Transducers
Masking can reduce the ranges over which marine mammals can detect
biologically relevant sounds in the presence of high-duty cycle
sources. Lower-duty cycle sonars have less of a masking effect as sonar
tones occur over a relatively short duration, thus the listener can
detect signals of interest during the quiet periods between cycles. The
LFA sonar duty cycle averages 7.5-10 percent with a maximum of 20
percent; however, single pulses range from 6 to 100 seconds with an
average of 60 seconds. Additionally, sonar tones occur over a
relatively narrow bandwidth, which means the signal is unlikely to
overlap more than a small portion of the vocalizations for most
species. LFA sonar signals are limited to the 100-500 Hz range. For
large mysticetes, the range of best hearing is estimated between 0.1
and 10 kHz, which overlaps with SURTASS LFA sonar sources.
Additionally, many of their vocalizations are below 1 kHz, which
overlaps with low-frequency sources. Any auditory impacts (TTS and AUD
INJ) or masking may affect communication due to low-frequency sonars.
As noted previously in the Marine Mammal Hearing Groups section
(table 3, figure 2, and figure 3, specifically), most marine mammal
taxa (with the exception of LF hearing specialists) have significantly
reduced hearing sensitivity in the 100-500 Hz range of SURTASS LFA
sonar. Specifically, the HF cetacean species weighting function curve
shows 17-40 dB reduced sensitivity in that frequency range (i.e., the
sound would be perceived as that much lower level than a sound in the
most susceptible portion of their hearing range), the underwater
pinniped weighting function curves show from 9-30-dB reductions, and
the VHF cetacean weighting function curve shows a 47-65 dB reduction at
frequencies from 200 to 500 Hz (i.e., generalized hearing range for
this hearing group starts at 200 Hz) and suggest even further reduced
sensitivity. Even the LF cetacean species have somewhat reduced
sensitivity in the 100 to 500 Hz range (0.5-6 dB). Any masking by LFA
sonar would be expected to coincide with the time they are in the
vicinity of a transmitting vessel (vessels would be transmitting, at
most, 8 hours per day) and overlapping with only a small portion of the
hearing range (given the narrow bandwidth). LFA sonar could overlap in
frequency with mysticete vocalizations; however, LFA sonar overlaps
little or not at all with vocalizations for most other marine mammal
species, and especially not with high-frequency echolocation calls of
odontocetes. For example, in the presence of LFA sonar, humpback whales
were observed to increase the length of their songs (Fristrup et al.,
2003; Miller et al., 2000), potentially due to the overlap in
frequencies between the whale song and the LFA sonar.
High-frequency (10-100 kHz) sonars, including the HF/M3 source
(frequency range of 30-40 kHz), fall within the best hearing and
vocalization ranges of most odontocetes; however, the HF/M3 source is
an intermittent source with a low duty cycle, thus less likely to
overlap both hearing and vocalizations, and high frequency sounds
attenuate more rapidly in the water due to absorption than do lower
frequency sounds, thus producing a smaller zone of potential masking
than mid- and low-frequency sounds. While high-frequency sonar has the
potential to mask marine mammal vocalizations under certain conditions,
reduction in available communication space or ability to locate prey is
unlikely because of the small zone of effect.
For other mysticetes, the range of best hearing and vocalizations
is typically between 1 and 30 kHz, which overlaps with mid- and high-
frequency sonar sources. Masking from high-frequency sonar sources
would be less likely to affect communication for these mysticetes than
impacts due to low-frequency sonars. Odontocetes that use echolocation
to hunt may experience masking of the echoes needed to find their prey
when foraging near low-frequency and mid-frequency sonar sources.
Communication sounds could also be masked by these sources. This effect
is likely to be temporary in offshore areas where these sources would
operate. Odontocetes with very high frequency hearing, such as harbor
porpoises, may experience masking of echolocation and communication
calls from close-proximity very-high-frequency sources, but these
effects are likely to be transient and temporary in the case of the HF/
M3, given the small impact zone. Pinnipeds may also experience masking
due to low- and mid-frequency sources because their communication calls
range from approximately 0.1-30 kHz. Some species of pinnipeds
communicate primarily in air and would not experience masking due to
underwater sonar use. Any impacts from masking would generally be
expected to occur within the same areas for which direct behavioral
disturbance from the SURTASS sources is quantified in the Estimated
Take of Marine Mammals section.
Masking Due to Vessel Noise
Masking is more likely to occur in the presence of broadband,
relatively continuous noise sources such as vessels; however, we note
that this rule contemplates no more than four vessels traversing an
ocean basin at greater than 22 km from shore (away from where marine
mammal densities are higher), resulting in a very low likelihood of any
meaningful masking resulting from the noise of the vessels themselves.
Several studies have shown decreases in marine mammal communication
space and changes in behavior as a result of the presence of vessel
noise. For example, NARWs were observed to shift the frequency content
of their calls upward while reducing the rate of calling in areas of
increased anthropogenic noise (Parks et al., 2007) as well as
increasing the amplitude (intensity) of their calls (Parks, 2009; Parks
et al., 2011). Fournet et al. (2018) observed that humpback whales in
Alaska responded to increasing ambient sound levels (natural and
anthropogenic) by increasing the source levels of their calls (non-song
[[Page 11652]]
vocalizations). Clark et al. (2009) also observed that right whales'
communication space decreased by up to 84 percent in the presence of
vessels. Cholewiak et al. (2018) also observed loss in communication
space in Stellwagen National Marine Sanctuary for NARWs, fin whales,
and humpback whales with increased ambient noise and shipping noise.
Gabriele et al. (2018) modeled the effects of vessel traffic sound on
communication space in Glacier Bay National Park in Alaska and found
that typical summer vessel traffic in Glacier Bay National Park causes
losses of communication space to singing whales (reduced by 13-28
percent), calling whales (18-51 percent), and roaring seals (32-61
percent), particularly during daylight hours and even in the absence of
cruise ships. Dunlop (2019) observed that an increase in vessel noise
reduced modeled communication space and resulted in significant
reduction in group social interactions in Australian humpback whales.
However, communication signal masking did not fully explain this change
in social behavior in the model, indicating there may also be an
additional effect of the physical presence of the vessel on social
behavior (Dunlop, 2019). Although humpback whales off Australia did not
change the frequency or duration of their vocalizations in the presence
of ship noise, their source levels were lower than expected based on
source level changes to wind noise, potentially indicating some signal
masking (Dunlop, 2016). Multiple delphinid species have also been shown
to increase the minimum or maximum frequencies of their whistles in the
presence of anthropogenic noise and reduced communication space (e.g.,
Holt et al., 2009; Holt et al., 2011; Gervaise et al., 2012; Williams
et al., 2014; Hermannsen et al., 2014; Papale et al., 2015; Liu et al.,
2017).
Other Physiological Responses
Physiological stress is a natural and adaptive process that helps
an animal survive changing conditions. When an animal perceives a
potential threat, whether or not the stimulus actually poses a threat,
a stress response is triggered (Selye, 1950; Moberg, 2000; Sapolsky,
2005). Once an animal's central nervous system perceives a threat, it
mounts a biological response or defense that consists of a combination
of behavioral responses, autonomic nervous system responses,
neuroendocrine responses, or immune responses.
The primary distinction between stress (which is adaptive and does
not normally place an animal at risk) and distress is the biotic cost
of the response. During a stress response, an animal uses glycogen
stores that can be quickly replenished once the stress is alleviated.
In such circumstances, the cost of the stress response would not pose
serious fitness consequences. However, when an animal does not have
sufficient energy reserves to satisfy the energetic costs of a stress
response, energy resources must be diverted from other biotic
functions. For example, when a stress response diverts energy away from
growth in young animals, those animals may experience stunted growth.
When a stress response diverts energy from a fetus, an animal's
reproductive success and its fitness will suffer. In these cases, the
animals will have entered a pre-pathological or pathological state
which is called ``distress'' or ``allostatic loading'' (McEwen and
Wingfield, 2003). This pathological state of distress will last until
the animal replenishes its energetic reserves sufficiently to restore
normal function.
According to Moberg (2000), in the case of many stressors, an
animal's first and sometimes most economical (in terms of biotic costs)
response is behavioral avoidance of the potential stressor or avoidance
of continued exposure to a stressor. An animal's second line of defense
to stressors involves the sympathetic part of the autonomic nervous
system and the classical ``fight or flight'' response, which includes
the cardiovascular system, the gastrointestinal system, the exocrine
glands, and the adrenal medulla to produce changes in heart rate, blood
pressure, and gastrointestinal activity that humans commonly associate
with ``stress.'' These responses have a relatively short duration and
may or may not have significant long-term effect on an animal's
welfare.
An animal's third line of defense to stressors involves its
neuroendocrine systems or sympathetic nervous systems; the system that
has received the most study has been the hypothalamus-pituitary-adrenal
(HPA) system (also known as the HPA axis in mammals or the
hypothalamus-pituitary-interrenal axis in fish and some reptiles).
Unlike stress responses associated with the autonomic nervous system,
virtually all neuro-endocrine functions that are affected by stress,
including immune competence, reproduction, metabolism, and behavior,
are regulated by pituitary hormones. Stress-induced changes in the
secretion of pituitary hormones have been implicated in failed
reproduction (Moberg, 1987; Rivier and Rivest, 1991), altered
metabolism (Elasser et al., 2000), reduced immune competence (Blecha,
2000), and behavioral disturbance (Moberg, 1987; Blecha, 2000).
Increases in the circulation of glucocorticosteroids (cortisol,
corticosterone, and aldosterone in marine mammals; see Romano et al.
(2004)) have been equated with stress for many years.
Marine mammals naturally experience stressors within their
environment and as part of their life histories. Changing weather and
ocean conditions, exposure to disease and naturally occurring toxins,
lack of prey availability, and interactions with predators all
contribute to the stress a marine mammal experiences (Atkinson et al.,
2015). Breeding cycles, periods of fasting, social interactions with
members of the same species, and molting (for pinnipeds) are also
stressors, although they are natural components of an animal's life
history. Anthropogenic activities have the potential to provide
additional stressors beyond those that occur naturally (e.g., fishery
interactions, pollution, tourism, ocean noise) (Fair et al., 2014;
Meissner et al., 2015; Rolland et al., 2012).
Relationships between these physiological mechanisms, animal
behavior, and the costs of stress responses are well-studied through
controlled experiments for both laboratory and free-ranging animals
(e.g., Holberton et al., 1996; Hood et al., 1998; Jessop et al., 2003;
Krausman et al., 2004; Lankford et al., 2005; Reneerkens et al., 2002;
Thompson and Hamer, 2000). However, it should be noted that our
understanding of the functions of various stress hormones (e.g.,
cortisol), is based largely upon observations of the stress response in
terrestrial mammals. Atkinson et al., (2015) note that the endocrine
response of marine mammals to stress may not be the same as that of
terrestrial mammals because of the selective pressures marine mammals
faced during their evolution in an ocean environment. For example, due
to the necessity of breath-holding while diving and foraging at depth,
the physiological role of epinephrine and norepinephrine (the
catecholamines) in marine mammals might be different than in other
mammals. Relatively little information exists on the linkage between
anthropogenic sound exposure and stress in marine mammals, and even
less information exists on the ultimate consequences of sound-induced
stress responses (either acute or chronic). Most studies to date have
focused on acute responses to sound either by measuring catecholamines,
a neurohormone, or
[[Page 11653]]
heart rate as a proxy for an acute stress response.
The ability to make predictions from stress hormones about impacts
on individuals and populations exposed to various forms of natural and
anthropogenic stressors relies on understanding the linkages between
changes in stress hormones and resulting physiological impacts.
Currently, the sound characteristics that correlate with specific
stress responses in marine mammals are poorly understood, as are the
ultimate consequences of these changes. Several research efforts have
improved the understanding of, and the ability to predict, how
stressors ultimately affect marine mammal populations (e.g., King et
al., 2015; New et al., 2013a; Pirotta et al., 2015a; Pirotta et al.,
2022b). This includes determining how and to what degree various types
of anthropogenic sound cause stress in marine mammals and understanding
what factors may mitigate those physiological stress responses. Factors
potentially affecting an animal's response to a stressor include life
history, sex, age, reproductive status, overall physiological and
behavioral adaptability, and whether they are na[iuml]ve or experienced
with the sound (e.g., prior experience with a stressor may result in a
reduced response due to habituation) (Finneran and Branstetter, 2013;
St. Aubin and Dierauf, 2001). Because there are many unknowns regarding
the occurrence of acoustically induced stress responses in marine
mammals, any physiological response (e.g., hearing loss or injury) or
significant behavioral response is assumed to be associated with a
stress response.
Non-impulsive sources of sound can cause direct physiological
effects including noise-induced loss of hearing sensitivity (or
``threshold shift'') or other auditory injury, nitrogen decompression,
acoustically-induced bubble growth, and injury due to sound-induced
acoustic resonance. Separately, an animal's behavioral response to an
acoustic exposure might lead to physiological effects that might
ultimately lead to injury or death, which is discussed later in the
Stranding and Mortality section.
Heart Rate Response
Several experimental studies have measured the heart rate response
of a variety of marine mammals. For example, Miksis et al. (2001)
observed increases in heart rates of captive bottlenose dolphins to
which known calls of other dolphins were played, although no increase
in heart rate was observed when background tank noise was played back.
However, it cannot be determined whether the increase in heart rate was
due to stress or social factors, such as expectation of an encounter
with a known conspecific. Similarly, a young captive beluga's heart
rate increased during exposure to noise, with increases dependent upon
the frequency band of noise and duration of exposure, and with a sharp
decrease to normal or below normal levels upon cessation of the
exposure (Lyamin et al., 2011). Spectral analysis of heart rate
variability corroborated direct measures of heart rate (Bakhchina et
al., 2017). This response might have been in part due to the conditions
during testing, the young age of the animal, and the novelty of the
exposure. A year later the exposure was repeated at a slightly higher
received level and there was no heart rate response, indicating the
beluga whale had potentially habituated to the noise exposure.
Kvadsheim et al. (2010a) measured the heart rate of captive hooded
seals during exposure to sonar signals and found an increase in the
heart rate of the seals during exposure periods versus control periods
when the animals were at the surface. When the animals dove, the normal
dive-related heart rate decrease was not impacted by the sonar
exposure. Similarly, Thompson et al. (1998) observed a rapid, short-
lived decrease in heart rates in wild harbor and grey seals exposed to
seismic air guns (cited in Gordon et al. (2003)).
Two captive harbor porpoises showed significant bradycardia
(reduced heart rate), below that which occurs with diving, when they
were exposed to pinger-like sounds with frequencies between 100-140 kHz
(Teilmann et al., 2006). The bradycardia was found only in the early
noise exposures and the porpoises acclimated quickly across successive
noise exposures. Elmegaard et al. (2021) also found that initial
exposures to sonar sweeps produced bradycardia but did not elicit a
startle response in captive harbor porpoises. As with Teilmann et al.
(2006), the cardiac response disappeared over several repeat exposures
suggesting rapid acclimation to the noise. In the same animals, 40-kHz
noise pulses induced startle responses but without a change in heart
rate. Bakkeren et al. (2023) found no change in the heart rate of a
harbor porpoise during exposure to masking noise (\1/3\ octave band
noise, centered frequency of 125 kHz, maximum received level of 125 dB
re 1 [mu]Pa) during an echolocation task but showed significant
bradycardia while blindfolded for the same task. The authors attributed
the change in heart rate to sensory deprivation, although no strong
conclusions about acoustic masking could be made since the animal was
still able to perform the echolocation task in the presence of the
masking noise. Williams et al. (2022) observed periods of increased
heart rate variability in narwhals during seismic air gun impulse
exposure, but profound bradycardia was not noted. Conversely, Williams
et al. (2017) found that a profound bradycardia persisted in narwhals,
even though exercise effort increased dramatically as part of their
escape response following release from capture and handling.
Limited evidence across several different species suggests that
increased heart rate might occur as part of the acute stress response
of marine mammals that are at the surface. However, the decreased heart
rate typical of diving marine mammals can be enhanced in response to an
acute stressor, suggesting that the context of the exposure is critical
to understanding the cardiac response. Furthermore, in instances where
a cardiac response was noted, there appears to be rapid habituation
when repeat exposures occur. Additional research is required to
understand the interaction of dive bradycardia, noise-induced cardiac
responses, and the role of habituation in marine mammals.
Stress Hormone and Immune Response
What is known about the function of the various stress hormones is
based largely upon observations of the stress response in terrestrial
mammals. The endocrine response of marine mammals to stress may not be
the same as that of terrestrial mammals because of the selective
pressures marine mammals faced during their evolution in an ocean
environment (Atkinson et al., 2015). For example, due to the necessity
of breath-holding while diving and foraging at depth, the physiological
role of epinephrine and norepinephrine (the catecholamines) might be
different in marine versus other mammals.
Catecholamines increase during breath-hold diving in seals, co-
occurring with a reduction in heart rate, peripheral vasoconstriction
(i.e., constriction of blood vessels), and an increased reliance on
anaerobic metabolism during extended dives (Hance et al., 1982;
Hochachka et al., 1995; Hurford et al., 1996); the catecholamine
increase is not associated with increased heart rate, glycemic release,
and increased oxygen consumption typical of terrestrial mammals.
Captive belugas demonstrated no catecholamine response to the playback
of oil drilling
[[Page 11654]]
sounds (Thomas et al., 1990b) but showed a small but statistically
significant increase in catecholamines following exposure to impulsive
sounds produced from a seismic water gun (Romano et al., 2004). A
captive bottlenose dolphin exposed to the same sounds did not
demonstrate a catecholamine response but did demonstrate a
statistically significant elevation in aldosterone (Romano et al.,
2004); however, the increase was within the normal daily variation
observed in this species (St. Aubin et al., 1996) and was likely of
little biological significance. Aldosterone has been speculated to not
only contribute to electrolyte balance, but possibly also the
maintenance of blood pressure during periods of vasoconstriction
(Houser et al., 2011). In marine mammals, aldosterone is thought to
play a role in mediating stress (St. Aubin and Dierauf, 2001; St. Aubin
and Geraci, 1989).
Yang et al. (2021) measured cortisol concentrations in two captive
bottlenose dolphins and found significantly higher concentrations after
exposure to 140 dB re 1 [mu]Pa impulsive noise playbacks. Two out of
six tested indicators of immune system function underwent acoustic
dose-dependent changes, suggesting that repeated exposures or sustained
stress response to impulsive sounds may increase an affected
individual's susceptibility to pathogens. Unfortunately, absolute
values of cortisol were not provided, and it is not possible from the
study to tell if cortisol rose to problematic levels (e.g., see normal
variation and changes due to handling in Houser et al. (2021) and
Champagne et al. (2018)). Exposing dolphins to a different acoustic
stressor yielded contrasting results. Houser et al. (2020) measured
cortisol and epinephrine obtained from 30 captive bottlenose dolphins
exposed to simulated Navy MFAS and found no correlation between SPL and
stress hormone levels, even though sound exposures were as high as 185
dB re 1 [mu]Pa. In the same experiment (Houser et al., 2013b),
behavioral responses were shown to increase in severity with increasing
received SPLs. These results suggest that behavioral responses to sonar
signals are not necessarily indicative of a hormonal stress response.
Whereas a limited amount of work has addressed the potential for
acute sound exposures to produce a stress response, almost nothing is
known about how chronic exposure to acoustic stressors affects stress
hormones in marine mammals, particularly as it relates to survival or
reproduction. In what is probably the only study of chronic noise
exposure in marine mammals associating changes in a stress hormone with
changes in anthropogenic noise, Rolland et al. (2012) compared the
levels of cortisol metabolites in NARW feces collected before and after
September 11, 2001. Following the events of September 11, 2001,
shipping was significantly reduced in the region where fecal
collections were made, and regional ocean background noise declined.
Fecal cortisol metabolites significantly decreased during the period of
reduced ship traffic and ocean noise (Rolland et al., 2012). Rolland et
al. (2017) also compared acute (death by vessel strike) to chronic
(entanglement or live stranding) stressors in NARW and found that
whales subject to chronic stressors had higher levels of glucocorticoid
stress hormones (cortisol and corticosterone) than either healthy
whales or those killed by ships. It was presumed that whales subjected
to acute stress may have died too quickly for increases in fecal
glucocorticoids to be detected.
Considerably more work has been conducted in an attempt to
determine the potential effect of vessel disturbance on smaller
cetaceans, particularly killer whales (Bain, 2002; Erbe, 2002; Lusseau,
2006; Noren et al., 2009; Pirotta et al., 2015b; Read et al., 2014;
Rolland et al., 2012; Williams et al., 2009; Williams et al., 2014a;
Williams et al., 2014b; Williams et al., 2006b). Most of these efforts
focused primarily on estimates of metabolic costs associated with
altered behavior or inferred consequences of boat presence and noise
but did not directly measure stress hormones. However, Ayres et al.
(2012) investigated Southern Resident killer whale fecal thyroid
hormone and cortisol metabolites to assess two potential threats to the
species' recovery: lack of prey (salmon) and impacts from exposure to
the physical presence of vessel traffic (but without measuring vessel
traffic noise). Ayres et al. (2012) concluded from these stress hormone
measures that the lack of prey overshadowed any population-level
physiological impacts on Southern Resident killer whales due to vessel
traffic. Lemos et al. (2022) investigated the potential for vessel
traffic to affect gray whales. By assessing gray whale fecal cortisol
metabolites across years in which vessel traffic was variable, Lemos et
al. (2022) found a direct relationship between the presence/density of
vessel traffic and fecal cortisol metabolite levels. Unfortunately, no
direct noise exposure measurements were made on any individual making
it impossible to tell if other natural and anthropogenic factors could
also be related to the results. Collectively, these studies indicate
the difficulty in determining which factors primarily influence the
secretion of stress hormones, including the separate and additive
effects of vessel presence and vessel noise. While vessel presence
could contribute to the variation in fecal cortisol metabolites in
NARWs and gray whales, there are other potential influences on fecal
hormone metabolites, so it is difficult to establish a direct link
between ocean noise and fecal hormone metabolites.
Non-Auditory Injury
Non-auditory injury, or direct injury, is considered unlikely to
occur in the context of the Navy's proposed activities. Here, we
discuss less direct non-auditory injury impacts, including acoustically
induced bubble formation, and injury from sonar-induced acoustic
resonance. Neither the Navy, nor NMFS expects physical or non-auditory
injury or mortality to any of the marine mammal species in the Study
Area due to the proposed activities.
One theoretical cause of injury to marine mammals is rectified
diffusion (Crum and Mao, 1996), the process of increasing the size of a
bubble by exposing it to a sound field. This process could be
facilitated if the environment in which the ensonified bubbles exist is
supersaturated with gas. Repetitive diving by marine mammals can cause
the blood and some tissues to accumulate gas to a greater degree than
is supported by the surrounding environmental pressure (Ridgway and
Howard, 1979). The deeper and longer dives of some marine mammals (for
example, beaked whales) are theoretically predicted to induce greater
supersaturation (Houser et al., 2001b). If rectified diffusion were
possible in marine mammals exposed to high-level sound, conditions of
tissue supersaturation could theoretically speed the rate and increase
the size of bubble growth. Subsequent effects due to tissue trauma and
emboli would presumably mirror those observed in humans suffering from
decompression sickness. Acoustically-induced (or mediated) bubble
growth and other pressure-related physiological impacts are addressed
below but are not expected to result from the Navy's proposed
activities.
It is unlikely that the short duration (in combination with the
source levels) of sonar pings would be long enough to drive bubble
growth to any substantial size, if such a phenomenon occurs. However,
an alternative but related hypothesis has also been suggested: stable
bubbles could be destabilized by
[[Page 11655]]
high-level sound exposures such that bubble growth then occurs through
static diffusion of gas out of the tissues. In such a scenario the
marine mammal would need to be in a gas-supersaturated state for a long
enough period of time for bubbles to become of a problematic size.
Recent research with ex vivo supersaturated bovine tissues suggested
that, for a 37 kHz signal, a sound exposure of approximately 215 dB re
1 [mu]Pa would be required before microbubbles became destabilized and
grew (Crum et al., 2005). Assuming spherical spreading loss and a
nominal sonar source level of 235 dB re 1 [mu]Pa at 1 m, a whale would
need to be within 10 m of the sonar dome to be exposed to such sound
levels. Furthermore, tissues in the study were supersaturated by
exposing them to pressures of 400-700 kilopascals for periods of hours
and then releasing them to ambient pressures. Assuming the
equilibration of gases with the tissues occurred when the tissues were
exposed to the high pressures, levels of supersaturation in the tissues
could have been as high as 400-700 percent. These levels of tissue
supersaturation are substantially higher than model predictions for
marine mammals (Fahlman et al., 2009; Fahlman et al., 2014; Houser et
al., 2001; Saunders et al., 2008). It is improbable that this mechanism
is responsible for stranding events or traumas associated with beaked
whale strandings because both the degree of supersaturation and
exposure levels observed to cause microbubble destabilization are
unlikely to occur, either alone or in concert.
Yet another hypothesis has speculated that rapid ascent to the
surface following exposure to a startling sound might produce tissue
gas saturation sufficient for the evolution of nitrogen bubbles (i.e.,
decompression sickness) (Jepson et al., 2003; Fernandez et al., 2005).
In this scenario, the rate of ascent would need to be sufficiently
rapid to compromise behavioral or physiological protections against
nitrogen bubble formation. Alternatively, Tyack et al. (2006) studied
the deep diving behavior of beaked whales and concluded that: ``Using
current models of breath-hold diving, we infer that their natural
diving behavior is inconsistent with known problems of acute nitrogen
supersaturation and embolism.'' Collectively, these hypotheses can be
referred to as ``hypotheses of acoustically mediated bubble growth.''
Although theoretical predictions suggest the possibility for
acoustically mediated bubble growth, there is considerable disagreement
among scientists as to its likelihood (Piantadosi and Thalmann, 2004;
Evans and Miller, 2003; Cox et al., 2006; Rommel et al., 2006). Crum
and Mao (1996) hypothesized that received levels would have to exceed
190 dB in order for there to be the possibility of significant bubble
growth due to supersaturation of gases in the blood (i.e., rectified
diffusion). Work conducted by Crum et al. (2005) demonstrated the
possibility of rectified diffusion for short duration signals, but at
SELs and tissue saturation levels that are highly improbable to occur
in diving marine mammals. To date, energy levels predicted to cause in
vivo bubble formation within diving cetaceans have not been evaluated
(NOAA, 2002b). Jepson et al. (2003, 2005) and Fernandez et al. (2004,
2005) concluded that in vivo bubble formation, which may be exacerbated
by deep, long-duration, repetitive dives may explain why beaked whales
appear to be relatively vulnerable to MFAS/HFAS exposures. It has also
been argued that traumas from some beaked whale strandings are
consistent with gas emboli and bubble-induced tissue separations
(Jepson et al., 2003); however, there is no conclusive evidence of this
(Rommel et al., 2006). Based on examination of sonar-associated
strandings, Bernaldo de Quiros et al. (2019) list diagnostic features,
the presence of all of which suggest gas and fat embolic syndrome for
beaked whales stranded in association with sonar exposure.
As described in additional detail in appendix D the 2025 SURTASS
Draft SEIS/OEIS, marine mammals generally are thought to deal with
nitrogen loads in their blood and other tissues, caused by gas exchange
from the lungs under conditions of high ambient pressure during diving,
through anatomical, behavioral, and physiological adaptations (Hooker
et al., 2012). Although not a direct injury, variations in marine
mammal diving behavior or avoidance responses have been hypothesized to
result in nitrogen off-gassing in super-saturated tissues, possibly to
the point of deleterious vascular and tissue bubble formation (Hooker
et al., 2012; Jepson et al., 2003; Saunders et al., 2008) with
resulting symptoms similar to decompression sickness; however, the
process is still not well understood.
In 2009, Hooker et al. tested two mathematical models to predict
blood and tissue tension N2 (P<INF>N2</INF>) using field data from
three beaked whale species: northern bottlenose whales, goose-beaked
whales, and Blainville's beaked whales. The researchers aimed to
determine if physiology (body mass, diving lung volume, and dive
response) or dive behavior (dive depth and duration, changes in ascent
rate, and diel behavior) would lead to differences in P<INF>N2</INF>
levels and thereby decompression sickness risk between species. In
their study, they compared results for previously published time depth
recorder data (Hooker and Baird, 1999; Baird et al., 2006, 2008) from
goose-beaked whale, Blainville's beaked whale, and northern bottlenose
whale. They reported that diving lung volume and extent of the dive
response had a large effect on end-dive P<INF>N2</INF>. Also, results
showed that dive profiles had a larger influence on end-dive
P<INF>N2</INF> than body mass differences between species. Despite diel
changes (i.e., variation that occurs regularly every day or most days)
in dive behavior, P<INF>N2</INF> levels showed no consistent trend.
Model output suggested that all three species live with tissue
P<INF>N2</INF> levels that would cause a significant proportion of
decompression sickness cases in terrestrial mammals. The authors
concluded that the dive behavior of goose-beaked whale was different
from both Blainville's beaked whale and northern bottlenose whale and
resulted in higher predicted tissue and blood N2 levels (Hooker et al.,
2009). They also suggested that the prevalence of goose-beaked whales
stranding after naval sonar exercises could be explained by either a
higher abundance of this species in the affected areas or by possible
species differences in behavior and/or physiology related to MF active
sonar (Hooker et al., 2009).
Bernaldo de Quiros et al. (2012) showed that, among stranded
whales, deep diving species of whales had higher abundances of gas
bubbles compared to shallow diving species. Kvadsheim et al. (2012)
estimated blood and tissue P<INF>N2</INF> levels in species
representing shallow, intermediate, and deep diving cetaceans following
behavioral responses to sonar and their comparisons found that deep
diving species had higher end-dive blood and tissue N2 levels,
indicating a higher risk of developing gas bubble emboli compared with
shallow diving species. Fahlmann et al. (2014) evaluated dive data
recorded from sperm, killer, long-finned pilot, Blainville's, and
goose-beaked whales before and during exposure to low-frequency (1-2
kHz), as defined by the authors, and mid-frequency (2-7 kHz) active
sonar in an attempt to determine if either differences in dive behavior
or physiological responses to sonar are plausible risk factors for
bubble formation. The authors suggested that CO<INF>2</INF> may
initiate bubble formation and growth, while elevated levels of N2 may
[[Page 11656]]
be important for continued bubble growth. The authors also suggest that
if CO<INF>2</INF> plays an important role in bubble formation, a
cetacean escaping a sound source may experience increased metabolic
rate, CO<INF>2</INF> production, and alteration in cardiac output,
which could increase risk of gas bubble emboli. However, as discussed
in Kvadsheim et al. (2012), the actual observed behavioral responses to
sonar from the species in their study (sperm, killer, long-finned
pilot, Blainville's beaked, and goose-beaked whales) did not imply any
significantly increased risk of decompression sickness due to high
levels of N2. Therefore, further information is needed to understand
the relationship between exposure to stimuli, behavioral response
(discussed in more detail below), elevated N2 levels, and gas bubble
emboli in marine mammals. The hypotheses for gas bubble formation
related to beaked whale strandings is that beaked whales potentially
have strong avoidance responses to MFAS because they sound similar to
their main predator, the killer whale (Cox et al., 2006; Southall et
al., 2007; Zimmer and Tyack, 2007; Baird et al., 2008; Hooker et al.,
2009). Further investigation is needed to assess the potential validity
of these hypotheses.
To summarize, while there are several hypotheses, there is little
data directly connecting intense, anthropogenic underwater sounds with
non-auditory physical effects in marine mammals. The available data do
not support identification of a specific exposure level above which
non-auditory effects can be expected (Southall et al., 2007) or any
meaningful quantitative predictions of the numbers (if any) of marine
mammals that might be affected in these ways. In addition, such
effects, if they occur at all, would be expected to be limited to
situations where marine mammals were exposed to high powered sounds at
very close range over a prolonged period of time, which is not expected
to occur based on the speed of the vessels operating sonar in
combination with the speed and behavior of marine mammals in the
vicinity of sonar.
An object exposed to its resonant frequency will tend to amplify
its vibration at that frequency, a phenomenon called acoustic
resonance. Acoustic resonance has been proposed as a potential
mechanism by which a sonar or sources with similar operating
characteristics could damage tissues of marine mammals. In 2002, NMFS
convened a panel of government and private scientists to investigate
the potential for acoustic resonance to occur in marine mammals (NOAA,
2002). They modeled and evaluated the likelihood that Navy MFAS (2-10
kHz) caused resonance effects in beaked whales that eventually led to
their stranding. The workshop participants concluded that resonance in
air-filled structures was not likely to have played a primary role in
the Bahamas stranding in 2000. They listed several reasons supporting
this finding including (among others): (1) tissue displacements at
resonance are estimated to be too small to cause tissue damage; (2)
tissue-lined air spaces most susceptible to resonance are too large in
marine mammals to have resonant frequencies in the ranges used by MFAS
or LFA sonar; (3) lung resonant frequencies increase with depth, and
tissue displacements decrease with depth so if resonance is more likely
to be caused at depth it is also less likely to have an affect there;
and (4) lung tissue damage has not been observed in any mass, multi-
species stranding of beaked whales. The frequency at which resonance
was predicted to occur in the animals' lungs was 50 Hz, well below the
frequencies used by the MFAS systems associated with the Bahamas event.
The workshop participants focused on the March 2000 stranding of beaked
whales in the Bahamas as high-quality data were available, but the
workshop report notes that the results apply to other sonar-related
stranding events. For the reasons given by the 2002 workshop
participants, we do not anticipate injury due to sonar-induced acoustic
resonance from the Navy's proposed activity.
Further Potential Effects of Behavioral Disturbance on Marine Mammal
Fitness
The different ways in which marine mammals respond to sound are
sometimes indicators of the ultimate effect that exposure to a given
stimulus will have on the well-being (survival, reproduction, etc.) of
an animal. The long-term consequences of disturbance, hearing loss,
chronic masking, and acute or chronic physiological stress are
difficult to predict because of the different factors experienced by
individual animals, such as context of stressor exposure, underlying
health conditions, and other environmental or anthropogenic stressors.
Linking these non-lethal effects on individuals to changes in
population growth rates requires long-term data, which is lacking for
many populations. We summarize several studies below, but there are few
quantitative marine mammal data relating the exposure of marine mammals
to sound to effects on reproduction or survival, though data exists for
terrestrial species to which we can draw comparisons for marine
mammals. Several authors have reported that disturbance stimuli may
cause animals to abandon nesting and foraging sites (Sutherland and
Crockford, 1993), may cause animals to increase their activity levels
and suffer premature deaths or reduced reproductive success when their
energy expenditures exceed their energy budgets (Daan et al., 1996;
Feare, 1976; Mullner et al., 2004), or may cause animals to experience
higher predation rates when they adopt risk-prone foraging or migratory
strategies (Frid and Dill, 2002). Each of these studies addressed the
consequences of animals shifting from one behavioral state (e.g.,
resting or foraging) to another behavioral state (e.g., avoidance or
escape behavior) because of human disturbance or disturbance stimuli.
Lusseau and Bejder (2007) present data from three long-term studies
illustrating the connections between disturbance from whale-watching
boats and population-level effects in cetaceans. In Shark Bay
Australia, the abundance of bottlenose dolphins was compared within
adjacent control and tourism sites over three consecutive 4.5-year
periods of increasing tourism levels. Between the second and third time
periods, in which tourism doubled, dolphin abundance decreased by 15
percent in the tourism area and did not change significantly in the
control area. In Fiordland, New Zealand, two populations (Milford and
Doubtful Sounds) of bottlenose dolphins with tourism levels that
differed by a factor of seven were observed and significant increases
in travelling time and decreases in resting time were documented for
both. Consistent short-term avoidance strategies were observed in
response to tour boats until a threshold of disturbance was reached
(average 68 minutes between interactions), after which the response
switched to a longer-term habitat displacement strategy. For one
population, tourism occurred only in a part of the home range. However,
tourism occurred throughout the home range of the Doubtful Sound
population and once boat traffic increased beyond the 68-minute
threshold (resulting in abandonment of their home range/preferred
habitat), reproductive success drastically decreased (i.e., increased
stillbirths) and abundance decreased significantly (from 67 to 56
individuals in a short period). Last, in a study of Northern Resident
killer whales off Vancouver Island, exposure to boat traffic was shown
to reduce foraging opportunities and increase traveling
[[Page 11657]]
time. A simple bioenergetics model was applied to show that the reduced
foraging opportunities equated to a decreased energy intake of 18
percent, while the increased traveling incurred an increased energy
output of 3-4 percent, which suggests that a management action based on
avoiding interference with foraging might be particularly effective.
An important variable to consider is duration of disturbance.
Severity scales used to assess behavioral responses of marine mammals
to acute sound exposures are not appropriate to apply to sustained or
chronic exposures, which requires considering the health of a
population over time rather than a focus on immediate impacts to
individuals (Southall et al., 2021). For example, short-term costs
experienced over the course of a week by an otherwise healthy
individual may be recouped over time after exposure to the stressor
ends. These short-term costs would be unlikely to result in long-term
consequences to that individual or to that individual's population.
Comparatively, long-term costs accumulated by otherwise healthy
individuals over an entire season, year, or throughout a life stage
(e.g., pup, juvenile, adult) would be less easily recouped and more
likely to result in long-term consequences to that individual or
population.
Marine mammals exposed to frequent or intense anthropogenic
activities may leave the area, habituate to the activity, or tolerate
the disturbance and remain in the area (Wartzok et al., 2003). Highly
resident or localized populations may also stay in an area of
disturbance because the cost of displacement is higher than the cost of
remaining in the area (Forney et al., 2017). As such, an apparent lack
of response (e.g., no displacement or avoidance of a sound source) does
not necessarily indicate there is no cost to the individual or
population, as some resources or habitats may be of such high value
that animals may choose to stay, even when experiencing the
consequences of stress, masking, or hearing loss (Forney et al., 2017).
Longer term displacement can lead to changes in abundance or
distribution patterns of the species in the affected region (Bejder et
al., 2006b; Blackwell et al., 2004; Teilmann et al., 2006). For
example, gray whales in Baja California, Mexico, abandoned a historical
breeding lagoon in the mid-1960s due to an increase in dredging and
commercial shipping operations, and only repopulated the lagoon after
shipping activities had ceased for several years (Bryant et al., 1984).
Mysticetes in the northeast tended to adjust to vessel traffic over
several years, trending towards more neutral behavioral responses to
passing vessels (Watkins, 1986), indicating that some animals may
habituate to high levels of human activity. A study on bottlenose
dolphin responses to vessel approaches found that lesser responses in
populations of dolphins regularly subjected to high levels of vessel
traffic could be a sign of habituation, or it could be that the more
sensitive animals in this population previously abandoned the area of
higher human activity (Bejder et al., 2006a).
Population characteristics (e.g., whether a population is open or
closed to immigration and emigration) can influence sensitivity to
disturbance as well; closed populations could not withstand a higher
probability of disturbance compared to open populations with no
limitation on food (New et al., 2020). Predicting population trends or
long-term displacement patterns due to anthropogenic disturbance is
challenging due to limited information and survey data for many species
over sufficient spatiotemporal scales, as well as a full understanding
of how other factors, such as oceanographic oscillations, affect marine
mammal presence (Moore and Barlow, 2013; Barlow, 2016; Moore and
Barlow, 2017).
Population models are necessary to understand and link short-term
effects to individuals from disturbance (anthropogenic impacts or
environmental change) to long-term population consequences. Population
models require inputs for the population size and changes in vital
rates of the population (e.g., the mean values for survival age,
lifetime reproductive success, recruitment of new individuals into the
population), to predict changes in population dynamics (e.g.,
population growth rate). These efforts often rely on bioenergetic
models, or energy budget models, which analyze energy intake from food
and energy costs for life functions, such as maintenance, growth, and
reproduction, either at the individual or population level (Pirotta,
2022), and model sensitivity analyses have identified the most
consequential parameters, including prey characteristics, feeding
processes, energy expenditure, body size, energy storage, and lactation
capability (Pirotta, 2022). However, there is a high level of
uncertainty around many parameters in these models (H[uuml]tt et al.,
2023).
The U.S. National Research Council (NRC) committee on
Characterizing Biologically Significant Marine Mammal Behavior
developed an initial conceptual model to link acoustic disturbance to
population effects and inform data and research needs (NRC, 2005). This
Population Consequences of Acoustic Disturbance, or PCAD, conceptual
model linked the parameters of sound exposure, behavior change, life
function immediately affected, vital rates, and population effects. In
its report, the committee found that the relationships between vital
rates and population effects were relatively well understood, but that
the relationships between the other components of the model were not
well-known or easily observed.
Following the PCAD framework (NRC, 2005), an Office of Naval
Research (ONR) working group developed the Population Consequences of
Disturbance (PCoD), outlining an updated conceptual model of the
relationships linking disturbance to changes in behavior and
physiology, health, vital rates, and population dynamics. The PCoD
model considers all types of disturbance, not solely anthropogenic or
acoustic, and incorporates physiological changes, such as stress or
injury, along with behavioral changes as a direct result of disturbance
(National Academies of Sciences Engineering and Medicine, 2017). In
this framework, behavioral and physiological changes can have direct
(acute) effects on vital rates, such as when changes in habitat use or
increased stress levels raise the probability of mother-calf separation
or predation; they can have indirect and long-term (chronic) effects on
vital rates, such as when changes in time/energy budgets or increased
disease susceptibility affect health, which then affects vital rates;
or they can have no effect to vital rates (New et al., 2014; Pirotta et
al., 2018a). In addition to outlining this general framework and
compiling the relevant literature that supports it, the authors chose
four example species for which extensive long-term monitoring data
exist (southern elephant seals, NARW, Ziphiidae beaked whales, and
bottlenose dolphins) and developed state-space energetic models that
can be use
[…truncated; see source link]Indexed from Federal Register on March 10, 2026.
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