Proposed Rule2026-03691
Taking and Importing Marine Mammals; Taking Marine Mammals Incidental to Geophysical Surveys in the Gulf of America
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
February 24, 2026
Issuing agencies
Commerce DepartmentNational Oceanic and Atmospheric Administration
Abstract
NMFS has received a request for the reimplementation of incidental take regulations (ITR) governing the incidental taking of marine mammals during geophysical survey activity conducted in the Gulf of America (GOA). Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting comments on its proposed rule and will consider public comments relevant to this proposed rule prior to issuing any final rule.
Full Text
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[Federal Register Volume 91, Number 36 (Tuesday, February 24, 2026)]
[Proposed Rules]
[Pages 9014-9086]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-03691]
[[Page 9013]]
Vol. 91
Tuesday,
No. 36
February 24, 2026
Part III
Department of Commerce
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National Oceanic and Atmospheric Administration
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50 CFR Part 217
Taking and Importing Marine Mammals; Taking Marine Mammals Incidental
to Geophysical Surveys in the Gulf of America; Proposed Rule
Federal Register / Vol. 91, No. 36 / Tuesday, February 24, 2026 /
Proposed Rules
[[Page 9014]]
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DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
50 CFR Part 217
[Docket No. 260220-0051]
RIN 0648-BO19
Taking and Importing Marine Mammals; Taking Marine Mammals
Incidental to Geophysical Surveys in the Gulf of America
AGENCY: National Marine Fisheries Service (NMFS), National Oceanic and
Atmospheric Administration (NOAA), Commerce.
ACTION: Proposed rule; request for comments.
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SUMMARY: NMFS has received a request for the reimplementation of
incidental take regulations (ITR) governing the incidental taking of
marine mammals during geophysical survey activity conducted in the Gulf
of America (GOA). Pursuant to the Marine Mammal Protection Act (MMPA),
NMFS is requesting comments on its proposed rule and will consider
public comments relevant to this proposed rule prior to issuing any
final rule.
DATES: Comments and information must be received no later than March
26, 2026.
ADDRESSES: Submit all electronic public comments via the Federal e-
Rulemaking Portal. Visit <a href="https://www.regulations.gov">https://www.regulations.gov</a> and enter NOAA-
NMFS-2025-0638 in the Search box. Click on the ``Comment'' icon,
complete the required fields, and enter or attach your comments. A
plain language summary of the rule is also available on the Federal e-
Rulemaking Portal.
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 on
<a href="https://www.regulations.gov">https://www.regulations.gov</a> without change. All personal identifying
information (e.g., name, address), 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).
FOR FURTHER INFORMATION CONTACT: Ben Laws, Office of Protected
Resources, NMFS, (301) 427-8401.
SUPPLEMENTARY INFORMATION:
Purpose and Need for Regulatory Action
On January 19, 2021 (86 FR 5322), in response to a petition request
from BOEM, NMFS issued a final rule implementing ITRs under the MMPA,
16 U.S.C. 1361 et seq., governing the take of marine mammals incidental
to the conduct of geophysical survey activities in the GOA.\1\ The ITRs
provide a framework for authorization of incidental take through
Letters of Authorization (LOAs) upon request from individual applicants
planning specific geophysical survey activities The ITRs became
effective on April 19, 2021, and are effective through April 19, 2026
(86 FR 5322, January 19, 2021).
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\1\ Pursuant to Executive Order 14172, ``Restoring Names That
Honor American Greatness,'' and Department of the Interior
Secretarial Order 3423, ``The Gulf of America,'' the body of water
formerly known as the Gulf of Mexico is now called the Gulf of
America. Accordingly, NMFS amended the incidental take regulations
to reflect the change. See 90 FR 38001 (August 7, 2025).
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NMFS subsequently discovered that the 2021 rule was based on
erroneous take estimates. We conducted another rulemaking to reassess
the statutorily required findings for issuance of the 2021 ITRs using
correct take estimates and other newly available and pertinent
information relevant to the analyses supporting some of the findings in
the 2021 final rule and the taking allowable under the regulations. We
issued a final rule affirming those findings in April 2024, effective
through April 19, 2026 (89 FR 31488, April 24, 2024). The 2024 rule did
not result in any changes to the existing ITRs.
On March 25, 2025, NMFS received an application from the EnerGeo
Alliance (EnerGeo) requesting development of ITRs governing the taking
of marine mammals incidental to geophysical survey activity conducted
in the GOA over the course of 5 years following the expiration of the
existing ITRs. Following receipt of NMFS' comments on the draft
application on April 15, 2025, EnerGeo submitted revised versions of
the application on July 14, August 8, and August 12, 2025, the last of
which was determined to be adequate and complete. NMFS determined at
that time, based on the date of submission of the adequate and complete
application, that it was unlikely a new rulemaking process could be
completed prior to expiration of the existing ITRs on April 19, 2026.
On August 28, 2025, NMFS Office of Protected Resources (OPR)
received a request from NMFS Office of Policy (Policy) for
reimplementation of the current ITR to avoid a lapse in ITRs offering
incidental take coverage for GOA geophysical survey activities. The
request notes that the pending April 2026 expiration of the current
ITRs would affect regulatory certainty through loss of an efficient
permitting framework, and that reimplementation of the existing ITRs on
the basis of the same specified activity defined in the initial 2021
final rule and associated estimates of incidental take evaluated in the
2024 corrective rulemaking is consistent with the MMPA and appropriate
pursuant to Executive Orders 14156, ``Declaring a National Energy
Emergency,'' and 14154, ``Unleashing American Energy.'' On October 20,
2025, BOEM (the original petitioner for the current ITRs) submitted a
request to be included in the process as a co-petitioner.
NMFS has received multiple requests from industry survey operators
relating to specific survey activities that would extend beyond the
expiration date of the current ITRs, establishing the ongoing need for
the ITRs. The requested reimplementation of regulations would continue
the current established framework for authorization of incidental take
through LOAs until superseded by a new ITR promulgated on the basis of
the separate EnerGeo request.
Legal Authority for the Action
Section 101(a)(5)(A) of the MMPA (16 U.S.C. 1371(a)(5)(A)) directs
the Secretary of Commerce 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 for up to 5 years if,
after notice and public comment, the agency makes certain findings and
issues regulations that set forth permissible methods of taking
pursuant to that activity and other means of effecting the ``least
practicable adverse impact'' (LPAI) on the affected species or stocks
and their habitat (see the discussion below in the Proposed Mitigation
section), as well as monitoring and reporting requirements. Under NMFS'
implementing regulations for section 101(a)(5)(A), NMFS issues LOAs to
individuals (including entities) seeking authorization for take under
the activity-specific incidental take regulations (50 CFR 216.106).
Severability
In the event a court declares NMFS' interpretation of small numbers
to be invalid, NMFS intends that the remaining aspects of the rule and
ITR be severable. This is because the negligible impact analysis for
this rule is the
[[Page 9015]]
biologically relevant inquiry, and that analysis is based on the total
annual estimated taking for all activities the regulations will govern.
The issuance of LOAs to authorize the incidental take of marine
mammals, subject to the mitigation, monitoring, and reporting
requirements in those LOAs, is based on a finding that the total taking
over the five-year period will have a negligible impact on the affected
species or stocks; and that the mitigation and related monitoring will
effect the least practicable adverse impact on those species or stocks.
The small numbers standard is a statutory requirement that could be
satisfied on an LOA by LOA basis in accordance with the ruling of a
court that invalidates the interpretation set forth in this proposed
rule. NMFS is including a provision in the proposed regulatory text to
that effect.
Summary of Major Provisions Within the Regulations
Following is a summary of the major provisions of this proposed
rule regarding geophysical survey activities. The regulations contain
requirements for mitigation, monitoring, and reporting, including:
<bullet> Standard detection-based mitigation measures, including
use of visual and acoustic observation to detect marine mammals and
shutdown of acoustic sources in certain circumstances;
<bullet> A time-area restriction designed to avoid effects to
bottlenose dolphins in times and places of particular importance;
<bullet> Vessel strike avoidance measures; and
<bullet> Monitoring and reporting requirements.
These measures are unchanged from those included in the current
ITRs. See 50 CFR 217.180 et seq.
Background
Sections 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361 et seq.)
direct 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 issued or, if
the taking is limited to harassment, a notice of a proposed
authorization is provided to the public for review.
An incidental take authorization shall be granted if NMFS finds
that the taking will have a negligible impact on the species or
stock(s), will not have an unmitigable adverse impact on the
availability of the species or stock(s) for subsistence uses (where
relevant), and if the permissible methods of taking and requirements
pertaining to the mitigation, monitoring, and reporting of such takings
are set forth.
NMFS has defined ``negligible impact'' in 50 CFR 216.103 as an
impact resulting from the specified activity that cannot be reasonably
expected to, and is not reasonably likely to, adversely affect the
species or stock through effects on annual rates of recruitment or
survival. The MMPA states that the term ``take'' means to harass, hunt,
capture, kill or attempt to harass, hunt, capture, or kill any marine
mammal.
Except with respect to certain activities not pertinent here, the
MMPA defines ``harassment'' as: any act of pursuit, torment, or
annoyance, which (i) has the potential to injure a marine mammal or
marine mammal stock in the wild (Level A harassment); or (ii) has the
potential to disturb a marine mammal or marine mammal stock in the wild
by causing disruption of behavioral patterns, including, but not
limited to, migration, breathing, nursing, breeding, feeding, or
sheltering (Level B harassment).
On January 19, 2021, we issued a final rule with ITRs to govern the
unintentional taking of marine mammals incidental to geophysical survey
activities conducted in U.S. waters of the GOA over the course of the
statutory maximum of 5 years (86 FR 5322, January 19, 2021). NMFS
subsequently discovered that the 2021 rule was based on erroneous take
estimates. We conducted another rulemaking to reassess the statutorily
required findings for issuance of the 2021 ITRs using correct take
estimates and other newly available and pertinent information relevant
to the analyses supporting some of the findings in the 2021 final rule
and the taking allowable under the regulations. We issued a final rule
affirming those findings in April 2024 (89 FR 31488, April 24, 2024).
The 2024 rule did not result in any changes to the existing ITRs, which
provide a framework for authorization of incidental take through LOAs
upon request from individual applicants planning specific geophysical
survey activities. The existing ITRs are in effect through April 19,
2026.
On March 25, 2025, NMFS received an application from EnerGeo
requesting development of ITRs governing the taking of marine mammals
incidental to geophysical survey activity conducted in the GOA over the
course of 5 years following the date of issuance. Following receipt of
NMFS' comments on the draft application on April 15, 2025, EnerGeo
submitted revised versions of the application on July 14, August 8, and
August 12, 2025. On September 24, 2025 (90 FR 45936), we published a
notice of receipt of the request in the Federal Register, requesting
comments and information related to the request.
On August 28, 2025, NMFS OPR received a request from NMFS Policy
for reimplementation of the current ITR. The request notes that the
pending April 2026 expiration of the current ITR would affect
regulatory certainty with loss of an efficient permitting framework,
and that reimplementation of the existing ITR on the basis of the same
specified activity defined in the initial 2021 final rule and
associated estimates of incidental take evaluated in the 2024
corrective rulemaking is consistent with the MMPA and appropriate
pursuant to Executive Orders 14156, ``Declaring a National Energy
Emergency,'' and 14154, ``Unleashing American Energy.'' On September 3,
2025 (90 FR 42569), we published a notice of receipt of the request in
the Federal Register, requesting comments and information related to
the request. All comments received are available online at <a href="https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america">https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america</a>. Among the
comments was a letter from EnerGeo and other industry trade
associations expressing support for NMFS' proposed issuance of
reimplemented ITRs until superseded by a new ITR promulgated on the
basis of the separate EnerGeo request. Please see the letters for full
comments.
On October 20, 2025, BOEM (the original petitioner for the current
ITRs) submitted a request to be included in the process as a co-
petitioner, expressing support for the requested reimplementation of
the existing ITRs. Both the NMFS Policy and BOEM requests are available
online at: <a href="https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america">https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america</a>.
This proposed rule provides analysis of the same activities and
activity levels considered for the 2021 final rule, which were
unchanged in the 2024 final rule, and utilizes the same modeling
methodology described in the 2024 final rule. We incorporate the best
available information, including information that was newly evaluated
in the 2024 final rule and any information that is newly available
since issuance of the 2024 final rule. The 2024 final rule incorporated
expanded modeling results relative to the 2021 final rule that
[[Page 9016]]
estimate take utilizing the existing methodology but also consider the
effects of using smaller airgun arrays (relative to the proxy source
originally defined by BOEM) that are currently prevalent as evidenced
by LOA applications received by NMFS to date (see <a href="https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america">https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america</a>).
There are no changes to the nature or level of the specified
activities within or across years or to the geographic scope of the
activity. There is no new information pertaining to the estimates of
marine mammal take presented in the 2024 final rule and, therefore, no
changes to those take numbers. Based on our assessment of the specified
activity in light of the revised take estimates and other new
information, we have preliminarily determined that the 2024 ITRs at 50
CFR 217.180 et seq., which include the required mitigation and
associated monitoring measures, satisfy the MMPA requirement to
prescribe the means of effecting the LPAI on the affected species or
stocks and their habitat, and therefore, do not change those
regulations, nor do we change the requirements pertaining to monitoring
and reporting.
National Environmental Policy Act (NEPA)
In 2017, BOEM produced a final Programmatic Environmental Impact
Statement (PEIS) to evaluate the direct, indirect, and cumulative
impacts of geological and geophysical survey activities in the GOA,
pursuant to requirements of NEPA. The PEIS is available online at:
<a href="https://www.boem.gov/Gulf-of-Mexico-Geological-and-Geophysical-Activities-Programmatic-EIS/">https://www.boem.gov/Gulf-of-Mexico-Geological-and-Geophysical-Activities-Programmatic-EIS/</a>. NOAA, through NMFS, participated in
preparation of the PEIS as a cooperating agency due to its legal
jurisdiction and special expertise in conservation and management of
marine mammals, including its authority to authorize incidental take of
marine mammals under the MMPA.
In 2020, NMFS prepared a Record of Decision (ROD): (1) to adopt
BOEM's Final PEIS to support NMFS' analysis associated with issuance of
incidental take authorizations pursuant to section 101(a)(5)(A) or (D)
of the MMPA and the regulations governing the taking and importing of
marine mammals (50 CFR part 216); and (2) to announce and explain the
basis for NMFS' decision to review and potentially issue incidental
take authorizations under the MMPA on a case-by-case basis, if
appropriate.
The 2017 NOAA NEPA Companion Manual required supplements to
Environmental Impact Statements if (1) the agency made substantial
changes in the proposed action that are relevant to environmental
concerns or (2) there were significant new circumstances or information
relevant to environmental issues and bearing on the proposed action or
its impacts. For the 2024 final rule, NMFS considered these criteria
and the criteria relied upon for the 2020 ROD to determine whether any
new circumstances or information were ``significant,'' thereby
requiring supplementation of the 2017 PEIS. NMFS reevaluated its
findings related to the MMPA negligible impact standard and the LPAI
standard governing its regulations in light of the corrected take
estimates and other relevant new information. Based on that evaluation,
NMFS reaffirmed its negligible impact determinations and determined
that the existing regulations prescribed the means of effecting the
LPAI on the affected species or stocks and their habitat, and therefore
made no changes to the regulations. NMFS considered updated take
estimates that corrected the take estimate errors and incorporated
other new information, e.g., modeling of a more representative airgun
array and updated marine mammal density information. NMFS also
consulted scientific publications from 2021 through 2024, data that
were collected by the agency and other entities after the PEIS was
completed, field reports, reports produced under the BOEM-funded Gulf
of Mexico Marine Assessment Program for Protected Species (GoMMAPPS)
project), and other sources (e.g., updated NMFS Stock Assessment
Reports (SARs)). In addition, NMFS considered new circumstances and
information related to updated information on Rice's whales in the
action area (population abundance, mortality and sources of mortality,
distribution and occurrence) and any new data, analysis, or information
on the effects of geophysical survey activity on marine mammals and
relating to the effectiveness and practicability of measures to reduce
the risk associated with impacts of such survey activity. Based on the
review applying the 2017 supplementation standard and the 2020 ROD
criteria, NMFS determined for its 2024 final rule that supplementation
of the 2017 PEIS was not warranted.
In 2025, NOAA revised its NEPA procedures. As required by the 2025
procedures, environmental documents must be supplemented when (1) the
agency makes substantial changes to the proposed activity or decision
that are relevant to environmental concerns; or (2) the agency decides,
in its discretion, that there are substantial new circumstances or
information about the significance of the adverse effects that bear on
the proposed activity or decision or its effects. Under this standard,
NMFS has again considered whether there are any substantial new
circumstances or information that bear on this proposed action or its
impacts. For NMFS' consideration of new circumstances and information,
NMFS has consulted any new scientific information available since
issuance of the 2024 final rule. Again, NMFS has not made any changes
to the proposed action relevant to environmental concerns, and has made
no changes to the regulations. Based on the current review, NMFS has
again determined preliminarily that supplementation of the 2017 PEIS is
not warranted.
Summary of the Proposed Action
This proposed rule provides analysis of the same activities and
activity levels considered for the 2024 final rule, and utilizes the
same modeling methodology described in the 2024 final rule. There are
no changes to the nature or level of the specified activities within or
across years or to the geographic scope of the activity. Based on our
preliminary assessment of the specified activity in light of the take
estimates, which remain unchanged, we have determined that the
specified activity will have a negligible impact on the affected
species or stocks of marine mammals.\2\ Additionally, the regulations
at 50 CFR 217.180 satisfy the MMPA requirement to prescribe the means
of effecting the least practicable adverse impact on the affected
species or stocks and their habitat and contain monitoring and
reporting requirements pertaining to the taking. Therefore, as
requested, we propose to reimplement those regulations.
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\2\ There are no relevant subsistence uses implicated by this
action. Therefore the taking from the specified activity will not
have an unmitigable adverse impact on the availability of the
species for taking for relevant subsistence uses. See 16 U.S.C.
1371(a)(5)(A).
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Description of the Specified Activity
Overview
The specified activity for this proposed action as requested by the
NMFS' Policy petition is unchanged from the specified activity
considered for the 2021 and 2024 rules, consisting of geophysical
surveys conducted for a variety of reasons. Actual total amounts of
effort (including by survey type and
[[Page 9017]]
location) are not known in advance of receiving LOA requests, but take
in excess of what is analyzed in this rule would not be authorized.
Applicants seeking authorization for take of marine mammals incidental
to survey activities outside the geographic scope of the rule (i.e.,
within the former Gulf of Mexico Energy Security Act (GOMESA) (Sec.
104, Pub. L. 109-432) \3\ moratorium area) would need to pursue a
separate MMPA incidental take authorization (see figure 1).
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\3\ The Congressional moratorium in GOMESA was in place until
June 30, 2022. On September 8, 2020, the President withdrew, under
section 12 of the Outer Continental Shelf Lands Act, the same area
covered by the prior GOMESA moratorium from disposition by leasing
for 10 years, beginning on July 1, 2022, and ending on June 30,
2032.
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EnerGeo's 2025 ITR petition suggests that the existing level of
effort estimates, by survey type and location, are a reasonable
representation of the activities expected to occur under our proposed
ITR reimplementation rule (which EnerGeo supports). That petition,
available online at: <a href="https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america">https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america</a>, carries forward the same survey types and similar
estimated annual levels of effort by survey type and location as
specified over a 10-year period in BOEM's 2016 petition (as adjusted in
2020 by BOEM to account for removal from consideration of the area then
under a Congressional leasing moratorium under GOMESA). The most
notable difference is EnerGeo's estimate that approximately 40 percent
of forecast survey effort may be accomplished using less
environmentally impactful alternative sources to airgun arrays (e.g.,
tuned pulse or dual barbell sources; additional descriptions of these
source types may be found in Federal Register notices of LOA issuance
under the existing ITR, e.g., 86 FR 37309, July 15, 2021; 87 FR 55790,
September 12, 2022; 88 FR 72739, October 23, 2023). NMFS will address
these changes to survey effort in a future rulemaking on EnerGeo's
petition. For the current rulemaking, we have determined the specified
activity that is the subject of this proposed rule is a reasonable
projection on which to proceed.
Geophysical surveys are conducted to obtain information on marine
seabed and subsurface geology for a variety of reasons, including to
obtain data for hydrocarbon and mineral exploration and production; aid
in siting of oil and gas structures, facilities, and pipelines;
identify possible seafloor or shallow depth geologic hazards; and
locate potential archaeological resources and benthic habitats that
should be avoided.
Deep penetration seismic surveys using airgun arrays as an acoustic
source (sound sources are described in the Detailed Description of
Activities section) are a primary method of obtaining geophysical data
used to characterize subsurface structure. These surveys are designed
to illuminate deeper subsurface structures and formations. A deep
penetration survey uses an acoustic source suited to provide data on
geological formations that may be thousands of meters (m) beneath the
seafloor, as compared with a shallow penetration or high resolution
geophysical (HRG) survey that may be intended to evaluate shallow
subsurface formations or the seafloor itself (e.g., for hazards).
Deep penetration surveys may be two-dimensional (2D) or three-
dimensional (3D), and there are a variety of survey methodologies
designed to provide the specific data of interest. 2D surveys are
designed to acquire data over large areas (thousands of square miles)
in order to screen for potential hydrocarbon prospectivity, and provide
a cross-sectional image of the structure. In contrast, 3D surveys may
use similar acoustic sources but are designed to cover smaller areas
with greater resolution (e.g., with closer survey line spacing),
providing a volumetric image of underlying geological structures.
Repeated 3D surveys are referred to as four-dimensional (4D), or time-
lapse, surveys that assess the depletion of a reservoir.
Shallow penetration and high-resolution surveys are designed to
highlight seabed and near-surface potential obstructions, archaeology,
and geohazards that may have safety implications during rig
installation or well and development facility siting. Shallow
penetration surveys may use a small airgun array, single airgun, or
similar sources, while high-resolution surveys (which are limited to
imaging the seafloor itself) may use a variety of sources, such as sub-
bottom profilers, single or multibeam echosounders, or side-scan
sonars.
Dates and Duration
The specified activities may occur at any time during the 5-year
period of validity of the proposed regulations. Actual dates and
duration of individual surveys are not known. Although the proposed
period of validity is for 5 years, we reiterate the requested
reimplementation of regulations would continue only until superseded by
a new ITR promulgated on the basis of the separate EnerGeo request.
Specified Geographical Region
Generally speaking, projected survey activity may occur within U.S.
waters within the GOA, outside of the former GOMESA moratorium area.
The specified geographical region (with modeling zones and depicting
the area withdrawn from leasing consideration) is depicted in figure 1.
[[Page 9018]]
[GRAPHIC] [TIFF OMITTED] TP24FE26.010
Figure 1--Specified Geographical Region
Detailed Description of Activities
An airgun is a device used to emit acoustic energy pulses into the
seafloor, and generally consists of a steel cylinder that is charged
with high-pressure air. There are different types of airguns;
differences between types of airguns are generally in the mechanical
parts that release the pressurized air, and the bubble and acoustic
energy released are effectively the same. Airguns are typically
operated at a firing pressure of 2,000 pounds per square inch (psi).
Release of the compressed air into the water column generates a signal
that reflects (or refracts) off the seafloor and/or subsurface layers
having acoustic impedance contrast. Individual airguns are available in
different volumetric sizes and, for deep penetration seismic surveys,
are towed in arrays (i.e., a certain number of airguns of varying sizes
in a certain arrangement) designed according to a given company's
method of data acquisition, seismic target, and data processing
capabilities.
Airgun arrays are typically configured in subarrays of 6-12 airguns
each. The airgun array is typically towed at a speed of approximately
4.5 to 5 knots (kn). The output of an airgun array is directly
proportional to airgun firing pressure or to the number of airguns, and
is expressed as the cube root of the total volume of the array.
Airguns are considered to be low-frequency acoustic sources,
producing sound with energy in a frequency range from less than 10
hertz (Hz) to 2 kHz (though there may be energy at higher frequencies),
with most energy radiated at frequencies below 500 Hz. Frequencies of
interest to industry are below approximately 100 Hz. The amplitude of
the acoustic wave emitted from the source is equal in all directions
(i.e., omnidirectional) for a single airgun, but airgun arrays do
possess some directionality due to phase delays between guns in
different directions. Airgun arrays are typically tuned to maximize
functionality for data acquisition purposes, meaning that sound
transmitted in horizontal directions and at higher frequencies is
minimized to the extent possible.
When fired, a brief (~0.1 second) pulse of sound is emitted by all
airguns in an array nearly simultaneously, in order to increase the
amplitude of the overall source pressure signal. The combined signal
amplitude and directivity is dependent on the number and sizes of
individual airguns and their geometric positions within the array. The
airguns are silent during the intervening periods, with the array
typically fired on a fixed distance (or shot point) interval. The
intervals are optimized for water depth and the distance of important
geological features below seafloor, but a typical interval in
relatively deep water might be approximately every 10-20 seconds (or
25-50 m, depending on vessel speed). The return signal is recorded by a
listening device, and later analyzed with computer interpretation and
mapping systems used to depict the subsurface. There must be enough
time between shots for the sound signals to propagate down to and
reflect from the feature of interest, and then to propagate upward to
be received on hydrophones or geophones. Reverberation of sound from
previous shots must also be given time to dissipate. The receiving
hydrophones can be towed behind or in front of the airgun array (may be
towed from the source vessel or from a separate receiver vessel), or
ocean bottom nodes (OBN) containing geophone receivers can be deployed
on the seabed. Receivers may be displaced several kilometers (km)
horizontally away from the source, so horizontal propagation time is
also considered in setting the interval between shots.
[[Page 9019]]
Sound levels for airgun arrays are typically modeled or measured at
some distance from the source and a nominal source level then back-
calculated. Because these arrays constitute a distributed acoustic
source rather than a single point source (i.e., the ``source'' is
actually comprised of multiple sources with some predetermined spatial
arrangement), the highest sound levels measurable at any location in
the water will be less than the nominal source level. At sufficient
distance--in the far field--the array may be perceived as a single
point source but individual sources, each with less intensity than that
of the whole, may be discerned at closer distances (Caldwell and
Dragoset (2000) define the far field as greater than 250 m; though this
distance is dependent on the array dimensions). Therefore, back-
calculated source levels are not typically considered to be accurate
indicators of the true maximum amplitude of the output in the far
field, which is what is typically of concern in assessing potential
impacts to marine mammals. In addition, the effective source level for
sound propagating in near-horizontal directions (i.e., directions
likely to impact most marine mammals in the vicinity of an array) is
likely to be substantially lower (e.g., 15-24 decibels (dB); Caldwell
and Dragoset, 2000) than the nominal source level applicable to
downward propagation because of the directional nature of the sound
from the airgun array. The horizontal propagation of sound is reduced
by noise cancellation effects created when sound from neighboring
airguns on the same horizontal plane partially cancel each other out.
Alternative sources to conventional airgun arrays are increasingly
used in deep penetration surveys. These sources, such as the tuned
pulse source (TPS) or dual barbell sources, are expected to present
lower potential for impacts to marine mammals but they operate on the
same basic principles as traditional airgun sources in that they use
compressed air to create a bubble in the water column which then goes
through a series of collapses and expansions creating primarily low-
frequency sounds. Because of the increasing potential for use of these
sources, we describe them briefly here to show that they (and their
potential impacts) fall within the scope of this proposed rule.
However, the acoustic exposure modeling supporting this rule, and the
estimated marine mammal take numbers evaluated herein, assume that
airgun sources are used during all projected survey effort.
The difference between the TPS and airgun sources is that the TPS
releases a larger volume of air, but at lower pressure. This creates a
larger bubble resulting in more of the energy being concentrated in
low-frequencies. The release of the air is also ``tuned'' so that the
primary signal has an extended rise time and lower peak pressure level
than that of a traditional airgun array source. Field data confirm that
the TPS produces more sound at lower frequencies (approximately 2-4 Hz)
compared to an airgun source, while producing much less sound (lower
decibel levels) at frequencies above 4 Hz, meaning that the source
produces significantly reduced energy at frequencies used by marine
mammals for hearing and communication. This means that even for species
in the low-frequency hearing group (mysticete whales) most affected by
seismic survey sounds, the TPS is expected to have less impact than a
traditional airgun array in terms of overlap with frequencies the
species use. Potential impacts on high- and very high-frequency hearing
groups will be reduced even more.
Dual barbell sources consist of one physical element with two large
chambers, similarly creating a larger bubble resulting in more of the
energy being concentrated in low frequencies. In addition to
concentrating energy at lower frequencies, these sources are expected
to produce lower overall sound levels than conventional airgun sources.
The number of airguns in an array is highly influential on overall
sound energy output, because the output increases approximately
linearly with the number of airgun elements. In this case, because the
same air volume is used to operate two very large guns, rather than
tens of smaller guns, the array produces lower sound levels than a
conventional array of equivalent total volume.
Survey protocols generally involve a predetermined set of survey,
or track, lines. The seismic acquisition vessel(s) (source vessel) will
travel down a linear track for some distance until a line of data is
acquired, then turn and acquire data on a different track. In some
cases, data is acquired as the source vessel(s) turns continuously
rather than moving on a linear track (i.e., coil surveys). The spacing
between track lines and the length of track lines can vary greatly,
depending on the objectives of a survey. Spacing and length of tracks
varies by survey.
The general activities described here could occur pre- or post-
leasing and/or on- or off-lease. Pre-lease surveys are more likely to
involve larger-scale activity designed to explore or evaluate geologic
formations. Post-lease activities may also include deep penetration
surveys, but would be expected to be smaller in spatial and temporal
scale as they are associated with specific leased blocks. Shallow
penetration and HRG surveys are more likely to be associated with
specific leased blocks and/or facilities, with HRG surveys used along
pipeline routes and to search for archaeological resources and/or
benthic communities.
2D and 3D Surveys (Deep Penetration Surveys)--Deep penetration
surveys may use an airgun array(s) as the acoustic source and may be 2D
or 3D (with repeated 3D surveys termed 4D). Surveys may be designed as
either multi-source (i.e., multiple arrays towed by one or more source
vessel(s)) or single source.
We described previously the basic differences between 2D and 3D
surveys. A typical 2D survey deploys a single array, whereas a 3D
vessel may deploy multiple source arrays. Among 3D surveys in
particular, there are a variety of survey designs employed to acquire
the specific data of interest. Conventional, single-vessel 3D surveys
are referred to as narrow azimuth (NAZ) surveys. Survey techniques
using multiple source vessels, often referred to as wide-azimuth (WAZ)
surveys, help to provide better data quality than that achievable using
traditional NAZ surveys, including better illumination, higher signal-
to-noise ratios, and higher resolution. This is useful in imaging
subsurface areas containing complex geologic structures, particularly
those beneath salt bodies with irregular geometries.
In summary, 3D survey design involves a vessel with one or more
acoustic sources covering an area of interest with relatively tight
spatial configuration. In order to provide richer, more useful data,
particularly in areas with more difficult geology, survey designs
become more complicated with additional source and/or receiver vessels
operating in potentially increasingly complicated choreographies. The
time required to complete one pass of a trackline for a single NAZ
vessel and the time required for one pass by a multi-vessel entourage
conducting a WAZ survey will be essentially the same. Turn times will
be somewhat longer during multi-vessel surveys to ensure that all
vessels are properly aligned prior to beginning the next trackline.
Coil surveys, described previously, reduce the total survey time due to
elimination of the trackline-turn methodology. Note that, while coil
surveys occur infrequently in the GOA, the coil survey simulation is
applicable to a variety of survey types that are
[[Page 9020]]
conducted within smaller areas than 2D and 3D survey types.
Borehole Seismic Surveys--The placement of seismic sensors in a
drilled well or borehole is another way data can be acquired. These
surveys, typically referred to as vertical seismic profiles (VSP),
provide information about geologic structure, lithology, and fluids
that is intermediate between that obtained from sea surface surveys and
well-log scale information (well logging is the process of recording
various physical, chemical, electrical, or other properties of the
rock/fluid mixtures penetrated by drilling a borehole). VSP surveying
is conducted by placing receivers at many (50-200) depths in a wellbore
and recording both direct-arriving and reflection energy from an
acoustic source. The acoustic source usually is a single airgun or
small airgun array hung from a platform or deployed from a source
vessel. The airguns used for VSPs may be the same or similar to those
used for 2D and 3D surveys; however, the number of airguns and the
total volume of an array used are typically less. Some VSP surveys take
less than a day, and most are completed in a few days. Borehole seismic
surveys include 2D VSPs, 3D VSPs, and other types of surveys.
Shallow Penetration/HRG Surveys--These surveys are conducted to
provide data informing initial site evaluation, drilling rig
emplacement, and platform or pipeline design and emplacement.
Identification of geohazards (e.g., gas hydrates, buried channels) is
necessary to avoid drilling and facilities emplacement problems, and
operators are required to identify and avoid archaeological resources
and certain benthic communities. In most cases, conventional 2D and 3D
deep penetration surveys do not have the correct resolution to provide
the required information. Shallow penetration surveys typically use
small airgun arrays, paired or single airguns, or non-airgun impulsive
sources such as sparkers or boomers. HRG surveys generally use
electromechanical sources, including sources that are not likely to
cause incidental take of marine mammals, such as sub bottom profilers,
echosounders, and side-scan sonars (Ruppel et al., 2022).
Representative Sound Sources
Because the specifics of acoustic sources to be used cannot be
known in advance of receiving LOA requests from industry operators, it
is necessary to define representative acoustic source parameters, as
well as representative survey patterns. The supporting modeling for the
2021 ITR considered two specific airgun array sizes/configurations
(4,130 and 8,000 in\3\ arrays) as well as a single, 90-in\3\ airgun.
For the 2024 rule, modeling of a third representative airgun array size
(5,110-in\3\) was also specifically considered. In its petition for the
2021 ITR, BOEM determined realistic representative proxy sound sources
and survey patterns. We note that EnerGeo's 2025 petition for a new ITR
carries forward these assumed proxies regarding survey patterns, as
well as the 5,110-in\3\ array modeled for the 2024 rule, as
representative of ongoing industry survey activities in the GOA.
Acoustic exposure modeling for the 8,000-in\3\ airgun array and 90-
in\3\ single airgun, which provided support for the 2021 rule, was
described in detail in ``Acoustic Propagation and Marine Mammal
Exposure Modeling of Geological and Geophysical Sources in the Gulf of
Mexico'' and ``Addendum to Acoustic Propagation and Marine Mammal
Exposure Modeling of Geological and Geophysical Sources in the Gulf of
Mexico'' (Zeddies et al., 2015, 2017a). Additional information,
including evaluation of the 4,130-in\3\ airgun array, was provided in
``Gulf of Mexico Acoustic Exposure Model Variable Analysis'' (Zeddies
et al., 2017b).
Modeling of the more representative 5,110-in\3\ airgun array for
NMFS' 2024 rule (in view of LOA applications received to date under the
current ITR) was described in a 2022 memorandum (Weirathmueller et al.,
2022). These reports provide full detail regarding the modeled acoustic
sources and survey types and are available online at:
<a href="http://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america">www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-america</a>.
Representative sources for the modeling include the three different
airgun arrays, the single airgun, and an acoustic source package
including a sub-bottom profiler in combination with multibeam
echosounder and side-scan sonar. Two major survey types were
considered: large-area seismic (including 2D, 3D NAZ, 3D WAZ, and coil
surveys) and small-area, high-resolution geotechnical (including single
airgun surveys and HRG surveys using the aforementioned package of
sources). The nominal airgun sources used for analysis of this proposed
rule include a small single airgun (90-in\3\ airgun) and a large airgun
array (8,000-in\3\). While the 5,110-in\3\ airgun array is considered
most representative of the airgun sources that are likely to be used
during deep penetration surveys during the period of effectiveness of
this proposed ITR, the 8,000-in\3\ airgun array results in larger take
numbers for most species for which acoustic exposures were modeled, and
therefore provide the basis for the analysis herein, thus allowing the
flexibility for applicants to use larger sources when survey objectives
dictate. The modeling for the alternative 4,130- and 5,110-in\3\ arrays
provides more realistic estimates of take for use in survey-specific
LOAs, depending on the actual acoustic sources planned for use (see
Letters of Authorization section). We note that while high-resolution
geophysical sources were included for consideration in the 2021 final
rule to allow for take authorization if necessary, these types of
sources would not typically be expected to cause the incidental take of
marine mammals (Ruppel et al., 2022).
New technologies and/or uses of existing technologies may come into
practice during the period of validity of these proposed regulations.
As under the 2021 and 2024 final rules, NMFS will evaluate any such
developments on a case-specific basis to determine whether expected
impacts on marine mammals are consistent with those described or
referenced in this document and, therefore, whether any anticipated
take incidental to use of those new technologies or practices may
appropriately be authorized under the existing regulatory framework.
See Letters of Authorization for additional information.
Estimated Levels of Effort
Actual total amounts of effort by survey type and location cannot
be known in advance of receiving LOA requests from survey operators.
Therefore, BOEM's 2017 PEIS provided projections of survey level of
effort for the different survey types for a 10-year period (and BOEM
refined those projections following removal of the GOMESA area from the
scope of activity in 2020). As noted above, these estimated levels of
effort remain representative of expected survey activity on an ongoing
basis and, therefore, are carried forward unchanged. Table 1 provides
those effort projections for the next 5-year period.
In order to provide some spatial resolution to the projections of
survey effort and to provide reasonably similar areas within which
acoustic modeling might be conducted, the geographic region was divided
into seven zones, largely on the basis of water depth, seabed slope,
and defined BOEM planning area boundaries. Shelf regions typically
extend from shore to approximately 100-200 m water depths
[[Page 9021]]
where bathymetric relief is gradual. The slope starts where the seabed
relief is steeper and extends into deeper water. In the GOA water
deepens from 100-200 m to 1,500-2,500 m over as little as a 50 km
horizontal distance. As the slope ends, water depths become more
consistent, though depths can vary from 2,000 to 3,300 m. Three primary
bathymetric areas were defined as shelf (0-200 m water depth), slope
(200-2,000 m), and deep (>2,000 m).
Available information regarding cetacean density in the GOA shows
that, in addition to water depth, animal distribution tends to vary
from east to west in the GOA and appears correlated with the width of
shelf and slope areas from east to west. The western region is
characterized by a relatively narrow shelf and moderate-width slope.
The central region has a moderate-width shelf and moderate-width slope,
and the eastern region has a wide shelf and a very narrow slope.
Therefore, BOEM's western, central, and eastern planning area divisions
provide appropriate longitudinal separations for the shelf and slope
areas. Due to relative consistency in both physical properties and
predicted animal distribution, the deep area was not subdivided. As
shown in figure 1, zones 1-3 represent the shelf area (from east to
west), zones 4-6 represent the slope area (from east to west), and zone
7 is the deep area. Removal of the GOMESA moratorium area from the
scope of activity entirely eliminated zone 1 from consideration, and
reduced zone 4 by approximately 98 percent and zone 7 by 33 percent.
Smaller portions of zones 2 and 5 were also removed from consideration
(figure 1).
[[Page 9022]]
[GRAPHIC] [TIFF OMITTED] TP24FE26.011
[[Page 9023]]
Description of Marine Mammals in the Area of the Specified Activities
Table 2 lists all species with expected potential for occurrence in
the GOA and summarizes information related to the population or stock,
including potential biological removal (PBR). PBR, defined by the MMPA
as the maximum number of animals, not including natural mortalities,
that may be removed from a marine mammal stock while allowing that
stock to reach or maintain its optimum sustainable population, is
considered in concert with known sources of ongoing anthropogenic
mortality (as described in NMFS' stock assessment reports (SAR)). For
status of species, we provide information regarding U.S. regulatory
status under the MMPA and Endangered Species Act (ESA).
In some cases, species are treated as guilds. In general ecological
terms, a guild is a group of species that have similar requirements and
play a similar role within a community. However, for purposes of stock
assessment or density modeling, certain species may be treated together
as a guild because they are difficult to distinguish visually and many
observations are ambiguous. For example, NMFS' GOA SARs assess stocks
of Mesoplodon spp. and Kogia spp. as guilds. Following this approach,
we consider beaked whales and Kogia spp. as guilds. In this rule,
reference to ``beaked whales'' includes the goose-beaked whale \4\ and
Blainville's and Gervais' beaked whales, and reference to ``Kogia
spp.'' includes both the dwarf and pygmy sperm whale.
---------------------------------------------------------------------------
\4\ Note that this species is referred to in NMFS' SARs as the
``Cuvier's beaked whale.''
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The use of guilds herein follows the best available density
information (i.e., Garrison et al., 2023). The density models treat
beaked whales and Kogia spp. as guilds and consolidate four species
into an undifferentiated blackfish guild. These species include the
melon-headed whale, false killer whale, pygmy killer whale, and killer
whale. The model authors determined that, for this group of species,
there were insufficient sightings of any individual species to generate
a species-specific model (Garrison et al., 2023). Therefore, reference
to blackfish hereafter includes the melon-headed whale, false killer
whale, pygmy killer whale, and killer whale.\5\ Twenty-one species
(with 24 managed stocks) have the potential to co-occur with the
prospective survey activities. All managed stocks in this region are
assessed in NMFS' U.S. Atlantic SARs. All values presented in table 2
are the most recent available. For more information, please see
information presented in the SARs (available online at: <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports</a>).
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\5\ This rule provides a single take estimate for the melon-
headed whale, false killer whale, pygmy killer whale, and killer
whale grouped together as the ``blackfish.'' This approach reflects
the best available scientific information (Garrison et al., 2023).
These species are encountered only occasionally during any given
vessel survey, and these relatively infrequent encounters make it
difficult to fit species-specific detection and habitat models. For
each of these models, the authors detail analyses and decisions
relevant to model development, as well as notes of caution regarding
use of the models given the associated uncertainty resulting from
development of a model based on few sightings. The Garrison et al.
(2023) models are based on survey data from 2003 to 2019. Notably,
surveys conducted after 2009 were conducted in ``passing'' mode,
where the ship did not deviate from the trackline to approach and
verify species identifications for detected marine mammal groups,
resulting in an increase in observed marine mammal groups that could
not be identified to species. As a result of these factors, the
model authors determined it appropriate to develop a single spatial
model based on sightings of unidentified blackfish, in addition to
the relatively few sightings where species identification could be
confirmed.
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[[Page 9024]]
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[[Page 9025]]
[GRAPHIC] [TIFF OMITTED] TP24FE26.013
[[Page 9026]]
[GRAPHIC] [TIFF OMITTED] TP24FE26.014
In table 2 above, we report two sets of abundance estimates: those
from NMFS' SARs and those predicted by habitat-based cetacean density
models. Please see footnote 3 of table 2 for more detail. NMFS' SAR
estimates are typically generated from the most recent shipboard and/or
aerial surveys conducted. GOA oceanography is dynamic, and the spatial
scale of the GOA is small relative to the ability of most cetacean
species to travel. U.S. waters only comprise about 40 percent of the
entire GOA, and 65 percent of GOA oceanic waters are south of the U.S.
EEZ. Studies based on abundance and distribution surveys restricted to
U.S. waters are unable to detect temporal shifts in distribution beyond
U.S. waters that might account for any changes in abundance within U.S.
waters. NMFS' SAR estimates also in some cases do not incorporate
correction for detection bias. Therefore, for cryptic or long-diving
species (e.g., beaked whales, Kogia spp., sperm whales), they should
generally be considered underestimates (see footnotes 5 and 7 of table
2).
The model-based abundance estimates represent the output of
predictive models derived from multi-year observations and associated
environmental parameters and which incorporate corrections for
detection bias (the same models and data from which the density
estimates are derived). Incorporating more data over multiple years of
observation can yield different results in either direction, as the
result is not as readily influenced by fine-scale shifts in species
habitat preferences or by the absence of a species in the study area
during a given year. NMFS' SAR abundance estimates show substantial
year-to-year variability in some cases. Incorporation of correction for
detection bias should systematically result in greater abundance
predictions. For these reasons, the model-based estimates are generally
more realistic and, for the purposes of assessing estimated exposures
relative to abundance--used in this case to understand the scale of the
predicted takes compared to the population--NMFS generally believes
that the model-based abundance predictions are the best available
information and most appropriate because they were used to generate the
exposure estimates and therefore provide the most relevant comparison.
As part of our evaluation of the environmental baseline, which is
considered as part of the negligible impact analysis, we consider any
known areas of importance as marine mammal habitat. We also consider
other relevant information, such as unusual mortality events (UME) and
the 2010 Deepwater Horizon oil spill.
Habitat--Important habitat areas may include areas of known
importance for reproduction, feeding, or migration, or areas where
small and resident populations are known to occur. They may have
independent regulatory status such as designated critical habitat for
ESA-listed species (as defined by section 3 of the ESA) or be
identified through other means (e.g., recognized Biologically Important
Areas (BIA)).
No critical habitat has yet been designated for the Rice's whale,
though a proposed rule to do so was published (88 FR 47453, July 24,
2023). The proposal references the same supporting information
discussed herein in suggesting that GOA continental slope waters
between 100 and 400 m water depth be designated as critical habitat. In
addition, a BIA has been recognized since 2015 (LaBrecque et al.,
2015).
Our knowledge of Rice's whale distribution is based on a
combination of historic and contemporary sightings, passive acoustic
detections, and spatial modeling. The Rice's whale was historically
typically observed only within a narrowly circumscribed area within the
eastern GOA, leading to the area being described as a year-round BIA by
LaBrecque et al. (2015). In sightings data available to support that
description, whales were observed only between approximately the 100-
and 300-m isobaths in the eastern GOA from the head of the De Soto
Canyon (south of Pensacola, Florida) to northwest of Tampa Bay, Florida
(Maze-Foley and Mullin, 2006; Waring et al., 2016; Rosel and Wilcox,
2014; Rosel et al., 2016). NOAA's ESA status review of the species
(formerly the GOM Bryde's whale) (Rosel et al., 2016) expanded the 2015
BIA description by stating that, due to the depth of some sightings,
the area is appropriately defined to the 400-m isobath and westward to
Mobile Bay, Alabama, in order to provide some buffer around the deeper
sightings and to include all sightings in the northeastern GOA. This
area is now considered to mark a core habitat area for the species,
versus its entire range within the GOA (as described in the 2023
proposed critical habitat designation). The core habitat area contains
the highest known densities of Rice's whale and has defined the
movements of previously tagged Rice's whales.
More recent scientific data, including visual and acoustic
detections, now indicate that Rice's whales occupy waters along the
continental shelf and slope and adjacent waters throughout the U.S.
GOA, and in particular, waters between 100 and 400 m deep. The widest
swath of habitat occurs in the species' aforementioned core habitat
area in the northeastern GOA, south and west of Alabama and Florida.
However, a contiguous strip of habitat also extends south of the core
habitat area toward the Florida Keys, and westward along the
continental shelf and slope offshore of Mississippi, Louisiana, and
Texas (Garrison et al., 2023). Passive acoustic monitoring (PAM)
recordings have been especially valuable for confirming the species'
year-round presence in the central and western GOA (Soldevilla et al.,
2022, 2024), helping to offset the limited visual
[[Page 9027]]
survey effort in those locations. The shallowest and deepest waters
where Rice's whales have been confirmed visually to date are 117 m and
408 m, respectively, but Rice's whales may use waters that are deeper
or shallower than those values at times, and unconfirmed sightings from
protected species observers (PSOs) have occurred at a wider range of
locations and depths (Barkaszi and Kelley, 2018, 2024).
Current understanding regarding Rice's whale occurrence in the
central and western GOA is largely based on passive acoustic detections
(Soldevilla et al., 2022; 2024). As background, a NOAA survey reported
observation of a Rice's whale in the western GOA in 2017 (Garrison et
al., 2020). Genetic analysis of a skin biopsy that was collected from
the whale confirmed it to be a Rice's whale. There had not previously
been a genetically verified sighting of a Rice's whale in the western
GOA, and given the importance of this observation, additional survey
effort was conducted in an attempt to increase effort in the area.
However, no additional sightings were recorded (note that there were
two sightings of unidentified large baleen whales in 1992 in the
western GOA, recorded as Balaenoptera sp. or Bryde's/sei whale (Rosel
et al., 2021)). Subsequently, during 2023 survey effort in the western
GOA, a sighting of what has been described as a group of two probable
Rice's whales was recorded (<a href="https://www.fisheries.noaa.gov/science-blog/successful-final-leg-gulf-america-marine-mammal-and-seabird-vessel-survey">https://www.fisheries.noaa.gov/science-blog/successful-final-leg-gulf-america-marine-mammal-and-seabird-vessel-survey</a>). In addition, there are occasional sightings by PSOs of
baleen whales in the GOA that may be Rice's whales. Rosel et al. (2021)
reviewed 13 whale sightings reported by PSOs in the GOA from 2010-2014
that were recorded as baleen whales. No sightings were close enough for
the PSOs to see the diagnostic three lateral ridges on the whales'
rostrums required to confirm them as Rice's whales. Rosel et al. ruled
out five of the sightings as more likely being sperm whales based on
water depth and descriptions of the whales' behavior. The remaining
eight sightings may have been Rice's whales based on one or more lines
of evidence (i.e., photographs, behavioral description, and/or water
depth consistent with Rice's whales). Of these sightings, three
occurred in the northeastern GOA core habitat area, while the remaining
five occurred along the GOA shelf break south of Louisiana.
The acoustic detections provide evidence of year-round Rice's whale
presence outside of the northeastern GOA core habitat area. Soldevilla
et al. (2022) deployed autonomous passive acoustic recorders at 5 sites
along the GOA shelf break in predicted Rice's whale habitat (Roberts et
al., 2016) for 1 year (2016-2017) to (1) determine if Rice's whales
occur in waters beyond the northeastern GOA and, if so, (2) evaluate
their seasonal occurrence and site fidelity at the five sites. Over the
course of the 1-year study, sporadic, year-round recordings of calls
assessed as belonging to Rice's whales were made south of Louisiana
within approximately the same depth range (200-400 m), indicating that
some Rice's whales occurred regularly in waters beyond the northeastern
GOA core habitat area during the study period. Based on the detection
range of the sonobuoys and acoustic monitors used in the study, actual
occurrence could be in water depths up to 500 m (M. Soldevilla, pers.
comm.), though the deepest confirmed Rice's whale sighting is at 408 m
water depth. Data were successfully collected at four of the five
sites; of those four sites, Rice's whale calls were detected at three.
Detection of calls ranged from 1 to 16 percent of total days at the
three sites. Calls were present in all seasons at two sites, with no
obvious seasonality. It remains unknown whether animals are moving
between the northwestern and the northeastern GOA or whether these
represent different groups of animals (Soldevilla et al., 2022).
A subsequent follow-up study (Soldevilla et al., 2024) similarly
involved deployment of autonomous passive acoustic recorders for
approximately 1 year (2019-2020) at two shelf break sites, including
one central GOA site included in the previous study and one new site
further west, offshore Corpus Christi, Texas (recorders were also
deployed at a site in Mexican waters for almost 2 years (2020-2022)).
The study objectives were to (1) determine if Rice's whales occur in
Mexican waters and to (2) evaluate how frequently they occur at all
three sites. Rice's whale calls were detected on 33 and 25 percent of
days at the central and western GOA sites, respectively, with calls
recorded throughout the year, though no distinct seasonality was
detected. These findings reflect an increase in the frequency and
number of detections at the central GOA site compared with the 2016-
2017 study. The authors note that these findings highlight persistence
of Rice's whale detections at this site over multiple years, as well as
variability among years (Soldevilla et al., 2024). Rice's whale calls
were also detected at the site in Mexican waters (see Soldevilla et al.
(2024) for additional discussion). The authors also describe
differences in Rice's whale call types recorded in the eastern GOA
compared with those recorded in the western GOA, suggesting that whales
may indeed have a broader distribution than the northeastern GOA
(Soldevilla et al., 2024).
The rate of call detections throughout the year is considerably
higher in the eastern GOA than at the central/western GOA site where
calls were most commonly detected, with at least 8.3 calls/hour among
four eastern GOA sites within the core habitat area over 110 deployment
days (Rice et al., 2014) compared to 0.27 calls/hour over the 299-day
deployment at the central/western GOA site where calls were detected
most frequently in the 2016-2017 study. Approximately 2,000 total calls
were detected at the central/western GOA site over 10 months in 2016-
2017, compared to more than 66,000 total detections at the eastern GOA
deployment site over 11 months (i.e., approximately 30 times more calls
were detected at the eastern GOA site; Soldevilla et al., 2022).
Although ambient noise conditions were higher at the central/western
GOA site, thus influencing maximum detection range, accounting for this
difference in conditions would be expected to result in only 4-8 times
as many call detections if all other factors (including presence and
number of whales) were consistent (versus 30 times as many detections).
Overall, Soldevilla et al. (2022) assessed that there seem to be fewer
whales or more sparsely spaced whales in the central/western GOA
compared to the eastern GOA, with calls present on fewer days, lower
call detection rates, and far fewer call detections in the central/
western GOA.
The passive acoustic data discussed above provide evidence that
waters 100-400 m deep in the central and western GOA are Rice's whale
habitat and are being used by Rice's whales in all seasons. This could
imply that the population size is larger than previously estimated, or
it could indicate that some individual Rice's whales have a broader
distribution in the GOA than previously understood (Soldevilla et al.,
2024). Either way, the acoustic findings, combined with the low numbers
of visual sightings in the central and western GOA, suggest that
density and abundance of Rice's whales in the central and western GOA
are less than in the core habitat in the northeastern GOA. Therefore,
while we expect that some individual Rice's whales occur outside the
core habitat area and/or that whales from the northeastern GOA core
[[Page 9028]]
habitat area occasionally travel outside the area, the currently
available data are not sufficient to make inferences about Rice's whale
density and abundance in the central and western GOA. More research is
needed to answer key questions about Rice's whale density, abundance,
habitat use, demography, and stock structure in the central and western
GOA.
While these acoustic data and few confirmed sightings support the
presence of Rice's whales in western and central GOA waters (within the
100-400 m water depth), the information is consistent with the
predictions of Rice's whale density modeling, on which basis NMFS has
anticipated and evaluated the potential for and effects of takes of
Rice's whale in western and central GOA waters. Little is known about
the number of whales that may be present, the nature of these
individuals' use of the habitat, or the timing, duration, or frequency
of occurrence for individual whales. Conversely, the importance of
northeastern GOA waters to Rice's whale recovery is clear (Rosel et
al., 2016). A comparison of acoustic and sightings data from the
central/western and eastern GOA, even acknowledging the limitations of
those data, suggests that occurrence of whales in the northeastern GOA
core habitat is significantly greater and that the area provides the
habitat of greatest importance to the species.
Finally, we acknowledge the ``core distribution area'' described in
the 2024 final rule. Delineation of the core distribution area was an
effort by NMFS SEFSC (Rosel and Garrison, 2022) to more systematically
delimit the previously described core habitat area, including through
the addition of buffers around confirmed sightings and location data
from tagged whales to account for potential uncertainty in whale
locations and possible movements from those locations. However, the
result of this precautionary approach was that areas outside of Rice's
whale habitat (NMFS, 2023) were included in the core distribution area.
We discussed the relevance of this area in relation to our
understanding of Rice's whale habitat in detail in the 2024 final rule.
In summary, while the actual Rice's whale core habitat area (i.e., the
aforementioned area containing the majority of Rice's whale sightings,
containing the movements of previously tagged whales, and where the
volume and rate of acoustic detections is highest) is entirely outside
the geographic scope of the rule; 5 percent of the core distribution
area overlaps the scope of this rule. Within that small portion of the
core distribution area, 76 percent covers waters shallower than 100 m
(36 percent) or deeper than 400 m (40 percent), i.e., three-quarters of
the area covers waters considered outside of most suitable Rice's whale
habitat. Therefore, we have determined that the ``core distribution
area'' described by Rosel and Garrison (2022) has no relevance within
the geographic scope of this rule beyond consideration of Rice's whale
habitat (assumed to be within waters 100-400 m in depth) throughout the
geographic scope. We do not further discuss the core distribution area.
Deepwater Horizon Oil Spill--In 2010, the Macondo well blowout and
explosion aboard the Deepwater Horizon drilling rig (also known as the
Deepwater Horizon explosion, oil spill, and response; hereafter
referred to as the DWH oil spill) caused oil, natural gas, and other
substances to flow into the GOA for 87 days before the well was sealed.
Total oil discharge was estimated at 3.19 million barrels (134 million
gallons), resulting in the largest marine oil spill in history (DWH
NRDA Trustees, 2016). In addition, the response effort involved
extensive application of dispersants at the seafloor and at the
surface, and controlled burning of oil at the surface was also used
extensively as a response technique. The oil, dispersant, and burn
residue compounds continue to present ecological challenges in the
region. NMFS discussed the impacts of the DWH oil spill on marine
mammals in detail in its 2018 notice of proposed rulemaking (83 FR
29212; June 22, 2018), and we refer the reader to that document for
additional detail. The 2018 proposed rule provided detailed discussion
of the DWH oil spill. There is no new information regarding the DWH oil
spill. Estimates of annual mortality for many stocks over the period
2014-2018 include mortality attributed to the effects of the DWH oil
spill (see table 2) (Hayes et al., 2023), and these mortality estimates
are considered as part of the environmental baseline.
An Unusual Mortality Event (UME) affecting multiple cetacean
species in the northern GOA occurred from 2010 to 2014. Additional
information on the UME is available online at: <a href="https://www.fisheries.noaa.gov/national/marine-life-distress/2010-2014-cetacean-unusual-mortality-event-northern-gulf-mexico">https://www.fisheries.noaa.gov/national/marine-life-distress/2010-2014-cetacean-unusual-mortality-event-northern-gulf-mexico</a>. In summary, the
event included all cetaceans stranded during this time in Alabama,
Mississippi, and Louisiana and all cetaceans other than bottlenose
dolphins stranded in the Florida Panhandle (Franklin County through
Escambia County), with a total of 1,141 cetaceans stranded or reported
dead offshore. For reference, the same area experienced a normal
average of 75 strandings per year from 2002 to 2009 (Litz et al.,
2014). The majority of stranded animals were bottlenose dolphins,
though at least 10 additional species were reported as well. Since not
all cetaceans that die wash ashore where they may be found, the number
reported stranded is likely a fraction of the total number of cetaceans
that died during the UME. The UME investigation and the Deepwater
Horizon Natural Resource Damage Assessment determined that the DWH oil
spill was the most likely explanation of the persistent, elevated
stranding numbers in the northern GOA after the 2010 spill.
In summary, coastal and oceanic marine mammals were injured by
exposure to oil from the DWH spill. Nearly all of the stocks that
overlap with the oil spill footprint have demonstrable, quantifiable
injuries, and the remaining stocks (for which there is no quantifiable
injury) were also likely injured, though there is not currently enough
information to make a determination. Injuries included elevated
mortality rates, reduced reproduction, and disease. Due to these
effects, affected populations may require decades to recover absent
successful efforts at restoration (e.g., DWH NRDA Trustees, 2017). The
ability of the stocks to recover and the length of time required for
that recovery are tied to the carrying capacity of the habitat, and to
the degree of other population pressures. NMFS treats the effects of
the DWH oil spill as part of the baseline in considering the likely
resilience of these populations to the effects of the activities
considered in this proposed rule.
Marine Mammal Hearing
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). To reflect this, Southall et al.
(2007, 2019) recommended that marine mammals be divided into hearing
groups based on directly measured (behavioral or auditory evoked
potential techniques) or estimated hearing ranges
[[Page 9029]]
(behavioral response data, anatomical modeling, etc.). Generalized
hearing ranges were chosen based on the ~65 decibel (dB) threshold from
composite audiograms, previous analyses in NMFS (2018), and/or data
from Southall et al. (2007) and Southall et al. (2019).
[GRAPHIC] [TIFF OMITTED] TP24FE26.015
For more detail concerning these groups and associated frequency
ranges, please see NMFS (2024) for a review of available information.
Potential Effects of the 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 section later in this document includes a
quantitative analysis of the number of individuals that are expected to
be taken by this activity. The Negligible Impact Analysis and
Determination section considers the content of this section, the
Estimated Take section, and the Proposed Mitigation section, to draw
conclusions regarding the likely impacts of these activities on the
reproductive success or survivorship of individuals and whether those
impacts are reasonably expected to, or reasonably likely to, adversely
affect the species or stock through effects on annual rates of
recruitment or survival.
Description of Active Acoustic Sound Sources
This section contains a brief technical background on sound, the
characteristics of certain sound types, and on metrics used in this
proposal inasmuch as the information is relevant to the specified
activity and to a discussion of the potential effects of the specified
activity on marine mammals found later in this document.
Sound travels in waves, the basic components of which are
frequency, wavelength, velocity, and amplitude. Frequency is the number
of pressure waves that pass by a reference point per unit of time and
is measured in Hz or cycles per second. Wavelength is the distance
between two peaks or corresponding points of a sound wave (length of
one cycle). Higher frequency sounds have shorter wavelengths than lower
frequency sounds, and typically attenuate (decrease) more rapidly,
except in certain cases in shallower water. Amplitude is the height of
the sound pressure wave or the ``loudness'' of a sound and is typically
described using the relative unit of the dB. A sound pressure level
(SPL) in dB is described as the ratio between a measured pressure and a
reference pressure (for underwater sound, this is 1 micropascal
([mu]Pa)) and is a logarithmic unit that accounts for large variations
in amplitude; therefore, a relatively small change in dB corresponds to
large changes in sound pressure. The source level (SL) represents the
SPL referenced at a distance of 1 m from the source (referenced to 1
[mu]Pa) while the received level is the SPL at the listener's position
(referenced to 1 [mu]Pa).
Root mean square (RMS) is the quadratic mean sound pressure over
the duration of an impulse. RMS is calculated by squaring all of the
sound amplitudes, averaging the squares, and then taking the square
root of the average (Urick, 1983). RMS accounts for both positive and
negative values; squaring the pressures makes all values positive so
that they may be accounted for in the summation of pressure levels
(Hastings and Popper, 2005). This measurement is often used in the
context of discussing behavioral effects, in part because behavioral
effects, which often result from auditory cues, may be better expressed
through averaged units than by peak pressures.
Sound exposure level (SEL; represented as dB re 1 [mu]Pa\2\-s)
represents the total energy contained within a pulse and considers both
intensity and
[[Page 9030]]
duration of exposure. Peak sound pressure (also referred to as zero-to-
peak sound pressure or 0-p) is the maximum instantaneous sound pressure
measurable in the water at a specified distance from the source and is
represented in the same units as the RMS sound pressure. Another common
metric is peak-to-peak sound pressure (pk-pk), which is the algebraic
difference between the peak positive and peak negative sound pressures.
Peak-to-peak pressure is typically approximately 6 dB higher than peak
pressure (Southall et al., 2007).
When underwater objects vibrate or activity occurs, sound-pressure
waves are created. These waves alternately compress and decompress the
water as the sound wave travels. Underwater sound waves radiate in a
manner similar to ripples on the surface of a pond and may be either
directed in a beam or beams or may radiate in all directions
(omnidirectional sources), as is the case for pulses produced by the
airgun array considered here. The compressions and decompressions
associated with sound waves are detected as changes in pressure by
aquatic life and man-made sound receptors such as hydrophones.
Even in the absence of sound from the specified activity, the
underwater environment is typically loud due to ambient sound. Ambient
sound is defined as environmental background sound levels lacking a
single source or point (Richardson et al., 1995), and the sound level
of a region is defined by the total acoustical energy being generated
by known and unknown sources. These sources may include physical (e.g.,
wind and waves, earthquakes, ice, atmospheric sound), biological (e.g.,
sounds produced by marine mammals, fish, and invertebrates), and
anthropogenic (e.g., vessels, dredging, construction) sound. A number
of sources contribute to ambient sound, including the following
(Richardson et al., 1995):
Wind and waves--The complex interactions between wind and water
surface, including processes such as breaking waves and wave-induced
bubble oscillations and cavitation, are a main source of naturally
occurring ambient sound for frequencies between 200 Hz and 50 kHz
(Mitson, 1995). In general, ambient sound levels tend to increase with
increasing wind speed and wave height. Surf sound becomes important
near shore, with measurements collected at a distance of 8.5 km from
shore showing an increase of 10 dB in the 100 to 700 Hz band during
heavy surf conditions;
Precipitation--Sound from rain and hail impacting the water surface
can become an important component of total sound at frequencies above
500 Hz, and possibly down to 100 Hz during quiet times;
Biological--Marine mammals can contribute significantly to ambient
sound levels, as can some fish and snapping shrimp. The frequency band
for biological contributions is from approximately 12 Hz to over 100
kHz; and
Anthropogenic--Sources of anthropogenic sound related to human
activity include transportation (surface vessels), dredging and
construction, oil and gas drilling and production, seismic surveys,
sonar, explosions, and ocean acoustic studies. Vessel noise typically
dominates the total ambient sound for frequencies between 20 and 300
Hz. In general, the frequencies of anthropogenic sounds are below 1 kHz
and, if higher frequency sound levels are created, they attenuate
rapidly. Sound from identifiable anthropogenic sources other than the
activity of interest (e.g., a passing vessel) is sometimes termed
background sound, as opposed to ambient sound.
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
human 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 this 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 a given
activity may be a negligible addition to the local environment or could
form a distinctive signal that may affect marine mammals. Details of
source types are described in the following text.
Sounds are often considered to fall into one of two general types:
Pulsed and non-pulsed. The distinction between these two sound types is
important because they have differing potential to cause physical
effects, particularly with regard to hearing (e.g., NMFS, 2018; Ward,
1997 in Southall et al., 2007). Please see Southall et al. (2007) for
an in-depth discussion of these concepts.
Pulsed sound sources (e.g., airguns, explosions, gunshots, sonic
booms, impact pile driving) produce signals that are brief (typically
considered to be less than 1 second), broadband, atonal transients
(American National Standards Institute (ANSI), 1986, 2005; Harris,
1998; National Institute for Occupational Health and Safety (NIOSH),
1998; International Organization for Standardization, 2003) and occur
either as isolated events or repeated in some succession. Pulsed sounds
are all characterized by a relatively rapid rise from ambient pressure
to a maximal pressure value followed by a rapid decay period that may
include a period of diminishing, oscillating maximal and minimal
pressures, and generally have an increased capacity to induce physical
injury as compared with sounds that lack these features.
Non-pulsed sounds can be tonal, narrowband, or broadband, brief or
prolonged, and may be either continuous or non-continuous (ANSI, 1995;
NIOSH, 1998). Some of these non-pulsed sounds can be transient signals
of short duration but without the essential properties of pulses (e.g.,
rapid rise time). Examples of non-pulsed sounds include those produced
by vessels, aircraft, machinery operations such as drilling or
dredging, vibratory pile driving, and active sonar systems (such as
those used by the U.S. Navy). The duration of such sounds, as received
at a distance, can be greatly extended in a highly reverberant
environment.
Airgun arrays produce pulsed signals with energy in a frequency
range from about 10-2,000 Hz, with most energy radiated at frequencies
below 200 Hz. The amplitude of the acoustic wave emitted from the
source is equal in all directions (i.e., omnidirectional), but airgun
arrays do possess some directionality due to different phase delays
between guns in different directions. Airgun arrays are typically tuned
to maximize functionality for data acquisition purposes, meaning that
sound transmitted in horizontal directions and at higher frequencies is
minimized to the extent possible.
Acoustic Effects
Here, we discuss the effects of active acoustic sources on marine
mammals.
Potential Effects of Underwater Sound \6\--Anthropogenic sounds
cover a broad range of frequencies and sound
[[Page 9031]]
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 potentially result in one or more of the
following: Temporary or permanent hearing impairment; non-auditory
physical or physiological effects; behavioral disturbance; stress; and
masking (Richardson et al., 1995; Gordon et al., 2004; Nowacek et al.,
2007; Southall et al., 2007; G[ouml]tz et al., 2009). 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 hearing loss, as can
longer exposures to lower level sounds. Temporary or permanent loss of
hearing, if it occurs at all, will occur almost exclusively in cases
where a noise is within an animal's hearing frequency range. We first
describe specific manifestations of acoustic effects before providing
discussion specific to the use of airgun arrays.
---------------------------------------------------------------------------
\6\ Please refer to the information given previously
(Description of Active Acoustic Sound Sources) regarding sound,
characteristics of sound types, and metrics used in this document.
---------------------------------------------------------------------------
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 response.
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 or other 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 describe the more severe effects of certain non-auditory
physical or physiological effects only briefly as we do not expect that
use of airgun arrays are reasonably likely to result in such effects
(see below for further discussion). Potential effects from impulsive
sound sources can range in severity from effects such as behavioral
disturbance or tactile perception to physical discomfort, slight injury
of the internal organs and the auditory system, or mortality (Yelverton
et al., 1973). Non-auditory physiological effects or injuries that
theoretically might occur in marine mammals exposed to high level
underwater sound or as a secondary effect of extreme behavioral
reactions (e.g., change in dive profile as a result of an avoidance
reaction) 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). The survey activities considered here do not
involve the use of devices such as explosives or mid-frequency tactical
sonar that are associated with these types of effects.
Marine mammals, like all mammals, develop increased hearing
thresholds over time due to age-related degeneration of auditory
pathways and sensory cells of the inner ear. This natural, age-related
hearing loss is contrasted by noise-induced hearing loss
(M[oslash]ller, 2012). Marine mammals exposed to high-intensity sound
or to lower-intensity sound for prolonged periods can experience a
noise-induced hearing threshold shift (TS), which NMFS defines as a
change, usually an increase, in the threshold of audibility at a
specified frequency or portion of an individual's hearing range above a
previously established reference level as a result of noise exposure
(NMFS, 2018, 2024). The amount of TS is customarily expressed in dB.
Noise-induced hearing TS can be temporary (TTS) or permanent (PTS), and
higher-level sound exposures are more likely to cause PTS or other
auditory injury. As described in NMFS (2018, 2024) there are numerous
factors to consider when examining the consequence of TS, including,
but not limited to, the signal temporal pattern (e.g., impulsive or
non-impulsive), likelihood an individual would be exposed for a long
enough duration or to a high enough level to induce a TS, the magnitude
of the TS, time to recovery (seconds to minutes or hours to days), the
frequency range of the exposure (i.e., spectral content), the hearing
frequency range of the exposed species relative to the signal's
frequency spectrum (i.e., how an animal uses sound within the frequency
band of the signal; e.g., Kastelein et al., 2014), and the overlap
between the animal and the source (e.g., spatial, temporal, and
spectral).
Auditory Injury (AUD INJ)
NMFS (2024) defines AUD INJ as damage to the inner ear that can
result in destruction of tissue, such as the loss of cochlear neuron
synapses or auditory neuropathy (Houser 2021; Finneran 2024). AUD INJ
may or may not result in a PTS. PTS is subsequently defined as a
permanent, irreversible increase in the threshold of audibility at a
specified frequency or portion of an individual's hearing range above a
previously established reference level (NMFS, 2024). PTS does not
generally affect more than a limited frequency range, and an animal
that has incurred PTS has some level of hearing loss at the relevant
frequencies; typically, animals with PTS or other AUD INJ are not
functionally deaf (Au and Hastings, 2008; Finneran, 2016). For marine
mammals, AUD INJ is considered to be possible when sound exposures are
sufficient to produce 40 dB of TTS measured after exposure (Southall et
al. 2007, 1019). AUD INJ levels for marine mammals are estimates; with
the exception of a single study unintentionally inducing PTS in a
harbor seal (Phoca vitulina) (Kastak et al., 2008; Reichmuth et al.
2019), there are no empirical data measuring AUD INJ in marine mammals
largely due to the fact that, for various ethical reasons, experiments
involving anthropogenic noise exposure at levels inducing AUD INJ are
not typically pursued or authorized (NMFS, 2024).
Temporary Threshold Shift (TTS)
TTS is the mildest form of hearing impairment that can occur during
exposure to sound. TTS is a temporary, reversible increase in the
threshold of audibility at a specified frequency or portion of an
individual's hearing range above a previously established reference
level (NMFS, 2024) that represents primarily tissue fatigue (Henderson
et al., 2008), and is not considered an AUD INJ. Based on data from
marine mammal TTS measurements (see Southall et al., 2007, 2019), a TTS
of 6 dB is considered the minimum threshold shift clearly larger than
any day-to-day or session-to-session variation in a subject's normal
hearing ability (Finneran et al., 2000, 2002; Schlundt et al., 2000).
While experiencing TTS, the hearing threshold rises, and a sound must
be at a higher level in order to be heard.
In terrestrial and marine mammals, TTS can last from minutes or
hours to days (i.e., there is recovery back to baseline/pre-exposure
levels), can occur within a specific frequency range (i.e., an animal
might only have a temporary loss of hearing sensitivity within 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). In many cases, hearing sensitivity recovers
rapidly after exposure to the sound ends. While there
[[Page 9032]]
are data on sound levels and durations necessary to elicit mild TTS for
marine mammals, recovery is complicated to predict and dependent on
multiple factors.
Relationships between TTS and AUD INJ thresholds have not been
studied in marine mammals, and there are no measured PTS data for
cetaceans, but such relationships are assumed to be similar to those in
humans and other terrestrial mammals. AUD INJ typically occurs at
exposure levels at least several dB above that inducing mild TTS (e.g.,
a 40-dB threshold shift approximates AUD INJ onset (Kryter et al.,
1966; Miller, 1974), while a 6-dB threshold shift approximates TTS
onset (Southall et al., 2007, 2019). Based on data from terrestrial
mammals, a precautionary assumption is that the AUD INJ thresholds for
impulsive sounds (such as airgun pulses as received close to the
source) are at least 6 dB higher than the TTS threshold on a peak sound
pressure level (PK SPL) basis and AUD INJ cumulative SEL
(SEL<INF>24h</INF>) thresholds are 15 (impulsive sound criteria) to 20
dB (non-impulsive criteria) higher than TTS cumulative SEL thresholds
(Southall et al., 2007, 2019). Given the higher level of sound or
longer exposure duration necessary to cause AUD INJ as compared with
TTS, it is considerably less likely that AUD INJ could occur.
Marine mammal hearing plays a critical role in communication with
conspecifics and 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. 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 occurs 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.
Finneran et al. (2015) measured hearing thresholds in 3 captive
bottlenose dolphins before and after exposure to 10 pulses produced by
a seismic airgun in order to study TTS induced after exposure to
multiple pulses. Exposures began at relatively low levels and gradually
increased over a period of several months, with the highest exposures
at peak SPLs from 196 to 210 dB and cumulative (unweighted) SELs from
193 to 195 dB. No substantial TTS was observed. In addition, behavioral
reactions were observed that indicated that animals can learn behaviors
that effectively mitigate noise exposures (although exposure patterns
must be learned, which is less likely in wild animals than for the
captive animals considered in this study). The authors note that the
failure to induce more significant auditory effects was likely due to
the intermittent nature of exposure, the relatively low peak pressure
produced by the acoustic source, and the low-frequency energy in airgun
pulses as compared with the frequency range of best sensitivity for
dolphins and other high-frequency cetaceans.
Currently, TTS data only exist for four species of cetaceans
(bottlenose dolphin, beluga whale (Delphinapterus leucas), harbor
porpoise (Phocoena phocoena), and Yangtze finless porpoise (Neophocaena
asiaeorientalis)) exposed to a limited number of sound sources (i.e.,
mostly tones and octave-band noise) in laboratory settings (Finneran,
2015). In general, harbor porpoises have a lower TTS onset than other
measured cetacean species (Finneran, 2015). Additionally, the existing
marine mammal TTS data come from a limited number of individuals within
these species.
Critical questions remain regarding the rate of TTS growth and
recovery after exposure to intermittent noise and the effects of single
and multiple pulses. Data at present are also insufficient to construct
generalized models for recovery and determine the time necessary to
treat subsequent exposures as independent events. More information is
needed on the relationship between auditory evoked potential and
behavioral measures of TTS for various stimuli. For summaries of data
on TTS in marine mammals or for further discussion of TTS onset
thresholds, please see Southall et al. (2007, 2019), Finneran and
Jenkins (2012), Finneran (2015), and NMFS (2018, 2024).
Behavioral Effects--Behavioral disturbance may include a variety of
effects, including subtle changes in behavior (e.g., minor or brief
avoidance of an area or changes in vocalizations), more conspicuous
changes in similar behavioral activities, and more sustained and/or
potentially severe reactions, such as displacement from or abandonment
of high-quality habitat. Behavioral responses to sound are highly
variable and context-specific, and any reactions depend on numerous
intrinsic and extrinsic factors (e.g., species, state of maturity,
experience, current activity, reproductive state, auditory sensitivity,
time of day), as well as the interplay between factors (e.g.,
Richardson et al., 1995; Wartzok et al., 2003; Southall et al., 2007,
2019; Weilgart, 2007; Archer et al., 2010). Behavioral reactions can
vary not only among individuals but also within an individual,
depending on previous experience with a sound source, context, and
numerous other factors (Ellison et al., 2012), and can vary depending
on characteristics associated with the sound source (e.g., whether it
is moving or stationary, number of sources, distance from the source).
Please see appendices B-C of Southall et al. (2007) for a review of
studies involving marine mammal behavioral responses to sound.
Habituation can occur when an animal's response to a stimulus wanes
with repeated exposure, usually in the absence of unpleasant associated
events (Wartzok et al., 2003). Animals are most likely to habituate to
sounds that are predictable and unvarying. It is important to note that
habituation is appropriately considered as a ``progressive reduction in
response to stimuli that are perceived as neither aversive nor
beneficial,'' rather than as, more generally, moderation in response to
human disturbance (Bejder et al., 2009). The opposite process is
sensitization, when an unpleasant experience leads to subsequent
responses, often in the form of avoidance, at a lower level of
exposure. As noted, behavioral state may affect the type of response.
For example, animals that are resting may show greater behavioral
change in response to disturbing sound levels than animals that are
highly motivated to remain in an area for feeding (Richardson et al.,
1995; National Research Council (NRC), 2003; Wartzok et al., 2003).
Controlled experiments with captive marine mammals have shown
pronounced behavioral reactions, including avoidance of loud sound
sources (Ridgway et al., 1997). Observed responses of wild marine
mammals to loud pulsed sound sources (typically seismic airguns or
acoustic harassment devices) have been varied but often consist of
avoidance behavior or other behavioral changes suggesting discomfort
(Morton and Symonds, 2002; see also Richardson et al., 1995; Nowacek et
al., 2007). However, many delphinids approach acoustic source vessels
with no apparent discomfort or obvious behavioral change (e.g.,
Barkaszi et al., 2012, Barkaszi and Kelly, 2018).
Available studies show wide variation in response to underwater
sound;
[[Page 9033]]
therefore, it is difficult to predict specifically how any given sound
in a particular instance might affect marine mammals perceiving the
signal. If a marine mammal does react briefly to an underwater sound by
changing its behavior or moving a small distance, the impacts of the
change are unlikely to be significant to the individual, let alone the
stock or population. However, if a sound source displaces marine
mammals from an important feeding or breeding area for a prolonged
period, impacts on individuals and populations could be significant
(e.g., Lusseau and Bejder, 2007; Weilgart, 2007; NRC, 2005). There are
broad categories of potential response, which we describe in greater
detail here, that include alteration of dive behavior, alteration of
foraging behavior, effects to breathing, interference with or
alteration of vocalization, avoidance, and flight.
Changes in dive behavior can vary widely, and may consist of
increased or decreased dive times and surface intervals as well as
changes in the rates of ascent and descent during a dive (e.g., Frankel
and Clark, 2000; Ng and Leung, 2003; Nowacek et al., 2004; Goldbogen et
al., 2013a, b). Variations in dive behavior may reflect disruptions in
biologically significant activities (e.g., foraging) or they may be of
little biological significance. The impact of an alteration to dive
behavior resulting from an acoustic exposure depends on what the animal
is doing at the time of the exposure and the type and magnitude of the
response.
Disruption of feeding behavior can be difficult to correlate with
anthropogenic sound exposure, so it is usually inferred by observed
displacement from known foraging areas, the appearance of secondary
indicators (e.g., bubble nets or sediment plumes), or changes in dive
behavior. As for other types of behavioral response, the frequency,
duration, and temporal pattern of signal presentation, as well as
differences in species sensitivity, are likely contributing factors to
differences in response in any given circumstance (e.g., Croll et al.,
2001; Nowacek et al., 2004; Madsen et al., 2006; Yazvenko et al.,
2007a, b). A determination of whether foraging disruptions affect
fitness consequences would require information on or estimates of the
energetic requirements of the affected individuals and the relationship
between prey availability, foraging effort and success, and the life
history stage of the animal.
Visual tracking, PAM, and movement recording tags were used to
quantify sperm whale behavior prior to, during, and following exposure
to airgun arrays at received levels in the range 140-160 dB at
distances of 7-13 km, following a phase-in of sound intensity and full
array exposures at 1-13 km (Madsen et al., 2006; Miller et al., 2009).
Sperm whales did not exhibit horizontal avoidance behavior at the
surface. However, foraging behavior may have been affected. The sperm
whales exhibited 19 percent less vocal, or buzz, rate during full
exposure relative to post exposure, and the whale that was approached
most closely had an extended resting period and did not resume foraging
until the airguns had ceased firing. The remaining whales continued to
execute foraging dives throughout exposure; however, swimming movements
during foraging dives were 6 percent lower during exposure than control
periods (Miller et al., 2009). These data raise concerns that seismic
surveys may impact foraging behavior in sperm whales, although more
data are required to understand whether the differences were due to
exposure or natural variation in sperm whale behavior (Miller et al.,
2009).
Changes in respiration naturally vary with different behaviors and
alterations to breathing rate as a function of acoustic exposure and
can be expected to co-occur with other behavioral reactions, such as a
flight response or an alteration in diving. However, respiration rates
in and of themselves may be representative of annoyance or an acute
stress response. Various studies have shown that respiration rates may
either be unaffected or could increase, depending on the species and
signal characteristics, again highlighting the importance in
understanding species differences in the tolerance of underwater noise
when determining the potential for impacts resulting from anthropogenic
sound exposure (e.g., Kastelein et al., 2001, 2005, 2006; Gailey et
al., 2007, 2016).
Marine mammals vocalize for different purposes and across multiple
modes, such as whistling, echolocation click production, calling, and
singing. Changes in vocalization behavior in response to anthropogenic
noise can occur for any of these modes and may result from a need to
compete with an increase in background noise or may reflect increased
vigilance or a startle response. For example, in the presence of
potentially masking signals, humpback whales and killer whales have
been observed to increase the length of their songs or amplitude of
calls (Miller et al., 2000; Fristrup et al., 2003; Foote et al., 2004;
Holt et al., 2012), while right whales have been 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).
In some cases, animals may cease sound production during production of
aversive signals (Bowles et al., 1994).
Cerchio et al. (2014) used PAM to document the presence of singing
humpback whales off the coast of northern Angola and to
opportunistically test for the effect of seismic survey activity on the
number of singing whales. Two recording units were deployed between
March and December 2008 in the offshore environment; numbers of singers
were counted every hour. Generalized additive mixed models were used to
assess the effect of survey day (seasonality), hour (diel variation),
moon phase, and received levels of noise (measured from a single pulse
during each 10 minutes sampled period) on singer number. The number of
singers significantly decreased with increasing received level of
noise, suggesting that humpback whale communication was disrupted to
some extent by the survey activity.
Castellote et al. (2012) reported acoustic and behavioral changes
by fin whales in response to shipping and airgun noise. Acoustic
features of fin whale song notes recorded in the Mediterranean Sea and
northeast Atlantic Ocean were compared for areas with different
shipping noise levels and traffic intensities and during a seismic
airgun survey. During the first 72 hours of the survey, a steady
decrease in song received levels and bearings to singers indicated that
whales moved away from the acoustic source and out of the study area.
This displacement persisted for a time period well beyond the 10-day
duration of seismic airgun activity, providing evidence that fin whales
may avoid an area for an extended period in the presence of increased
noise. The authors hypothesize that fin whale acoustic communication is
modified to compensate for increased background noise and that a
sensitization process may play a role in the observed temporary
displacement.
Seismic pulses at average received levels of 131 dB re 1 [mu]Pa\2\-
s caused blue whales to increase call production (Di Iorio and Clark,
2009). In contrast, McDonald et al. (1995) tracked a blue whale with
seafloor seismometers and reported that it stopped vocalizing and
changed its travel direction at a range of 10 km from the acoustic
source vessel (estimated received level 143 dB pk-pk). Blackwell et al.
(2013) found that bowhead whale call rates dropped significantly at
onset of airgun use at sites with a median distance of 41-45 km from
the survey. Blackwell et al.
[[Page 9034]]
(2015) expanded this analysis to show that whales actually increased
calling rates as soon as airgun signals were detectable before
ultimately decreasing calling rates at higher received levels (i.e.,
10-minute cumulative SEL (SEL<INF>cum</INF>) of ~127 dB). Overall,
these results suggest that bowhead whales may adjust their vocal output
in an effort to compensate for noise before ceasing vocalization effort
and ultimately deflecting from the acoustic source (Blackwell et al.,
2013, 2015). These studies demonstrate that even low levels of noise
received far from the source can induce changes in vocalization and/or
behavior for mysticetes.
Avoidance is the displacement of an individual from an area or
migration path as a result of the presence of sound or other stressors,
and is one of the most obvious manifestations of disturbance in marine
mammals (Richardson et al., 1995). For example, gray whales are known
to change direction--deflecting from customary migratory paths--in
order to avoid noise from seismic surveys (Malme et al., 1984).
Humpback whales show avoidance behavior in the presence of an active
seismic array during observational studies and controlled exposure
experiments in western Australia (McCauley et al., 2000). Avoidance may
be short-term, with animals returning to the area once the noise has
ceased (e.g., Bowles et al., 1994; Goold, 1996; Stone et al., 2000;
Morton and Symonds, 2002; Gailey et al., 2007). Longer-term
displacement is possible, however, which may lead to changes in
abundance or distribution patterns of the affected species in the
affected region if habituation to the presence of the sound does not
occur (e.g., Bejder et al., 2006; Teilmann et al., 2006).
Forney et al. (2017) detail the potential effects of noise on
marine mammal populations with high site fidelity, including
displacement and auditory masking, noting that a lack of observed
response does not imply absence of fitness costs and that apparent
tolerance of disturbance may have population-level impacts that are
less obvious and difficult to document. Avoidance of overlap between
disturbing noise and areas and/or times of particular importance for
sensitive species may be critical to avoiding population-level impacts
because (particularly for animals with high site fidelity) there may be
a strong motivation to remain in the area despite negative impacts.
Forney et al. (2017) state that, for these animals, remaining in a
disturbed area may reflect a lack of alternatives rather than a lack of
effects.
Forney et al. (2017) specifically discuss beaked whales, stating
that until recently most knowledge of beaked whales was derived from
strandings, as they have been involved in atypical mass stranding
events associated with mid-frequency active (MFA) sonar training
operations. Given these observations and recent research, beaked whales
appear to be particularly sensitive and vulnerable to certain types of
acoustic disturbance relative to most other marine mammal species.
Individual beaked whales reacted strongly to experiments using
simulated MFA sonar at low received levels, by moving away from the
sound source and stopping foraging for extended periods. These
responses, if on a frequent basis, could result in significant fitness
costs to individuals (Forney et al., 2017). Additionally, difficulty in
detection of beaked whales due to their cryptic surfacing behavior and
silence when near the surface pose problems for mitigation measures
employed to protect beaked whales. Forney et al. (2017) specifically
state that failure to consider both displacement of beaked whales from
their habitat and noise exposure could lead to more severe biological
consequences.
A flight response is a dramatic change in normal movement to a
directed and rapid movement away from the perceived location of a sound
source. The flight response differs from other avoidance responses in
the intensity of the response (e.g., directed movement, rate of
travel). Relatively little information on flight responses of marine
mammals to anthropogenic signals exist, although observations of flight
responses to the presence of predators have occurred (Connor and
Heithaus, 1996). The result of a flight response could range from
brief, temporary exertion and displacement from the area where the
signal provokes flight to, in extreme cases, marine mammal strandings
(Evans and England, 2001). However, it should be noted that response to
a perceived predator does not necessarily invoke flight (Ford and
Reeves, 2008), and whether individuals are solitary or in groups may
influence the response.
Behavioral disturbance can also impact marine mammals in more
subtle ways. Increased vigilance may result in costs related to
diversion of focus and attention (i.e., when a response consists of
increased vigilance, it may come at the cost of decreased attention to
other critical behaviors such as foraging or resting). These effects
have generally not been demonstrated for marine mammals, but studies
involving fish and terrestrial animals have shown that increased
vigilance may substantially reduce feeding rates (e.g., Beauchamp and
Livoreil, 1997; Fritz et al., 2002; Purser and Radford, 2011). In
addition, chronic disturbance can cause population declines through
reduction of fitness (e.g., decline in body condition) and subsequent
reduction in reproductive success, survival, or both (e.g., Harrington
and Veitch, 1992; Daan et al., 1996; Bradshaw et al., 1998). However,
Ridgway et al. (2006) reported that increased vigilance in bottlenose
dolphins exposed to sound over a 5-day period did not cause any sleep
deprivation or stress effects.
Many animals perform vital functions, such as feeding, resting,
traveling, and socializing, on a diel cycle (24-hour cycle). Disruption
of such functions resulting from reactions to stressors, such as sound
exposure, are more likely to be significant if they last more than one
diel cycle or recur on subsequent days (Southall et al., 2007).
Consequently, a behavioral response lasting less than 1 day and not
recurring on subsequent days is not considered particularly severe
unless it could directly affect reproduction or survival (Southall et
al., 2007). Note that there is a difference between multi-day
substantive behavioral reactions and multi-day anthropogenic
activities. For example, just because an activity lasts for multiple
days does not necessarily mean that individual animals are either
exposed to activity-related stressors for multiple days or, further,
exposed in a manner resulting in sustained multi-day substantive
behavioral responses.
Stone (2015) reported data from at-sea observations during 1,196
seismic surveys from 1994 to 2010. When large arrays of airguns
(considered to be 500 in\3\ or more in that study) were firing, lateral
displacement, more localized avoidance, or other changes in behavior
were evident for most odontocetes. However, significant responses to
large arrays were found only for the minke whale and fin whale.
Behavioral responses observed included changes in swimming or surfacing
behavior, with indications that cetaceans remained near the water
surface at these times. Cetaceans were recorded as feeding less often
when large arrays were active. Behavioral observations of gray whales
during a seismic survey monitored whale movements and respirations pre-
, during, and post-seismic survey (Gailey et al., 2016). Behavioral
state and water depth were the best ``natural'' predictors of whale
movements and respiration and, after considering natural variation,
none of the response variables were significantly associated with
seismic survey or vessel sounds.
[[Page 9035]]
Stress Responses--An animal's perception of a threat may be
sufficient to trigger stress responses consisting of some combination
of behavioral responses, autonomic nervous system responses,
neuroendocrine responses, or immune responses (e.g., Seyle, 1950;
Moberg, 2000). In many cases, an animal's first and sometimes most
economical (in terms of energetic costs) response is behavioral
avoidance of the potential stressor. Autonomic nervous system responses
to stress typically involve changes in heart rate, blood pressure, and
gastrointestinal activity. These responses have a relatively short
duration and may or may not have a significant long-term effect on an
animal's fitness.
Neuroendocrine stress responses often involve the hypothalamus-
pituitary-adrenal system. Virtually all neuroendocrine 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, altered metabolism, reduced immune
competence, and behavioral disturbance (e.g., Moberg, 1987; Blecha,
2000). Increases in the circulation of glucocorticoids are also equated
with stress (Romano et al., 2004).
The primary distinction between stress (which is adaptive and does
not normally place an animal at risk) and distress is the 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 functions. This state of
distress will last until the animal replenishes its energetic reserves
sufficiently to restore normal function.
Relationships between these physiological mechanisms, animal
behavior, and the costs of stress responses are well-studied through
controlled experiments and 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). Stress responses due to
exposure to anthropogenic sounds or other stressors and their effects
on marine mammals have also been reviewed (Fair and Becker, 2000;
Romano et al., 2002b) and, more rarely, studied in wild populations
(e.g., Romano et al., 2002a). For example, Rolland et al. (2012) found
that noise reduction from reduced ship traffic in the Bay of Fundy was
associated with decreased stress in North Atlantic right whales. These
and other studies lead to a reasonable expectation that some marine
mammals will experience physiological stress responses upon exposure to
acoustic stressors and that it is possible that some of these would be
classified as ``distress.'' In addition, any animal experiencing TTS
would likely also experience stress responses (NRC, 2003).
Auditory Masking--Sound can disrupt behavior through masking, or
interfering with, an animal's ability to detect, recognize, or
discriminate between acoustic signals of interest (e.g., those used for
intraspecific communication and social interactions, prey detection,
predator avoidance, navigation) (Richardson et al., 1995; Erbe et al.,
2016). Masking occurs when the 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 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 existing ambient noise and propagation conditions.
Under certain circumstances, significant masking could disrupt
behavioral patterns, which in turn could affect fitness for survival
and reproduction. It is important to distinguish TTS and PTS, which
persist after the sound exposure, from masking, which occurs during the
sound exposure. Because masking (without resulting in a TS) is not
associated with abnormal physiological function, it is not considered a
physiological effect, but rather a potential behavioral effect.
The frequency range of the potentially masking sound is important
in predicting any potential behavioral impacts. For example, low-
frequency signals 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) 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, 2009; Holt
et al., 2009). Masking may be less in situations where the signal and
noise come from different directions (Richardson et al., 1995), through
amplitude modulation of the signal, or through other compensatory
behaviors (Houser and Moore, 2014). Masking can be tested directly in
captive species (e.g., Erbe, 2008), 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., Branstetter et al.,
2013).
Masking affects both senders and receivers of acoustic signals and
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 the world's ocean from pre-industrial
periods, with most of the increase from distant commercial shipping
(Hildebrand, 2009). All anthropogenic sound sources, but especially
chronic and lower-frequency signals (e.g., from vessel traffic),
contribute to elevated ambient sound levels, thus intensifying masking.
Masking effects of pulsed sounds (even from large arrays of
airguns) on marine mammal calls and other natural sounds are expected
to be limited, although there are few specific data on this. Because of
the intermittent nature and low duty cycle of seismic pulses, animals
can emit and receive sounds in the relatively quiet intervals between
pulses. However, in exceptional situations, reverberation occurs for
much or all of the interval between pulses (e.g., Simard et al., 2005;
Clark and Gagnon 2006), which could mask calls. Situations with
prolonged strong reverberation are infrequent. However, it is common
for reverberation to cause some lesser degree of elevation of the
background level between airgun pulses (e.g., Gedamke 2011; Guerra et
al., 2011, 2016; Klinck et al., 2012; Guan et al., 2015), and this
weaker reverberation presumably reduces the detection range of calls
and other natural sounds to some degree. Guerra et al. (2016)
[[Page 9036]]
reported that ambient noise levels between seismic pulses were elevated
as a result of reverberation at ranges of 50 km from the seismic
source. Based on measurements in deep water of the Southern Ocean,
Gedamke (2011) estimated that the slight elevation of background noise
levels during intervals between seismic pulses reduced blue and fin
whale communication space by as much as 36-51 percent when a seismic
survey was operating 450-2,800 km away. Based on preliminary modeling,
Wittekind et al. (2016) reported that airgun sounds could reduce the
communication range of blue and fin whales 2,000 km from the seismic
source. Nieukirk et al. (2012) and Blackwell et al. (2013) noted the
potential for masking effects from seismic surveys on large whales.
Some baleen and toothed whales are known to continue calling in the
presence of seismic pulses, and their calls usually can be heard
between the pulses (e.g., Nieukirk et al., 2012; Thode et al., 2012;
Br[ouml]ker et al., 2013; Sciacca et al., 2016). Cerchio et al. (2014)
suggested that the breeding display of humpback whales off Angola could
be disrupted by seismic sounds, as singing activity declined with
increasing received levels. In addition, some cetaceans are known to
change their calling rates, shift their peak frequencies, or otherwise
modify their vocal behavior in response to airgun sounds (e.g., Di
Iorio and Clark 2009; Castellote et al., 2012; Blackwell et al., 2013,
2015). The hearing systems of baleen whales are more sensitive to low-
frequency sounds than are the ears of the small odontocetes that have
been studied directly (e.g., MacGillivray et al., 2014). The sounds
important to small odontocetes are predominantly at much higher
frequencies than are the dominant components of airgun sounds, thus
limiting the potential for masking. In general, masking effects of
seismic pulses are expected to be minor, given the normally
intermittent nature of seismic pulses.
Vessel Noise
Vessel noise from survey vessels could affect marine animals in the
proposed survey areas. Houghton et al. (2015) proposed that vessel
speed is the most important predictor of received noise levels, and
Putland et al. (2017) also reported reduced sound levels with decreased
vessel speed. However, some energy is also produced at higher
frequencies (Hermannsen et al., 2014); low levels of high-frequency
sound from vessels has been shown to elicit responses in harbor
porpoise (Dyndo et al., 2015).
Vessel noise, through masking, can reduce the effective
communication distance of a marine mammal if the frequency of the sound
source is close to that used by the animal, and if the sound is present
for a significant fraction of time (e.g., Richardson et al., 1995;
Clark et al., 2009; Jensen et al., 2009; Gervaise et al., 2012; Hatch
et al., 2012; Rice et al., 2014; Dunlop, 2015; Jones et al., 2017;
Putland et al., 2017). In addition to the frequency and duration of the
masking sound, the strength, temporal pattern, and location of the
introduced sound also play a role in the extent of the masking
(Branstetter et al., 2013, 2016; Finneran and Branstetter 2013; Sills
et al., 2017). Branstetter et al. (2013) reported that time-domain
metrics are also important in describing and predicting masking.
Baleen whales are thought to be more sensitive to sound at these
low frequencies than are toothed whales (e.g., MacGillivray et al.,
2014), possibly causing localized avoidance of the survey area during
seismic operations. Many odontocetes show considerable tolerance of
vessel traffic, although they sometimes react at long distances if
confined by ice or shallow water, if previously harassed by vessels, or
have had little or no recent exposure to vessels (Richardson et al.,
1995). Pirotta et al. (2015) noted that the physical presence of
vessels, not just ship noise, disturbed the foraging activity of
bottlenose dolphins. There is little data on the behavioral reactions
of beaked whales to vessel noise, though they seem to avoid approaching
vessels (e.g., W[uuml]rsig et al., 1998) or dive for an extended period
when approached by a vessel (e.g., Kasuya, 1986).
In summary, survey vessel sounds would not be at levels expected to
cause anything more than possible localized and temporary behavioral
changes in marine mammals, and would not be expected to result in
significant negative effects on individuals or at the population level.
In addition, in all oceans of the world, large vessel traffic is
currently so prevalent that it is commonly considered a usual source of
ambient sound (NSF-USGS, 2011).
Vessel Strike
Vessel collisions with marine mammals, or vessel strikes, can
result in death or serious injury of the animal. Wounds resulting from
vessel strike may include massive trauma, hemorrhaging, broken bones,
or propeller lacerations (Knowlton and Kraus, 2001). An animal at the
surface may be struck directly by a vessel, a surfacing animal may hit
the bottom of a vessel, or an animal just below the surface may be cut
by a vessel's propeller. Superficial strikes may not kill or result in
the death of the animal. These interactions are typically associated
with large whales (e.g., fin whales), which are occasionally found
draped across the bulbous bow of large commercial vessels upon arrival
in port. Although smaller cetaceans are more maneuverable in relation
to large vessels than are large whales, they may also be susceptible to
strike. The severity of injuries typically depends on the size and
speed of the vessel, with the probability of death or serious injury
increasing as vessel speed increases (Knowlton and Kraus, 2001; Laist
et al., 2001; Vanderlaan and Taggart, 2007; Conn and Silber, 2013).
Impact forces increase with speed, as does the probability of a strike
at a given distance (Silber et al., 2010; Gende et al., 2011).
Pace and Silber (2005) also found that the probability of death or
serious injury increased rapidly with increasing vessel speed.
Specifically, the predicted probability of serious injury or death
increased from 45 to 75 percent as vessel speed increased from 10 to 14
knots (kn, 26 kilometer per hour (kph)), and exceeded 90 percent at 17
kn (31 kph). Higher speeds during collisions result in greater force of
impact, but higher speeds also appear to increase the chance of severe
injuries or death through increased likelihood of collision by pulling
whales toward the vessel (Clyne, 1999; Knowlton et al., 1995). In a
separate study, Vanderlaan and Taggart (2007) analyzed the probability
of lethal mortality of large whales at a given speed, showing that the
greatest rate of change in the probability of a lethal injury to a
large whale as a function of vessel speed occurs between 8.6 and 15 kn
(28 kph). The chances of a lethal injury decline from approximately 80
percent at 15 kn (28 kph) to approximately 20 percent at 8.6 kn (16
kph). At speeds below 11.8 kn (22 kph), the chances of lethal injury
drop below 50 percent, while the probability asymptotically increases
toward one hundred percent above 15 kn (28 kph).
Survey vessels will travel at a speed of 5 kn (9 kph) while towing
seismic survey gear. At this speed, both the possibility of striking a
marine mammal and the possibility of a strike resulting in serious
injury or mortality are discountable. At average transit speed, the
probability of serious injury or mortality resulting from a strike is
less than 50 percent. However, the likelihood of a strike actually
happening is again discountable. Vessel strikes, as analyzed in the
studies cited above, generally involve commercial shipping,
[[Page 9037]]
which is much more common in both space and time than is geophysical
survey activity. No such incidents have been reported for geophysical
survey vessels.
Although the likelihood of the vessel striking a marine mammal is
low, we propose a robust vessel strike avoidance protocol (see Proposed
Mitigation), which we believe eliminates any foreseeable risk of vessel
strike during transit. We anticipate that vessel collisions involving a
seismic data acquisition vessel towing gear, while not impossible,
represent unlikely, unpredictable events for which there are no
preventive measures. Given the proposed mitigation measures, the
relatively slow speed of the vessel towing gear, the presence of bridge
crew watching for obstacles at all times (including marine mammals),
and the presence of marine mammal observers, the possibility of vessel
strike is discountable and, further, were a strike of a large whale to
occur, it would be unlikely to result in serious injury or mortality.
No incidental take resulting from vessel strike is anticipated, and
this potential effect of the specified activity will not be discussed
further in the following analysis.
Stranding--When a living or dead marine mammal swims or floats onto
shore and becomes ``beached'' or incapable of returning to sea, the
event is a ``stranding'' (Geraci et al., 1999; Perrin and Geraci, 2002;
Geraci and Lounsbury, 2005; NMFS, 2007). The legal definition for a
stranding under the MMPA is that a marine mammal is dead and is on a
beach or shore of the United States; or in waters under the
jurisdiction of the United States (including any navigable waters); or
a marine mammal is alive and is on a beach or shore of the United
States and is unable to return to the water; on a beach or shore of the
United States and, although able to return to the water, is in need of
apparent medical attention; or in the waters under the jurisdiction of
the United States (including any navigable waters), but is unable to
return to its natural habitat under its own power or without
assistance.
Marine mammals strand for a variety of reasons, such as infectious
agents, biotoxicosis, starvation, fishery interaction, vessel strike,
unusual oceanographic or weather events, sound exposure, or
combinations of these stressors sustained concurrently or in series.
However, the cause or causes of most strandings are unknown (Geraci et
al., 1976; Eaton, 1979; Odell et al., 1980; Best, 1982). Numerous
studies suggest that the physiology, behavior, habitat relationships,
age, or condition of cetaceans may cause them to strand or might
predispose them to strand when exposed to another phenomenon. These
suggestions are consistent with the conclusions of numerous other
studies that have demonstrated that combinations of dissimilar
stressors commonly combine to kill an animal or dramatically reduce its
fitness, even though one exposure without the other does not produce
the same result (Chroussos, 2000; Creel, 2005; DeVries et al., 2003;
Fair and Becker, 2000; Foley et al., 2001; Moberg, 2000; Relyea, 2005a;
2005b, Romero, 2004; Sih et al., 2004).
There is no conclusive evidence that exposure to airgun noise
results in behaviorally-mediated forms of injury. Behaviorally-mediated
injury (i.e., mass stranding events) has been primarily associated with
beaked whales exposed to MFA sonar. MFA sonar and the alerting stimulus
used in Nowacek et al. (2004) are very different from the noise
produced by airguns. One should therefore not expect the same reaction
to airgun noise as to these other sources. As explained below, military
MFA sonar is very different from airguns, and one should not assume
that airguns will cause the same effects as MFA sonar (including
strandings).
To understand why military MFA sonar affects beaked whales
differently than airguns do, it is important to note the distinction
between behavioral sensitivity and susceptibility to auditory injury.
To understand the potential for auditory injury in a particular marine
mammal species in relation to a given acoustic signal, the frequency
range the species is able to hear is critical, as well as the species'
auditory sensitivity to frequencies within that range. Current data
indicate that not all marine mammal species have equal hearing
capabilities across all frequencies and, therefore, species are grouped
into hearing groups with generalized hearing ranges assigned on the
basis of available data (Southall et al., 2007, 2019). Hearing ranges
as well as auditory sensitivity/susceptibility to frequencies within
those ranges vary across the different groups. For example, in terms of
hearing range, the very high-frequency cetaceans (e.g., Kogia spp.)
have a generalized hearing range of frequencies between 200 Hz and 165
kHz, while high-frequency cetaceans--such as dolphins and beaked
whales--have a generalized hearing range between 150 Hz to 160 kHz.
Regarding auditory susceptibility within the hearing range, while high-
frequency cetaceans and very high-frequency cetaceans have roughly
similar hearing ranges, the very high-frequency group is much more
susceptible to noise-induced hearing loss during sound exposure, i.e.,
these species have lower thresholds for these effects than other
hearing groups (NMFS, 2018, 2024). Referring to a species as
behaviorally sensitive to noise simply means that an animal of that
species is more likely to respond to lower received levels of sound
than an animal of another species that is considered less behaviorally
sensitive. So, while dolphin species and beaked whale species--both in
the high-frequency cetacean hearing group--are assumed to generally
hear the same sounds equally well and be equally susceptible to noise-
induced hearing loss (auditory injury), the best available information
indicates that a beaked whale is more likely to behaviorally respond to
that sound at a lower received level compared to an animal from other
high-frequency cetacean species that are less behaviorally sensitive.
This distinction is important because, while beaked whales are more
likely to respond behaviorally to sounds than are many other species
(even at lower levels), they cannot hear the predominant, lower
frequency sounds from seismic airguns as well as sounds that have more
energy at frequencies that beaked whales can hear better (such as
military MFA sonar).
Military MFA sonar effects beaked whales differently than airguns
do because it produces energy at different frequencies than airguns.
High-frequency cetacean hearing is generically thought to be best
between 8.8 to 110 kHz, i.e., these cutoff values define the range
above and below which a species in the group is assumed to have
declining auditory sensitivity, until reaching frequencies that cannot
be heard (NMFS, 2018, 2024). However, beaked whale hearing is likely
best within a higher, narrower range (20-80 kHz, with best sensitivity
around 40 kHz), based on a few measurements of hearing in stranded
beaked whales (Cook et al., 2006; Finneran et al., 2009; Pacini et al.,
2011) and several studies of acoustic signals produced by beaked whales
(e.g., Frantzis et al., 2002; Johnson et al., 2004, 2006; Zimmer et
al., 2005). While precaution requires that the full range of audibility
be considered when assessing risks associated with noise exposure
(Southall et al., 2007, 2019), animals typically produce sound at
frequencies where they hear best. More recently, Southall et al. (2019)
suggested that certain species in the high-frequency hearing group
(beaked whales, sperm whales, and killer whales) are likely more
sensitive to lower frequencies
[[Page 9038]]
within the group's generalized hearing range than are other species
within the group, and state that the data for beaked whales suggest
sensitivity to approximately 5 kHz. However, this information is
consistent with the general conclusion that beaked whales (and other
high-frequency cetaceans) are relatively insensitive to the frequencies
where most energy of an airgun signal is found. Military MFA sonar is
typically considered to operate in the frequency range of approximately
3-14 kHz (D'Amico et al., 2009), i.e., outside the range of likely best
hearing for beaked whales but within or close to the lower bounds,
whereas most energy in an airgun signal is radiated at much lower
frequencies, below 500 Hz (Dragoset, 1990).
It is important to distinguish between energy (loudness, measured
in dB) and frequency (pitch, measured in Hz). In considering the
potential impacts of mid-frequency components of airgun noise (1-10
kHz, where beaked whales can be expected to hear) on marine mammal
hearing, one needs to account for the energy associated with these
higher frequencies and determine what energy is truly ``significant.''
Although there is mid-frequency energy associated with airgun noise (as
expected from a broadband source), airgun sound is predominantly below
1 kHz (Breitzke et al., 2008; Tashmukhambetov et al., 2008; Tolstoy et
al., 2009). As stated by Richardson et al. (1995), ``[. . .] most
emitted [seismic airgun] energy is at 10-120 Hz, but the pulses contain
some energy up to 500-1,000 Hz.'' Tolstoy et al. (2009) conducted
empirical measurements, demonstrating that sound energy levels
associated with airguns were at least 20 dB lower at 1 kHz (considered
``mid-frequency'') compared to higher energy levels associated with
lower frequencies (below 300 Hz) (``all but a small fraction of the
total energy being concentrated in the 10-300 Hz range'' (Tolstoy et
al., 2009)), and at higher frequencies (e.g., 2.6-4 kHz), power might
be less than 10 percent of the peak power at 10 Hz. Energy levels
measured by Tolstoy et al. (2009) were even lower at frequencies above
1 kHz. In addition, as sound propagates away from the source, it tends
to lose higher-frequency components faster than low-frequency
components (i.e., low-frequency sounds typically propagate longer
distances than high-frequency sounds) (Diebold et al., 2010). Although
higher-frequency components of airgun signals have been recorded, it is
typically in surface-ducting conditions (e.g., DeRuiter et al., 2006;
Madsen et al., 2006) or in shallow water, where there are advantageous
propagation conditions for the higher frequency (but low-energy)
components of the airgun signal (Hermannsen et al., 2015). This should
not be of concern because the likely behavioral reactions of beaked
whales that can result in acute physical injury would result from noise
exposure at depth (because of the potentially greater consequences of
severe behavioral reactions). In summary, the frequency content of
airgun signals is such that beaked whales will not be able to hear the
signals well (compared to MFA sonar), especially at depth where we
expect the consequences of noise exposure could be more severe.
Aside from frequency content, there are other significant
differences between MFA sonar signals and the sounds produced by
airguns that minimize the risk of severe behavioral reactions that
could lead to strandings or deaths at sea, e.g., significantly longer
signal duration, horizontal sound direction, typical fast and
unpredictable source movement. All of these characteristics of MFA
sonar tend towards greater potential to cause severe behavioral or
physiological reactions in exposed beaked whales that may contribute to
stranding. Although both sources are powerful, MFA sonar contains
significantly greater energy in the mid-frequency range, where beaked
whales hear better. Short-duration, high energy pulses--such as those
produced by airguns--have greater potential to cause damage to auditory
structures (though this is unlikely for high-frequency cetaceans, as
explained later in this document), but it is longer duration signals
that have been implicated in the vast majority of beaked whale
strandings. Faster, less predictable movements in combination with
multiple source vessels are more likely to elicit a severe, potentially
anti-predator response. Of additional interest in assessing the
divergent characteristics of MFA sonar and airgun signals and their
relative potential to cause stranding events or deaths at sea is the
similarity between the MFA sonar signals and stereotyped calls of
beaked whales' primary predator: the killer whale (Zimmer and Tyack,
2007). Although generic disturbance stimuli--as airgun noise may be
considered in this case for beaked whales--may also trigger
antipredator responses, stronger responses should generally be expected
when perceived risk is greater, as when the stimulus is confused for a
known predator (Frid and Dill, 2002). In addition, because the source
of the perceived predator (i.e., MFA sonar) will likely be closer to
the whales (because attenuation limits the range of detection of mid-
frequencies) and moving faster (because it will be on faster-moving
vessels), any antipredator response would be more likely to be severe
(with greater perceived predation risk, an animal is more likely to
disregard the cost of the response; Frid and Dill, 2002). Indeed, when
analyzing movements of a beaked whale exposed to playback of killer
whale predation calls, Allen et al. (2014) found that the whale engaged
in a prolonged, directed avoidance response, suggesting a behavioral
reaction that could pose a risk factor for stranding. Overall, these
significant differences between sound from MFA sonar and the mid-
frequency sound component from airguns and the likelihood that MFA
sonar signals will be interpreted in error as a predator are critical
to understanding the likely risk of behaviorally-mediated injury due to
seismic surveys.
The available scientific literature also provides a useful contrast
between airgun noise and MFA sonar regarding the likely risk of
behaviorally-mediated injury. There is strong evidence for the
association of beaked whale stranding events with MFA sonar use, and
particularly detailed accounting of several events is available (e.g.,
a 2000 Bahamas stranding event for which investigators concluded that
MFA sonar use was responsible; Evans and England, 2001). D'Amico et al.
(2009) reviewed 126 beaked whale mass stranding events over the period
from 1950 (i.e., from the development of modern MFA sonar systems)
through 2004. Of these, there were two events where detailed
information was available on both the timing and location of the
stranding and the concurrent nearby naval activity, including
verification of active MFA sonar usage, with no evidence for an
alternative cause of stranding. An additional 10 events were at minimum
spatially and temporally coincident with naval activity likely to have
included MFA sonar use and, despite incomplete knowledge of timing and
location of the stranding or the naval activity in some cases, there
was no evidence for an alternative cause of stranding. The U.S. Navy
has publicly stated agreement that five such events since 1996 were
associated in time and space with MFA sonar use, either by the U.S.
Navy alone or in joint training exercises with the North Atlantic
Treaty Organization. The U.S. Navy additionally noted that, as of 2017,
a 2014 beaked whale stranding event in Crete coincident with naval
exercises was under review and had not yet been
[[Page 9039]]
determined to be linked to sonar activities (U.S. Navy, 2017).
Separately, the International Council for the Exploration of the Sea
reported in 2005 that, worldwide, there have been about 50 known
strandings, consisting mostly of beaked whales, with a potential causal
link to MFA sonar (International Council for the Exploration for the
Sea, 2005). In contrast, very few such associations have been made to
seismic surveys, despite widespread use of airguns as a geophysical
sound source in numerous locations around the world.
A review of possible stranding associations with seismic surveys
(Castellote and Llorens, 2016) states that, ``[s]peculation concerning
possible links between seismic survey noise and cetacean strandings is
available for a dozen events but without convincing causal evidence.''
The authors' search of available information found 10 events worth
further investigation via a ranking system representing a rough metric
of the relative level of confidence offered by the data for inferences
about the possible role of the seismic survey in a given stranding
event. Only three of these events involved beaked whales. Whereas
D'Amico et al. (2009) used a 1-5 ranking system, in which ``1''
represented the most robust evidence connecting the event to MFA sonar
use, Castellote and Llorens (2016) used a 1-6 ranking system, in which
``6'' represented the most robust evidence connecting the event to the
seismic survey. As described above, D'Amico et al. (2009) found that
two events were ranked ``1'' and 10 events were ranked ``2'' (i.e., 12
beaked whale stranding events were found to be associated with MFA
sonar use). In contrast, Castellote and Llorens (2016) found that none
of the three beaked whale stranding events achieved their highest ranks
of 5 or 6. Of the 10 total events, none achieved the highest rank of 6.
Two events were ranked as 5: one stranding in Peru involving dolphins
and porpoises and a 2008 stranding in Madagascar. This latter ranking
can only be broadly associated with the survey itself, as opposed to
use of seismic airguns. An investigation of the 2008 Madagascar
stranding event, which did not involve beaked whales, concluded that
use of a high-frequency mapping system (12-kHz multibeam echosounder)
was the most plausible and likely initial behavioral trigger of the
event, which was likely exacerbated by several site- and situation-
specific secondary factors. The review panel found that seismic airguns
were used after the initial strandings and animals entering a lagoon
system, that airgun use clearly had no role as an initial trigger, and
that there was no evidence that airgun use dissuaded animals from
leaving (Southall et al., 2013).
However, one of these stranding events, involving two goose-beaked
beaked whales, was contemporaneous with and reasonably associated
spatially with a 2002 seismic survey in the Gulf of California, as was
the case for the 2007 Gulf of Cadiz seismic survey discussed by
Castellote and Llorens (also involving two goose-beaked beaked whales).
Neither event was considered a ``true atypical mass stranding''
(according to Frantzis (1998)) as used in the analysis of Castellote
and Llorens (2016). While we agree with Castellote and Llorens that
this lack of evidence associating seismic surveys and stranding events
should not be considered conclusive, it is clear that there is very
little evidence that seismic surveys should be considered as posing a
significant risk of acute harm to beaked whales or other high-frequency
cetaceans. We have considered the potential for the proposed surveys to
result in marine mammal stranding and, based on the best available
information, do not expect a stranding to occur.
Other Potential Impacts
Here, we briefly address the potential risks due to entanglement
and contaminant spills. We are not aware of any records of marine
mammal entanglement in towed arrays such as those considered here, and
we address measures designed to eliminate the potential for
entanglement in gear used by OBN surveys in Proposed Mitigation. The
discharge of trash and debris is prohibited (33 CFR 151.51 through
151.77) unless it is passed through a machine that breaks up solids
such that they can pass through a 25-mm mesh screen. All other trash
and debris must be returned to shore for proper disposal with municipal
and solid waste. Some personal items may be accidentally lost
overboard. However, U.S. Coast Guard and Environmental Protection Act
regulations require operators to become proactive in avoiding
accidental loss of solid waste items by developing waste management
plans, posting informational placards, manifesting trash sent to shore,
and using special precautions such as covering outside trash bins to
prevent accidental loss of solid waste. Entanglement risks are
essentially eliminated by the proposed requirements, and entanglement
risks are not discussed further in this document.
Marine mammals could be affected by accidentally spilled diesel
fuel from a vessel associated with proposed survey activities.
Quantities of diesel fuel on the sea surface may affect marine mammals
through various pathways: surface contact of the fuel with skin and
other mucous membranes, inhalation of concentrated petroleum vapors, or
ingestion of the fuel (direct ingestion or by the ingestion of
contaminated prey) (e.g., Geraci and St. Aubin, 1980, 1985, 1990).
However, the likelihood of a fuel spill during any particular
geophysical survey is considered to be remote, and the potential for
impacts to marine mammals would depend greatly on the size and location
of a spill and meteorological conditions at the time of the spill.
Spilled fuel would rapidly spread to a layer of varying thickness and
break up into narrow bands or windrows parallel to the wind direction.
The rate at which the fuel spreads would be determined by the
prevailing conditions such as temperature, water currents, tidal
streams, and wind speeds. Lighter, volatile components of the fuel
would evaporate to the atmosphere almost completely in a few days.
Evaporation rate may increase as the fuel spreads because of the
increased surface area of the slick. Rougher seas, high wind speeds,
and high temperatures also tend to increase the rate of evaporation and
the proportion of fuel lost by this process (Scholz et al., 1999). We
do not anticipate potentially meaningful effects to marine mammals as a
result of any contaminant spill resulting from the proposed survey
activities, and contaminant spills resulting from the specified
activity are not discussed further in this document.
Anticipated Effects on Marine Mammal Habitat
Physical Disturbance--Sources of seafloor disturbance related to
geophysical surveys that may impact marine mammal habitat include
placement of anchors, nodes, cables, sensors, or other equipment on or
in the seafloor for various activities. Equipment deployed on the
seafloor has the potential to cause direct physical damage and could
affect bottom-associated fish resources.
Placement of equipment, such as nodes, on the seafloor could damage
areas of hard bottom where direct contact with the seafloor occurs and
could crush epifauna (organisms that live on the seafloor or surface of
other organisms). Damage to unknown or unseen hard bottom could occur,
but because of the small area covered by most bottom-founded equipment,
the patchy distribution of hard bottom habitat, and typical BOEM permit
[[Page 9040]]
conditions related to avoidance of such areas, contact with unknown
hard bottom is expected to be rare and impacts minor. Seafloor
disturbance in areas of soft bottom can cause loss of small patches of
epifauna and infauna due to burial or crushing, and bottom-feeding
fishes could be temporarily displaced from feeding areas. Overall, any
effects of physical damage to habitat are expected to be minor and
temporary.
Effects to Prey--Marine mammal prey varies by species, season, and
location and, for some, is not well documented. Fish react to sounds
which are especially strong and/or intermittent low-frequency sounds,
and behavioral responses such as flight or avoidance are the most
likely effects. However, the reaction of fish to airguns depends on the
physiological state of the fish, past exposures, motivation (e.g.,
feeding, spawning, migration), and other environmental factors. Several
studies have demonstrated that airgun sounds might affect the
distribution and behavior of some fishes, potentially impacting
foraging opportunities or increasing energetic costs (e.g., Fewtrell
and McCauley, 2012; Pearson et al., 1992; Skalski et al., 1992;
Santulli et al., 1999; Paxton et al., 2017), though the bulk of studies
indicate no or slight reaction to noise (e.g., Miller and Cripps, 2013;
Dalen and Knutsen, 1987; Pe[ntilde]a et al., 2013; Chapman and Hawkins,
1969; Wardle et al., 2001; Jorgenson and Gyselman, 2009; Blaxter et
al., 1981; Cott et al., 2012; Boeger et al., 2006), and that, most
commonly, while there are likely to be impacts to fish as a result of
noise from nearby airguns, such effects will be temporary. For example,
investigators reported significant, short-term declines in commercial
fishing catch rate of gadid fishes during and for up to 5 days after
seismic survey operations, but the catch rate subsequently returned to
normal (Eng[aring]s et al., 1996; Eng[aring]s and Lokkeborg, 2002).
Other studies have reported similar findings (Hassel et al., 2004).
Skalski et al., (1992) also found a reduction in catch rates--for
rockfish (Sebastes spp.) in response to controlled airgun exposure--but
suggested that the mechanism underlying the decline was not dispersal
but rather decreased responsiveness to baited hooks associated with an
alarm behavioral response. A companion study showed that alarm and
startle responses were not sustained following the removal of the sound
source (Pearson et al., 1992). Therefore, Skalski et al. (1992)
suggested that the effects on fish abundance may be transitory,
primarily occurring during the sound exposure itself. In some cases,
effects on catch rates are variable within a study, which may be more
broadly representative of temporary displacement of fish in response to
airgun noise (i.e., catch rates may increase in some locations and
decrease in others) than any long-term damage to the fish themselves
(Streever et al., 2016).
SPLs of sufficient strength have been known to cause injury to fish
and fish mortality and, in some studies, fish auditory systems have
been damaged by airgun noise (McCauley et al., 2003; Popper et al.,
2005; Song et al., 2008). However, in most fish species, hair cells in
the ear continuously regenerate and loss of auditory function likely is
restored when damaged cells are replaced with new cells. Halvorsen et
al. (2012) showed that a TTS of 4-6 dB was recoverable within 24 hours
for one species. Impacts would be most severe when the individual fish
is close to the source and when the duration of exposure is long; both
of which are conditions unlikely to occur for this survey that is
necessarily transient in any given location and likely result in brief,
infrequent noise exposure to prey species in any given area. For
seismic surveys, the sound source is constantly moving, and most fish
would likely avoid the sound source prior to receiving sound of
sufficient intensity to cause physiological or anatomical damage. In
addition, ramp-up may allow certain fish species the opportunity to
move further away from the sound source.
A comprehensive review (Carroll et al., 2017) found that results
are mixed as to the effects of airgun noise on the prey of marine
mammals. While some studies suggest a change in prey distribution and/
or a reduction in prey abundance following the use of seismic airguns,
others suggest no effects or even positive effects in prey abundance.
As one specific example, Paxton et al. (2017), which describes findings
related to the effects of a 2014 seismic survey on a reef off of North
Carolina, showed a 78 percent decrease in observed nighttime abundance
for certain species. It is important to note that the evening hours
during which the decline in fish habitat use was recorded (via video
recording) occurred on the same day that the seismic survey passed, and
no subsequent data is presented to support an inference that the
response was long-lasting. Additionally, given that the finding is
based on video images, the lack of recorded fish presence does not
support a conclusion that the fish actually moved away from the site or
suffered any serious impairment. In summary, this particular study
corroborates prior studies indicating that a startle response or short-
term displacement should be expected.
Available data suggest that cephalopods are capable of sensing the
particle motion of sounds and detect low frequencies up to 1-1.5 kHz,
depending on the species, and so are likely to detect airgun noise
(Kaifu et al., 2008; Hu et al., 2009; Mooney et al., 2010; Samson et
al., 2014). Auditory injuries (lesions occurring on the statocyst
sensory hair cells) have been reported upon controlled exposure to low-
frequency sounds, suggesting that cephalopods are particularly
sensitive to low-frequency sound (Andr[eacute] et al., 2011;
Sol[eacute] et al., 2013). Behavioral responses, such as inking and
jetting, have also been reported upon exposure to low-frequency sound
(McCauley et al., 2000; Samson et al., 2014). Similar to fish, however,
the transient nature of the survey leads to an expectation that effects
will be largely limited to behavioral reactions and would occur as a
result of brief, infrequent exposures.
With regard to potential impacts on zooplankton, McCauley et al.
(2017) found that exposure to airgun noise resulted in significant
depletion for more than half the taxa present and that there were two
to three times more dead zooplankton after airgun exposure compared
with controls for all taxa, within 1 km of the airguns. However, the
authors also stated that in order to have significant impacts on r-
selected species (i.e., those with high growth rates and that produce
many offspring) such as plankton, the spatial or temporal scale of
impact must be large in comparison with the ecosystem concerned, and it
is possible that the findings reflect avoidance by zooplankton rather
than mortality (McCauley et al., 2017). In addition, the results of
this study are inconsistent with a large body of research that
generally finds limited spatial and temporal impacts to zooplankton as
a result of exposure to airgun noise (e.g., Dalen and Knutsen, 1987;
Payne, 2004; Stanley et al., 2011). Most prior research on this topic,
which has focused on relatively small spatial scales, has showed
minimal effects (e.g., Kostyuchenko, 1973; Booman et al., 1996;
S[aelig]tre and Ona, 1996; Pearson et al., 1994; Bolle et al., 2012).
A modeling exercise was conducted as a follow-up to the McCauley et
al. (2017) study (as recommended by McCauley et al.), in order to
assess the potential for impacts on ocean ecosystem dynamics and
zooplankton population dynamics (Richardson et al., 2017). Richardson
et al. (2017) found that for copepods with a short life cycle
[[Page 9041]]
in a high-energy environment, a full-scale airgun survey would impact
copepod abundance up to 3 days following the end of the survey,
suggesting that effects such as those found by McCauley et al. (2017)
would not be expected to be detectable downstream of the survey areas,
either spatially or temporally.
Notably, a subsequent study produced results inconsistent with
those of McCauley et al. (2017). Researchers conducted a field and
laboratory study to assess if exposure to airgun noise affects
mortality, predator escape response, or gene expression of the copepod
Calanus finmarchicus (Fields et al., 2019). Immediate mortality of
copepods was significantly higher, relative to controls, at distances
of 5 m or less from the airguns. Mortality 1 week after the airgun
pulse was significantly higher in the copepods placed 10 m from the
airgun but was not significantly different from the controls at a
distance of 20 m from the airgun. The increase in mortality, relative
to controls, did not exceed 30 percent at any distance from the airgun.
Moreover, the authors caution that even this higher mortality in the
immediate vicinity of the airguns may be more pronounced than what
would be observed in free-swimming animals due to increased flow speed
of fluid inside bags containing the experimental animals. There were no
sublethal effects on the escape performance or the sensory threshold
needed to initiate an escape response at any of the distances from the
airgun that were tested. Whereas McCauley et al. (2017) reported an SEL
of 156 dB at a range of 509-658 m, with zooplankton mortality observed
at that range, Fields et al. (2019) reported an SEL of 186 dB at a
range of 25 m, with no reported mortality at that distance. Regardless,
if we assume a worst-case likelihood of severe impacts to zooplankton
within approximately 1 km of the acoustic source, the brief time to
regeneration of the potentially affected zooplankton populations does
not lead us to expect any meaningful follow-on effects to the prey base
for marine mammals.
A review article concluded that, while laboratory results provide
scientific evidence for high-intensity and low-frequency sound-induced
physical trauma and other negative effects on some fish and
invertebrates, the sound exposure scenarios in some cases are not
realistic to those encountered by marine organisms during routine
seismic operations (Carroll et al., 2017). The review finds that there
has been no evidence of reduced catch or abundance following seismic
activities for invertebrates, and that there is conflicting evidence
for fish with catch observed to increase, decrease, or remain the same.
Further, where there is evidence for decreased catch rates in response
to airgun noise, these findings provide no information about the
underlying biological cause of catch rate reduction (Carroll et al.,
2017).
In summary, impacts of the specified activity on marine mammal prey
species will likely generally be limited to behavioral responses, the
majority of prey species will be capable of moving out of the area
during the survey, a rapid return to normal recruitment, distribution,
and behavior for prey species is anticipated, and, overall, impacts to
prey species will be minor and temporary. Prey species exposed to sound
might move away from the sound source, experience TTS, experience
masking of biologically relevant sounds, or show no obvious direct
effects. Mortality from decompression injuries is possible in close
proximity to a sound, but only limited data on mortality in response to
airgun noise exposure are available (Hawkins et al., 2014). The most
likely impacts for most prey species in the survey area would be
temporary avoidance of the area. The proposed survey would move through
an area relatively quickly, limiting exposure to multiple impulsive
sounds. In all cases, sound levels would return to ambient once the
survey moves out of the area or ends and the noise source is shut down
and, when exposure to sound ends, behavioral and/or physiological
responses are expected to end relatively quickly (McCauley et al.,
2000). The duration of fish avoidance of a given area after survey
effort stops is unknown, but a rapid return to normal recruitment,
distribution, and behavior is anticipated. While the potential for
disruption of spawning aggregations or schools of important prey
species can be meaningful on a local scale, the mobile and temporary
nature of typical surveys and the likelihood of temporary avoidance
behavior suggest that impacts would be minor.
Acoustic Habitat--Acoustic habitat is the soundscape--which
encompasses all of the sound present in a particular location and time,
as a whole--when considered from the perspective of the animals
experiencing it. Animals produce sound for, or listen for sounds
produced by, conspecifics (communication during feeding, mating, and
other social activities), other animals (finding prey or avoiding
predators), and the physical environment (finding suitable habitats,
navigating). Together, sounds made by animals and the geophysical
environment (e.g., produced by earthquakes, lightning, wind, rain,
waves) make up the natural contributions to the total acoustics of a
place. These acoustic conditions, termed acoustic habitat, are one
attribute of an animal's total habitat.
Soundscapes are also defined by, and acoustic habitat is influenced
by, the total contribution of anthropogenic sound. This may include
incidental emissions from sources such as vessel traffic, or may be
intentionally introduced to the marine environment for data acquisition
purposes (as in the use of airgun arrays). Anthropogenic noise varies
widely in its frequency content, duration, and loudness, and these
characteristics greatly influence the potential habitat-mediated
effects to marine mammals (please see also the previous discussion on
masking under Acoustic Effects), which may range from local effects for
brief periods of time to chronic effects over large areas and for long
durations. Depending on the extent of effects to habitat, animals may
alter their communications signals (thereby potentially expending
additional energy) or miss acoustic cues (either conspecific or
adventitious). For more detail on these concepts see, e.g., Barber et
al., 2010; Pijanowski et al., 2011; Francis and Barber, 2013; Lillis et
al., 2014.
Problems arising from a failure to detect cues are more likely to
occur when noise stimuli are chronic and overlap with biologically
relevant cues used for communication, orientation, and predator/prey
detection (Francis and Barber, 2013). Although the signals emitted by
seismic airgun arrays are generally low frequency, they would also
likely be of short duration and transient in any given area due to the
nature of these surveys. As described previously, exploratory surveys
such as these cover a large area but would be transient rather than
focused in a given location over time and therefore would not be
considered chronic in any given location.
Based on the information discussed herein, we conclude that impacts
of the specified activity are not likely to have more than short-term
adverse effects on any prey habitat or populations of prey species.
Further, any impacts to marine mammal habitat are not expected to
result in significant or long-term consequences for individual marine
mammals, or to contribute to adverse impacts on their populations.
Estimated Take
This section provides an estimate of the numbers and type of
incidental takes that may be expected to occur
[[Page 9042]]
under the specified activity, which informs NMFS' negligible impact
determinations. Realized incidental takes would be determined by the
actual levels of activity at specific times and places that occur under
any issued LOAs and by the actual acoustic source used. Take estimates
are available for the three different airgun array configurations
described previously. The highest modeled estimated take (annual and 5-
year total) for each species is analyzed for the negligible impact
analysis.
Except with respect to certain activities not pertinent here,
section 3(18) of the MMPA defines ``harassment'' as: any act of
pursuit, torment, or annoyance which (i) has the potential to injure a
marine mammal or marine mammal stock in the wild (Level A harassment);
or (ii) has the potential to disturb a marine mammal or marine mammal
stock in the wild by causing disruption of behavioral patterns,
including, but not limited to, migration, breathing, nursing, breeding,
feeding, or sheltering (Level B harassment). Harassment is the only
type of take expected to result from these activities. It is unlikely
that lethal takes would occur even in the absence of the mitigation and
monitoring measures, and no such takes are anticipated or will be
authorized.
Anticipated takes would primarily be by Level B harassment, as use
of the described acoustic sources, particularly airgun arrays, is
likely to disrupt behavioral patterns of marine mammals upon exposure
to sound at certain levels. There is also some potential for auditory
injury (Level A harassment) to result for LF and VHF species due to the
size of the predicted auditory injury zones for those species, though
none is predicted to occur for Rice's whales (the only LF cetacean in
the GOA). NMFS does not expect auditory injury to occur for HF species.
Detailed discussion of this determination is provided below.
Below, we summarize how the take that may be authorized was
estimated using acoustic thresholds, sound field modeling, and marine
mammal density data. In addition to discussion provided below, please
see associated companion documents available on NMFS' website, for
additional detail (Zeddies et al., 2015, 2017a; Weirathmueller et al.,
2022). A summary overview of the take estimation process, as well as
full discussion related to the development of estimated take numbers,
is provided below.
Acoustic Thresholds
NMFS uses acoustic thresholds that identify the received level of
underwater sound above which exposed marine mammals generally would be
reasonably expected to exhibit disruption of behavioral patterns (Level
B harassment) or to incur AUD INJ of some degree (Level A harassment).
Level B Harassment--NMFS carries forward the approach to evaluation
of potential take by Level B harassment used for the current ITRs.
Based on the practical need to use a relatively simple threshold based
on available information that is both predictable and measurable for
most activities, NMFS typically uses a generalized acoustic threshold
based on received level to estimate the onset of Level B harassment
(e.g., the historical 160 dB rms threshold for intermittent sources,
which include the impulsive sources evaluated herein). In this case,
NMFS identified a more complex probabilistic risk function for use in
evaluating the potential effects of the specified activity. This
function, first described in Wood et al. (2012), differs from the
single-step 160 dB rms criterion primarily by acknowledging the
potential for Level B harassment at exposures to received levels below
160 dB rms as well as the potential that animals exposed to received
levels above 160 dB rms will not respond in ways constituting Level B
harassment. The approach described by Wood et al. (2012) also accounts
for differential hearing sensitivity by incorporating the Type I
frequency-weighting functions described by Southall et al. (2007). The
broader Type I filters are appropriately retained for use in evaluating
potential behavioral disturbance in conjunction with the probabilistic
response function. The criteria are described in table 4.
[GRAPHIC] [TIFF OMITTED] TP24FE26.016
Level A harassment--Modeling supporting the 2021 and 2024 final
rules relied on NMFS' Technical Guidance for Assessing the Effects of
Anthropogenic Sound on Marine Mammal Hearing (Version 2.0; NMFS, 2018)
(table 5). Since issuance of those rules, NMFS completed Updated
Technical Guidance (NMFS, 2024) (table 6). Both versions of the
technical guidance identify dual criteria to assess auditory injury
(Level A harassment) to five different marine mammal groups (based on
hearing sensitivity) as a result of exposure to noise from two
different types of sources (impulsive or non-impulsive). This proposed
rule carries forward the modeling and resulting take estimates from the
existing ITR, based on the 2018 Technical Guidance (NMFS, 2018), based
on our determination that those estimates of Level A harassment remain
sufficiently representative of any incidents of Level A harassment that
may reasonably be expected to occur (described next).
[[Page 9043]]
[GRAPHIC] [TIFF OMITTED] TP24FE26.017
These thresholds are provided in tables 5 and 6. The references,
analysis, and methodology used in the development of the thresholds are
described in NMFS' 2018 Technical Guidance and NMFS' 2024 Updated
Technical Guidance, both of which may be accessed at: <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance</a>. The specified activity considered herein
includes the use of impulsive seismic sources (i.e., airguns).
[GRAPHIC] [TIFF OMITTED] TP24FE26.018
In summary, the peak pressure threshold for LF cetaceans increased
by 3 dB, while the cumulative SEL threshold (upon which estimates of
potential AUD INJ for LF cetaceans is based in this case) is unchanged.
As discussed below, no Level A harassment is likely to occur for HF
cetaceans, though we note that the cumulative SEL threshold for the
hearing group increased by 8 dB. The peak pressure threshold for VHF
cetaceans (upon which estimates of potential AUD INJ are based in this
case) is unchanged, while the cumulative SEL threshold increased by 4
dB (see tables 5 and 6). Regarding the underlying frequency
sensitivities, the generalized hearing range for LF cetaceans remains
essentially the same (currently estimated as 7 Hz-36 kHz versus 7 Hz-35
kHz in the 2018 Technical Guidance), while the current HF cetacean
hearing range is unchanged from that estimated for the previously named
mid-frequency hearing group. The current VHF cetacean hearing range was
changed more significantly, from 275 Hz-160 kHz (for the previously
named HF hearing group) to 200 Hz-165 kHz (see table 3). However,
because the potential for Level A harassment is best predicted by
exposures above the peak pressure threshold for VHF cetaceans, the
change to estimated hearing range, and changes to the auditory
weighting function, are not relevant, i.e., frequency weighting is not
a factor in evaluating exposures to peak pressure output from airgun
arrays. As the peak pressure threshold for this hearing group is
unchanged, no change would be expected to the previously estimated
instances of Level A harassment.
Although the operable cumulative SEL threshold for LF cetaceans is
unchanged, frequency weighting is relevant to evaluations of potential
exposure above the threshold. Changes to the LF cetacean weighting
function would be expected to result in slight increases to estimated
isopleth distances associated with the AUD INJ threshold, though these
would remain smaller than the proposed shutdown distance for Rice's
whales (see Proposed Mitigation). The existing take estimates, which
NMFS proposes to carry forward for this ITR, predict that no Level A
harassment will occur for Rice's whales. Given the very low likelihood
of injurious exposure for Rice's whales, in context of the proposed
mitigation requirements, NMFS has determined that the minor changes to
the Technical Guidance for LF cetaceans do not affect the likelihood of
Level A harassment and, therefore, there is no need to update related
quantitative estimates. There are no
[[Page 9044]]
changes to the existing estimates of potential Level A harassment for
any species.
Acoustic Exposure Modeling
Zeddies et al. (2015, 2017a) provided estimates of the annual
marine mammal acoustic exposures exceeding the aforementioned criteria
caused by sounds from geophysical survey activity in the GOA for 10
years of notional activity levels, using 8,000-in\3\ airguns and other
sources, as well as full detail regarding the original acoustic
exposure modeling conducted in support of BOEM's 2016 petition and
NMFS' analysis in support of the 2021 final rule. Zeddies et al.
(2017b) provided information regarding source and propagation modeling
related to the 4,130-in\3\ airgun array, and Weirathmueller et al.
(2022) provide detail regarding the modeling performed for the 5,110-
in\3\ airgun array. For full details of the modeling effort, see the
reports (available online at: <a href="https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-mexico">https://www.fisheries.noaa.gov/action/incidental-take-authorization-oil-and-gas-industry-geophysical-survey-activity-gulf-mexico</a>).
The modeling effort produced exposure estimates computed from
modeled sound levels as received by simulated animals (animats) in a
specific modeling area. As described previously, the GOA was divided
into seven modeling zones with six survey types simulated within each
zone to estimate the potential effects of each survey: shelf and slope
waters were divided into eastern, central, and western zones, plus a
single deep-water zone, to account for both the geospatial dependence
of acoustic fields and the geographic variations of animal
distributions. The selected boundaries considered sound propagation
conditions and species distribution to create regions of optimized
uniformity in both acoustic environment and animal density. Survey
types included deep penetration surveys using a large airgun array (2D,
3D NAZ, 3D WAZ, and coil survey types), shallow penetration surveys
using a single airgun (which were assumed to be a reasonable proxy for
surveys conducted using a boomer), and high resolution surveys. We do
not discuss HRG surveys further, as they are not considered likely to
result in incidental take of marine mammals.
The results from each zone were summed to provide GOA-wide
estimates of take for each marine mammal species for each survey type
for each notional year. To get these annual aggregate exposure
estimates, 24-hr average exposure estimates from each survey type were
multiplied by the number of expected survey days from BOEM's effort
projections. Because these projections are not season-specific, surveys
were assumed to be equally likely to occur at any time of the year and
at any location within a given zone.
Acoustic source emission levels and directivity of a single airgun
and an airgun array were modeled using JASCO Applied Sciences' Airgun
Array Source Model (AASM). AASM is capable of predicting airgun source
levels at frequencies up to 25 kHz, and produces a set of notional
signatures for each array element based on array layout; volume, tow
depth, and firing pressure for each element; and interactions between
different elements in the array. The signatures are summed to obtain
the far-field source signature of the entire array in the horizontal
plane, which is then filtered into one third-octave frequency bands to
compute the source levels of the array as a function of frequency band
and azimuthal angle in the horizontal plane (at the source depth),
after which it is considered to be an azimuth-dependent directional
point source in the far field.
Underwater sound propagation (i.e., transmission loss) as a
function of range from each source was modeled using JASCO's Marine
Operations Noise Model (MONM) for multiple propagation radials centered
at the source to yield 3D transmission loss fields in the surrounding
area. The MONM computes received per-pulse SEL for directional sources
at specified depths. MONM uses two separate models to estimate
transmission loss. At frequencies less than 2 kHz, MONM computes
acoustic propagation via a wide-angle parabolic equation (PE) solution
to the acoustic wave equation, based on a version of the U.S. Naval
Research Laboratory's Range-dependent Acoustic Model (RAM) modified to
account for an elastic seabed. MONM-RAM incorporates bathymetry,
underwater sound speed as a function of depth, and a geoacoustic
profile based on seafloor composition, and accounts for source
horizontal directivity. At frequencies greater than 2 kHz, MONM
accounts for increased sound attenuation due to volume absorption at
higher frequencies with the widely-used BELLHOP Gaussian beam ray-trace
propagation model. This component incorporates bathymetry and
underwater sound speed as a function of depth with a simplified
representation of the sea bottom, as sub-bottom layers have a
negligible influence on the propagation of acoustic waves with
frequencies above 1 kHz. MONM-BELLHOP accounts for horizontal
directivity of the source and vertical variation of the source beam
pattern. Both propagation models account for full exposure from a
direct acoustic wave, as well as exposure from acoustic wave
reflections and refractions (i.e., multi-path arrivals at the
receiver).
In order to accurately estimate exposure, a simulation must
adequately cover the various location- and season-specific
environments. The surveys may be conducted at any location within the
planning area and occur at any time of the year, so simulations must
adequately cover each area and time period. As noted, potential
exposures were modeled within the seven zones corresponding with shelf
and slope environments subdivided into western, central, and eastern
areas, as well as a single deep zone. The subdivision depth definitions
are: shelf, 0-200 m; slope, 200-2,000 m; and deep, greater than 2,000
m. Within each of the seven zones, a set of representative survey-
simulation rectangles for each of the survey types was defined, with
larger areas for the ``large-area'' surveys (i.e., deep penetration
airgun) and smaller areas for the ``small-area'' surveys (i.e., shallow
penetration airgun).
A set of 30 sites was selected to calculate acoustic propagation
loss grids as functions of source, range from the source, azimuth from
the source, and receiver depth. These were then used as inputs to the
acoustic exposure model. The environmental parameters and acoustic
propagation conditions represented by these 30 modeling sites were
chosen to be representative of the prevalent acoustic propagation
conditions within the survey extents. To account for seasonal variation
in propagation, winter and summer were both used to calculate exposure
estimates. Propagation during spring and fall was found to be almost
identical to the results for summer, so those seasons were represented
with the summer results. The primary seasonal influence on transmission
loss is the presence of a sound channel, or duct, near the surface in
winter.
All acoustic exposure modeling, including source and propagation
modeling, was redone in 2022 in support of the 2024 final rule to
address the additional airgun array configurations as well as to
incorporate updated data on marine mammal density and species
behavioral parameters, as described below in this section
(Weirathmueller et al., 2022). However, all aspects of the modeling
(including source, propagation, and animal movement modeling) were
performed in the same manner as described in Zeddies et al. (2015,
2017a, 2017b).
[[Page 9045]]
The 2022 modeling update, which is also used for this proposed
rule, incorporated revised species definition files consisting of
behavioral parameters (e.g., depth, travel rate, dive profile) for each
species that govern simulated animal (animat) movement within the
movement model (Weirathmueller et al., 2022). These updated acoustic
exposure modeling results allow NMFS to evaluate full results for all
three array configurations, providing for appropriate representation of
the range of actual acoustic sources planned for use during
consideration of LOA requests.
Marine Mammal Density Information--The best available scientific
information was considered in conducting marine mammal exposure
estimates (the basis for estimating take). This information consists of
habitat-based cetacean density models produced by NMFS' Southeast
Fisheries Science Center (Garrison et al., 2023). These models
incorporate survey data from 2003 through 2019 including data from
survey effort conducted during winter, allowing for increased temporal
resolution of model predictions relative to previously available marine
mammal density data. In addition, these are the first density models
that incorporate survey data collected after the DWH oil spill. New
models were produced for all taxa other than Fraser's dolphin and
rough-toothed dolphin, as the model authors determined that there were
too few detections of these species to support model development.
Therefore, we rely on previously available models (Roberts et al.,
2016) for these two species.
For species occurring in oceanic waters, the density models are
based upon data collected during vessel surveys conducted in 2003-2004,
2009, and 2017-2018 (and surveys conducted in 2019 for Rice's whale).
Survey effort was generally conducted in a survey region bounded by the
shelf break (approximately the 200-m isobath) to the north and the
boundary of the U.S. EEZ to the south. Separate models were created for
species occurring in shelf waters (Atlantic spotted dolphin and
bottlenose dolphin) based on seasonal aerial surveys conducted in 2011-
2012 and 2017-2018. Based on water depth, the shelf models were used to
predict acoustic exposures for these two species in zones 2 and 3 (with
zone 1 no longer part of the specified geographic region), and the
oceanic models were used to predict exposures in zones 4-7.
As discussed above, the density modeling effort treats beaked
whales and Kogia spp. as guilds, as sightings of these species are
typically difficult to resolve to the species level. In addition, the
model authors determined there to be too few sightings and/or too few
sightings resolved to species level for the melon-headed whale, false
killer whale, pygmy killer whale, and killer whale to produce
individual species models. Instead, a single blackfish model was
developed to produce guild-level predictions for these species
(Garrison et al., 2023).
Take Estimates
Exposure estimates above Level A and Level B harassment criteria,
originally developed by Zeddies et al. (2015, 2017a, 2017b) and updated
by Weirathmueller et al. (2022) in association with the activity
projections for the various annual effort scenarios, were generated
based on the specific modeling scenarios (including source and survey
geometry), i.e., 2D survey (1 x source array), 3D NAZ survey (2 x
source array), 3D WAZ survey (4 x source array), coil survey (4 x
source array).
Level A Harassment--Here, we summarize acoustic exposure modeling
results related to Level A harassment. Overall, there is a low
likelihood of take by Level A harassment for any species, though the
degree of this low likelihood is primarily influenced by the specific
hearing group. For HF and VHF cetaceans, potential auditory injury
would be expected to occur on the basis of instantaneous exposure to
peak pressure output from an airgun array while for LF cetaceans,
potential auditory injury would occur on the basis of the accumulation
of energy output over time by an airgun array. Importantly, the modeled
exposure estimates do not account for either aversion or the beneficial
impacts of the required mitigation measures.
Of even greater import for HF cetaceans is that the small
calculated Level A harassment zone size in conjunction with the
properties of sound fields produced by arrays in the near field versus
far field leads to a logical conclusion that Level A harassment is so
unlikely for species in this hearing group as to be discountable.
For HF cetaceans, the only potential injury zones will be based on
the peak pressure metric, as such zones will be larger than those
calculated on the basis of the cumulative SEL metric (which are
essentially non-existent for HF and VHF cetaceans). The estimated zone
size for the 230 dB peak threshold for HF cetaceans is only 18 m. In a
theoretical modeling scenario, it is possible for animats to engage
with such a small assumed zone around a notional point source and,
subsequently, for these interactions to scale to predictions of real-
world exposures given a sufficient number of predicted 24-hr survey
days in confluence with sufficiently high predicted real-world animal
densities. However, this is not a realistic outcome. The source level
of the array is a theoretical definition assuming a point source and
measurement in the far-field of the source. As described by Caldwell
and Dragoset (2000), an array is not a point source, but one that spans
a small area. In the far-field, individual elements in arrays will
effectively work as one source because individual pressure peaks will
have coalesced into one relatively broad pulse. The array can then be
considered a ``point source.'' For distances within the near-field,
i.e., approximately two to three times the array dimensions, pressure
peaks from individual elements do not arrive simultaneously because the
observation point is not equidistant from each element. The effect is
destructive interference of the outputs of each element, so that peak
pressures in the near-field will be significantly lower than the output
of the largest individual element. Here, the 230 dB peak isopleth
distances would be expected to be within the near-field of the arrays
where the definition of source level breaks down. Therefore, actual
locations within this distance (i.e., within 18 m) of the array center
where the sound level exceeds 230 dB peak SPL would not necessarily
exist. In general, Caldwell and Dragoset (2000) suggest that the near-
field for airgun arrays is considered to extend out to approximately
250 m.
In order to provide quantitative support for this theoretical
argument, we calculated expected maximum distances at which the near-
field would transition to the far-field for five specific, real-world
arrays (83 FR 63268, December 7, 2018). The average distance to the
near-field calculated for the five arrays, following the process
described below, was 203 m (range 80-417 m).
For a specific array one can estimate the distance at which the
near-field transitions to the far-field by:
[GRAPHIC] [TIFF OMITTED] TP24FE26.019
with the condition that D [Gt] [lambda], and where D is the distance, L
is the longest dimension of the array, and [lambda] is the wavelength
of the signal (Lurton, 2002). Given that [lambda] can be defined by:
[GRAPHIC] [TIFF OMITTED] TP24FE26.020
[[Page 9046]]
where f is the frequency of the sound signal and v is the speed of the
sound in the medium of interest, one can rewrite the equation for D as:
[GRAPHIC] [TIFF OMITTED] TP24FE26.021
and calculate D directly given a particular frequency and known speed
of sound (here assumed to be 1,500 meters per second in water, although
this varies with environmental conditions).
To determine the closest distance to the array at which the modeled
source level prediction is valid (i.e., maximum extent of the near-
field), we calculated D based on an assumed frequency of 1 kHz. A
frequency of 1 kHz is commonly used in near-field/far-field
calculations for airgun arrays, and based on representative airgun
spectrum data and field measurements of an airgun array used on the R/V
Marcus G. Langseth, nearly all (greater than 95 percent) of the energy
from airgun arrays is below 1 kHz (Tolstoy et al., 2009). Thus, using 1
kHz as the upper cut-off for calculating the maximum extent of the
near-field should reasonably represent the near-field extent in field
conditions.
If the largest distance to the peak sound pressure level threshold
was equal to or less than the longest dimension of the array (i.e.,
under the array), or within the near-field, then received levels that
meet or exceed the threshold in most cases are not expected to occur.
This is because within the near-field and within the dimensions of the
array, the specified source level is overestimated and not applicable.
In fact, until one reaches a distance of approximately three or four
times the near-field distance, the average intensity of sound at any
given distance from the array is still less than that based on
calculations that assume a directional point source (Lurton, 2002). For
example, an airgun array used on the R/V Marcus G. Langseth has an
approximate diagonal of 29 m, resulting in a near-field distance of 140
m at 1 kHz (NSF and USGS, 2011). Field measurements of this array
indicate that the source behaves like multiple discrete sources, rather
than a directional point source, beginning at approximately 400 m (deep
site) to 1 km (shallow site) from the center of the array (Tolstoy et
al., 2009), distances that are actually greater than four times the
calculated 140-m near-field distance. Within these distances, the
recorded rece
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