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Notice2026-06854

Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to a Marine Geophysical Survey in the Western Central Atlantic Ocean

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Published

April 9, 2026

Issuing agencies

Commerce DepartmentNational Oceanic and Atmospheric Administration

Abstract

NMFS has received a request from the Lamont-Doherty Earth Observatory of Columbia University (L-DEO) for authorization to take marine mammals incidental to a marine geophysical survey off the Eastern North American Margin in the Western Central Atlantic Ocean. Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting comments on its proposal to issue an incidental harassment authorization (IHA) to incidentally take marine mammals during the specified activities. NMFS is also requesting comments on a possible one-time, 1-year renewal that could be issued under certain circumstances and if all requirements are met, as described in Request for Public Comments at the end of this notice. NMFS will consider public comments prior to making any final decision on the issuance of the requested MMPA authorization and agency responses will be summarized in the final notice of our decision.

Full Text

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[Federal Register Volume 91, Number 68 (Thursday, April 9, 2026)]
[Notices]
[Pages 18024-18053]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-06854]

[[Page 18023]]

Vol. 91

Thursday,

No. 68

April 9, 2026

Part II

Department of Commerce

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National Oceanic and Atmospheric Administration

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Takes of Marine Mammals Incidental to Specified Activities; Taking
Marine Mammals Incidental to a Marine Geophysical Survey in the Western
Central Atlantic Ocean; Notice

Federal Register / Vol. 91, No. 68 / Thursday, April 9, 2026 /
Notices

[[Page 18024]]

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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

[RTID 0648-XE792]

Takes of Marine Mammals Incidental to Specified Activities;
Taking Marine Mammals Incidental to a Marine Geophysical Survey in the
Western Central Atlantic Ocean

AGENCY: National Marine Fisheries Service (NMFS), National Oceanic and
Atmospheric Administration (NOAA), Commerce.

ACTION: Notice; proposed incidental harassment authorization; request
for comments on proposed authorization and possible renewal.

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SUMMARY: NMFS has received a request from the Lamont-Doherty Earth
Observatory of Columbia University (L-DEO) for authorization to take
marine mammals incidental to a marine geophysical survey off the
Eastern North American Margin in the Western Central Atlantic Ocean.
Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting
comments on its proposal to issue an incidental harassment
authorization (IHA) to incidentally take marine mammals during the
specified activities. NMFS is also requesting comments on a possible
one-time, 1-year renewal that could be issued under certain
circumstances and if all requirements are met, as described in Request
for Public Comments at the end of this notice. NMFS will consider
public comments prior to making any final decision on the issuance of
the requested MMPA authorization and agency responses will be
summarized in the final notice of our decision.

DATES: Comments and information must be received no later than May 11,
2026.

ADDRESSES: Comments should be addressed to Permits and Conservation
Division, Office of Protected Resources, National Marine Fisheries
Service and should be submitted via email to <a href="/cdn-cgi/l/email-protection#cc85989ce2a4adbea0adafa4a9be8ca2a3adade2aba3ba">[email&#160;protected]</a>.
Electronic copies of the application and supporting documents, as well
as a list of the references cited in this document, may be obtained
online at: <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-research-and-other-activities">https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-research-and-other-activities</a>. In case of problems accessing these documents, please call
the contact listed below.
Instructions: NMFS is not responsible for comments sent by any
other method, to any other address or individual, or received after the
end of the comment period. Comments, including all attachments, must
not exceed a 25-megabyte file size. All comments received are a part of
the public record and will generally be posted online at <a href="https://www.fisheries.noaa.gov/permit/incidental-take-authorizations-under-marine-mammal-protection-act">https://www.fisheries.noaa.gov/permit/incidental-take-authorizations-under-marine-mammal-protection-act</a> without change. All personal identifying
information (e.g., name, address) voluntarily submitted by the
commenter may be publicly accessible. Do not submit confidential
business information or otherwise sensitive or protected information.

FOR FURTHER INFORMATION CONTACT: Jenna Harlacher, Office of Protected
Resources, NMFS, (301) 427-8401.

SUPPLEMENTARY INFORMATION:

Background

The MMPA prohibits the ``take'' of marine mammals, with certain
exceptions. 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 proposed or, if the taking is limited to harassment, a notice of a
proposed IHA is provided to the public for review.
Authorization for incidental takings shall be granted if NMFS finds
that the taking will have a negligible impact on the species or
stock(s) and will not have an unmitigable adverse impact on the
availability of the species or stock(s) for taking for subsistence uses
(where relevant). Further, NMFS must prescribe the permissible methods
of taking; other ``means of effecting the least practicable adverse
impact'' on the affected species or stocks and their habitat, paying
particular attention to rookeries, mating grounds, and areas of similar
significance, and on the availability of the species or stocks for
taking for certain subsistence uses (referred to as ``mitigation'');
and requirements pertaining to the monitoring and reporting of the
takings. The definitions of all applicable MMPA statutory terms used
above are included in the relevant sections below (see also 16 U.S.C.
1362; 50 CFR 216.3, 216.103).

National Environmental Policy Act

To comply with the National Environmental Policy Act of 1969 (NEPA;
42 U.S.C. 4321 et seq.) and NOAA Administrative Order (NAO) 216-6A,
NMFS must review our proposed action (i.e., the issuance of an IHA)
with respect to potential impacts on the human environment.
This action is consistent with categories of activities identified
in Categorical Exclusion B4 (IHAs with no anticipated serious injury or
mortality) of the Companion Manual for NAO 216-6A, which do not
individually or cumulatively have the potential for significant impacts
on the quality of the human environment and for which we have not
identified any extraordinary circumstances that would preclude this
categorical exclusion. Accordingly, NMFS has preliminarily determined
that the issuance of the proposed IHA qualifies to be categorically
excluded from further NEPA review.

Summary of Request

On February 3, 2025, NMFS received a request from L-DEO for an IHA
to take marine mammals incidental to a marine geophysical survey off
the Eastern North American Margin in the Western Central Atlantic
Ocean. After sending questions and confirming details with the
applicant, the L-DEO's application was deemed adequate and complete on
March 7, 2025. NMFS was then informed that the planned survey would be
deferred to a later date, and on February 19, 2026, L-DEO informed NMFS
that the survey would occur in 2026 and requested that NMFS resume
consideration of its IHA request. L-DEO's request is for take of 31
species of marine mammals by Level B harassment and, for Bryde's whale,
sei whale, minke whale and Kogia spp, Level A harassment. Neither L-DEO
nor NMFS expect serious injury or mortality to result from this
activity and, therefore, an IHA is appropriate.

Description of Proposed Activity

Overview

Researchers from L-DEO of Columbia University, University of Texas
at Austin Institute for Geophysics and Syracuse University, with
funding from the National Science Foundation, propose to conduct a
high-energy seismic survey using airguns as the acoustic source from
the research vessel Marcus G. Langseth (Langseth), which is owned and
operated by L-DEO. The proposed survey would occur off the Eastern
North American Margin in the Western Central Atlantic Ocean from July
to September 2026. The proposed survey would occur within international
waters, in water depths ranging from approximately 4,800 to 5,550
meters (m). To conduct this survey, the Langseth would tow a 36-airgun
array with a total discharge volume of ~6,600

[[Page 18025]]

cubic inches (in\3\, 108,155 cubic centimeters (cc)) at a depth of 12
m. The airgun array receiving systems for the different survey segments
would consist of a 15 kilometer (km) long solid-state hydrophone
streamer and 52 ocean bottom seismometers (OBS). The airguns would fire
at a shot interval of 50 m (~24 seconds (s)) during 2-dimensional (2-D)
multi-channel seismic (MCS) reflection surveys with the hydrophone
streamer and at a 200 m (~78 s) interval during OBS seismic refraction
surveys. Approximately 4,264 km of total survey trackline is proposed,
including 691 km of MCS seismic reflection data and 3,573 km of OBS
refraction data.
The purpose of the proposed survey is to collect seismic data
spanning the oceanic lithosphere from the onset of oceanization for ~50
million years of incipient seafloor spreading at the nascent Mid-
Atlantic Ridge to investigate mantle dynamics during the opening of the
Central Atlantic Ocean. Additional data would be collected using a
magnetometer, gravitometer, multibeam echosounder (MBES), a sub-bottom
profiler (SBP), and an acoustic doppler current profiler (ADCP), which
would be operated from the Langseth continuously during the seismic
surveys, including during transit. Expendable bathythermographs will
also be deployed throughout the survey. No take of marine mammals is
expected to result from use of this equipment.

Dates and Duration

The proposed survey is expected to last for approximately 42 days
from July through September 2026, with 20 days of seismic operations,
13 days of OBS deployment and retrieval, 4.5 days of contingency, and
4.5 days of transit.

Specific Geographic Region

The proposed survey would occur within approximately 27-33[deg] N
lat., 67-75[deg] W long., in international waters, in water depths
ranging from approximately 4,800 to 5,550 m. The region where the
survey is proposed to occur is depicted in figure 1; the tracklines
could occur anywhere within the polygon shown in figure 1.
Representative survey tracklines are shown; however, some deviation in
actual tracklines, including the order of survey operations, could be
necessary for reasons such as science drivers, poor data quality,
inclement weather, or mechanical issues with the vessel and/or
equipment. The Langseth would likely leave out of and return to port in
Charleston, South Carolina, which is approximately 600 km west of the
proposed survey area.
[GRAPHIC] [TIFF OMITTED] TN09AP26.002

[[Page 18026]]

Detailed Description of the Specified Activity

The procedures to be used for the proposed survey would be similar
to those used during previous seismic surveys conducted by L-DEO and
would use conventional seismic methodology. The survey would involve
one source vessel, the Langseth, which is owned and operated by L-DEO.
During the high-energy MCS seismic reflection and OBS seismic
refraction surveys, Langseth would tow 4 strings with 36 airguns,
consisting of a mixture of Bolt 1500LL and Bolt 1900LLX. During the
survey, all 4 strings, totaling 36 active airguns with a total
discharge volume of 6,600 in\3\ (108,155 cc), would be used. The four
airgun strings would be spaced 16 m apart, distributed across an area
of approximately 24 m x 16 m behind the Langseth, and would be towed
approximately 140 m behind the vessel. The airgun array configurations
are illustrated in figure 2-11 of National Science Foundation (NSF) and
the U.S. Geological Survey's (USGS) Programmatic Environmental Impact
Statement (PEIS; NSF-USGS 2011). (The PEIS is available online at:
<a href="https://www.nsf.gov/geo/oce/envcomp/usgs-nsf-marine-seismic-research/nsf-usgs-final-eis-oeis_3june2011.pdf">https://www.nsf.gov/geo/oce/envcomp/usgs-nsf-marine-seismic-research/nsf-usgs-final-eis-oeis_3june2011.pdf</a>.). The receiving system would
consist of a 15-km long solid-state hydrophone streamer and 52 OBSs. As
the airgun arrays are towed along the survey lines, the hydrophone
streamer would transfer the data to the on-board processing system for
the MCS survey, and the OBSs would receive and store the returning
acoustic signals internally for later analysis.
Approximately 4,264 km of seismic acquisition are proposed (691 km
of 2-D MCS seismic reflection data and 3,573 km of OBS refraction
data). All 52 OBSs will be deployed at the beginning of the survey and
recovered at the end after acquiring all seismic data.
In addition to the operations of the airgun array, the ocean floor
would be mapped with the Kongsberg EM 122 MBES and a Knudsen Chirp 3260
SBP. A Teledyne RDI 75 kilohertz (kHz) Ocean Surveyor ADCP would be
used to measure water current velocities, and acoustic pingers would be
used to retrieve OBSs. Take of marine mammals is not expected to occur
incidental to use of the MBES, SBP, and ADCP operations whether or not
the airguns are operating simultaneously with the other sources. Given
their characteristics (e.g., narrow downward-directed beam), marine
mammals would experience no more than one or two brief ping exposures,
if any exposure were to occur (Ruppel et al., 2022).
Proposed mitigation, monitoring, and reporting measures are
described in detail later in this document (please see Proposed
Mitigation and Proposed Monitoring and Reporting).

Description of Marine Mammals in the Area of Specified Activities

Sections 3 and 4 of the application summarize available information
regarding status and trends, distribution and habitat preferences, and
behavior and life history of the potentially affected species. NMFS
fully considered all of this information, and we refer the reader to
these descriptions, instead of reprinting the information. Additional
information regarding population trends and threats may be found in
NMFS' Stock Assessment Reports (SARs; <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments</a>) and
more general information about these species (e.g., physical and
behavioral descriptions) may be found on NMFS' website (<a href="https://www.fisheries.noaa.gov/find-species">https://www.fisheries.noaa.gov/find-species</a>).
L-DEO's proposed action area is approximately 230 km outside of the
U.S. EEZ. For marine mammal populations whose range includes both U.S.
jurisdictional waters and international waters where L-DEO's survey is
proposed to occur, table 1 summarizes information related to the stock,
including regulatory status under the MMPA and Endangered Species Act
(ESA), stock abundance, and potential biological removal (PBR), where
known (as described in NMFS' SARs). While no serious injury or
mortality is anticipated or proposed to be authorized here, PBR and
annual serious injury and mortality (M/SI) from anthropogenic sources
are included here as gross indicators of the status of the species or
stocks and other threats. All values presented in table 1 for stocks
that are assessed in the SARs are the most recent available at the time
of publication and are available in the draft 2024 SARs (available
online at: <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/draft-marine-mammal-stock-assessment-reports">https://www.fisheries.noaa.gov/national/marine-mammal-protection/draft-marine-mammal-stock-assessment-reports</a>).
Table 1 also includes a modeled abundance, which is the total
number of individuals estimated within the U.S Navy Atlantic Fleet
Training and Testing Area Marine Mammal Density (AFTT) study area. The
AFTT model is considered the best scientific information available on
the abundance and density of marine mammal populations that may occur
in the survey area.

Table 1--Species \1\ With Estimated Take From the Specified Activities
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ESA/ MMPA Stock abundance
status; (CV, Nmin, most Modeled Annual M/
Common name Scientific name Stock strategic (Y/ recent abundance abundance \4\ PBR SI \5\
N) \2\ survey) \3\
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Order Artiodactyla--Cetacea--Mysticeti (baleen whales)
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Family Balaenopteridae
(rorquals):
Fin Whale.................... Balaenoptera Western N Atlantic.. E, D, Y 6,802 (0.24, 5,573, \6\ 11,672 11 2.05
physalus. 2021).
Blue Whale................... Balaenoptera Western N Atlantic.. E, D, Y UNK (UNK, 402, 191 0.8 0
musculus. 2008) \7\.
Brydes Whale................. Balaenoptera edeni.. N/A................. -, -, N N/A................ 536 N/A N/A
Sei Whale.................... Balaenoptera Nova Scotia......... E, D, Y 6,292 (1.02, 3,098, \8\ 19,503 6.2 0.6
borealis. 2021).
Minke Whale.................. Balaenoptera Canadian Eastern -, -, N 21,968 (0.31, 13,784 170 9.4
acutorostrata. Coastal. 17,002, 2021).
Humpback Whale............... Megaptera Gulf of Maine....... -, -, N 1,396 (0, 1380, \9\ 3,569 22 12.15
novaeangliae. 2016).
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Odontoceti (toothed whales, dolphins, and porpoises)
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Family Physeteridae:
Sperm Whale.................. Physeter N Atlantic.......... E, D, Y 5,895 (0.29, 4,639, 64,015 9.28 0.2
macrocephalus:. 2021).
Family Kogiidae:
Dwarf Sperm Whale............ Kogia sima.......... Western N Atlantic.. -, -, N \10\ 9,474 (0.36, \10\ 26,043 57 unk
7,080, 2021).

[[Page 18027]]

Pygmy Sperm Whale............ Kogia breviceps..... Western N Atlantic.. -, -, N \10\ 9,474 (0.36, \10\ 26,043 57 unk
7,080, 2021).
Family Ziphiidae (beaked whales):
Blainville's Beaked Whale.... Mesoplodon Western N Atlantic.. -, -, N 2,936 (0.26, 2,374, \11\ 65,069 24 0
densirostris. 2021).
Goose-Beaked Whale........... Ziphius cavirostris. Western N Atlantic.. -, -, N 4,260 (0.24, 3,817, \11\ 65,069 38 0.2
2021).
Gervais' Beaked Whale........ Mesoplodon europaeus Western N Atlantic.. -, -, N 8,595 (0.24, 7,022, \11\ 65,069 70 0
2021).
True's Beaked Whale.......... Mesoplodon mirus.... Western N Atlantic.. -,-,N 4,480 (0.34, 3,391, \11\ 65,069 34 0.2
2021).
Family Delphinidae:
Rough-Toothed Dolphin........ Steno bredanensis... Western N Atlantic.. -, -, N \12\ UNK (UNK, UNK, 32,848 UND 0
2021).
Bottlenose Dolphin........... Tursiops truncatus.. Western N Atlantic -, -, N \13\ 64,587 (0.24, 418,151 507 28
Offshore. 52,801, 2021).
Atlantic White-Sided Dolphin. Lagenorhynchus Western N Atlantic.. -, -, N 93,233 (0.71, \14\ 175,299 544 28
acutus. 54,443, 2021).
Pantropical Spotted Dolphin.. Stenella attenuata.. Western N Atlantic.. -, D, N 2,757 (0.50, 1,856, 321,740 19 0
2021).
Atlantic Spotted Dolphin..... Stenella frontalis.. Western N Atlantic.. -, -, N 31,506 (0.28, 259,519 250 0
25,042, 2021).
Spinner Dolphin.............. Stenella Western N Atlantic.. -, D, N 3,181 (0.65, 1,930, 152,511 19 0
longirostris. 2021).
Clymene Dolphin.............. Stenella clymene.... Western N Atlantic.. -, -, N 21,778 (0.72, 181,209 126 0
12,622, 2021).
Striped Dolphin.............. Stenella Western N Atlantic.. -, -, N 48,274 (0.29, 412,729 529 0
coeruleoalba. 38,040, 2021).
Fraser's Dolphin............. Lagenodelphis hosei. Western N Atlantic.. -, -, N \15\ UNK (UNK, UNK, 19,585 UNK 0
2021).
Risso's Dolphin.............. Grampus griseus..... Western N Atlantic.. -, -, N 44,067 (0.19, 78,205 307 18
30,662, 2021).
Common Dolphin............... Delphinus delphis... Western N Atlantic.. -, -, N 93,100 (0.56, \16\ 473,260 1,452 414
59,897, 2021).
Melon-Headed Whale........... Peponocephala Western N Atlantic.. -, -, N \17\ UNK (UNK, UNK, 64,114 UNK 0
electra. 2021).
Pygmy Killer Whale........... Feresa attenuata.... Western N Atlantic.. -, -, N \18\ UNK (UNK, UNK, 9,001 UNK 0
2021).
False Killer Whale........... Pseudorca crassidens Western N Atlantic.. -, -, N \19\ 1,298 (0.72, 12,682 7.6 0
775, 2021).
Killer Whale................. Orcinus orca........ Western N Atlantic.. -, -, N \20\ UNK (UNK, UNK, 191 ......... .........
2016).
Short-Finned Pilot Whale..... Globicephala Western N Atlantic.. -, -, Y \21\ 18,726 (0.33, \22\ 264,907 143 218
macrorhynchus. 14,292, 2021).
Long-Finned Pilot Whale...... Globicephala melas.. Western N Atlantic.. -, -, N \23\ 39,215 (0.30, \22\ 264,907 306 5.7
30,627, 2021).
Family Phocoenidae (porpoises):
Harbor Porpoise.............. Phocoena phocoena... Gulf of Maine/Bay of -, -, N 85,765 (0.53, 94,583 649 142.4
Fundy. 56,420, 2021).
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\1\ Information on the classification of marine mammal species can be found on the web page for The Society for Marine Mammalogy's Committee on Taxonomy
(https:www//<a href="http://marinemammalscience.org/science-and-publications/list-marine-mammal-species-subspecies/">marinemammalscience.org/science-and-publications/list-marine-mammal-species-subspecies/</a>).
\2\ ESA status: Endangered (E), Threatened (T)/MMPA status: Depleted (D). A dash (-) indicates that the species is not listed under the ESA or
designated as depleted under the MMPA. Under the MMPA, a strategic stock is one for which the level of direct human-caused mortality exceeds PBR or
which is determined to be declining and likely to be listed under the ESA within the foreseeable future. Any species or stock listed under the ESA is
automatically designated under the MMPA as depleted and as a strategic stock.
\3\ NMFS marine mammal stock assessment reports online at: <a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports-region">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports-region</a>. CV is coefficient of variation; Nmin is the minimum estimate of stock abundance. In some cases, CV is not applicable.
\4\ Mean annual abundance for the U.S. Navy AFTT area, including the offshore survey area (based on Mannocci et al., 2017; Roberts et al., 2023; and
Marine Geospatial Ecology Lab 2023).
\5\ These values, found in NMFS's SARs, represent annual levels of human-caused mortality plus serious injury from all sources combined (e.g.,
commercial fisheries, vessel strike). Annual M/SI often cannot be determined precisely and is in some cases presented as a minimum value or range.
\6\ Mean abundance for August, 1992-2020 (mean abundance for September was lower).
\7\ Population estimate based upon photo-identification data collected from 1980 to 2008. An updated estimate of 39 blue whales exists based upon
shipboard and aerial surveys conducted from June to September 2016, however this estimate only includes the U.S. portion of the stock. Therefore, as
the estimate of 402 represents the entire stock, it is currently considered the best estimate.
\8\ Mean abundance for March to September, 1992-2020.
\9\ Mean abundance for April-November, 1992-2020.
\10\ Estimates for Kogia spp.
\11\ Mean abundance for all beaked whales, 1992-2020.
\12\ The abundance estimate for this species is based upon the average of the 2011 and 2016 abundance estimates. However, uncertainties in the abundance
estimate exist due to the low number of sightings (n=1 in 2011; n=0 in 2016), variance in encounter rates, and uncertainty in estimation of detection
probability.
\13\ Estimates may include sightings of the coastal form.
\14\ Mean abundance for September, 1992-2020. (Mean abundance for August was lower).
\15\ The total number of Fraser's dolphins off the eastern U.S coast is unknown. Present data are insufficient to calculate a minimum population
estimate for this stock.
\16\ Mean abundance for short-beaked common dolphin, 1992-2020.
\17\ The population size of this species is unknown as this species was rarely sighted during surveys. Present data are insufficient to calculate a
minimum population estimate for this stock.
\18\ The total number of pygmy killer whales off the eastern U.S coast is unknown. Present data are insufficient to calculate a minimum population
estimate for this stock.
\19\ Population estimate based upon abundance estimates of 753 (CV=1.13) and 545 (CV=0.68) generated from vessel surveys conducted in U.S. waters of the
western North Atlantic during the summer of 2021.
\20\ The total number of killer whales off the eastern U.S coast is unknown. Present data are insufficient to calculate a minimum population estimate
for this species.

[[Page 18028]]

\21\A key uncertainty exists in the population size estimate for this species based upon the assumption that the logistic regression model accurately
represents the relative distribution of short-finned vs. long-finned pilot whales.
\22\ Globicephala spp., 1992-2020.
\23\ Key uncertainties exist in the population size estimate for this species, including uncertain separation between short-finned and long-finned pilot
whales, small negative bias due to lack of abundance estimate in the region between US and the Newfoundland/Labrador survey area, and uncertainty due
to unknown precision and accuracy of the availability bias correction factor that was applied.

As indicated above, all 31 species in table 1 temporally and
spatially co-occur with the activity to the degree that take could
occur. All species that could occur in the proposed survey areas are
included in table 3 of the IHA application. While North Atlantic right
whales occur in continental shelf waters of the eastern United States
(U.S.), the spatial occurrence of these species is such that take is
not expected to occur, and they are not discussed further beyond the
explanation provided here. North Atlantic right whales mainly occur
within 90 km of shore and in water depths less than 45 m (Knowlton et
al., 2002). As the proposed survey will take place 600 km from shore in
water depths greater than 1,000 m, North Atlantic right whales are not
expected to be encountered.
In addition to what is included in sections 3 and 4 of the
application and NMFS' website, further detail informing the baseline
and regional occurrence for select species of particular or unique
vulnerability (i.e., information regarding ESA listed species) is
provided below.
Fin Whale--The fin whale is widely distributed in all the world's
oceans (Gambell 1985), although it is most abundant in temperate and
cold waters (Aguilar and Garc[iacute]a-Vernet 2018). Nonetheless, its
overall range and distribution are not well known (Jefferson et al.,
2015). Fin whales most commonly occur offshore but can also be found in
coastal areas (Jefferson et al., 2015). Most populations migrate
seasonally between temperate waters where mating and calving occur in
winter, and polar waters where feeding occurs in summer (Aguilar and
Garc[iacute]a-Vernet 2018).
In the North Atlantic, fin whales are found in summer from Baffin
Bay, Spitsbergen, and the Barents Sea, south to North Carolina and the
coast of Portugal (Rice 1998). In winter, they have been sighted from
Newfoundland to the Gulf of Mexico and the Caribbean, and from the
Faroes and Norway south to the Canary Islands (Rice 1998). Based on
geographic differences in fin whale calls, Delarue et al. (2014)
suggested that there are four distinct stocks in the Northwest
Atlantic, including a central North Atlantic stock that extends south
along the Mid-Atlantic Ridge. The four feeding stocks in the Northwest
Atlantic currently recognized by the North Atlantic Marine Mammal
Commission (NAMMCO 2023) are located off West Iceland (in the Central
Atlantic), Eastern Greenland, Western Greenland, and Eastern Canada;
there are an additional three stocks in the eastern Atlantic.
In the western North Atlantic, fin whales occur off the eastern
U.S. year-round, but generally north of Cape Hatteras (Davis et al.,
2020; Hayes et al., 2024). During winter, fin whales are sighted more
frequently on the shelf off the U.S. east coast than any other large
whale (Department of the Navy (DoN) 2008a, b). Fin whales have been
detected acoustically off North Carolina during all seasons, with the
greatest number of detections during winter (Davis et al., 2020; Palka
et al., 2021; Kowarski et al., 2022; Passive Acoustic Cetacean Map
2025). South of North Carolina, fin whales have only been detected
acoustically on the shelf during fall and winter (Davis et al., 2020;
Palka et al., 2021; Kowarski et al., 2022) and in the offshore waters
of the Blake Plateau from fall through spring (Palka et al., 2021;
Kowarski et al., 2022). Acoustic detections have been made just west of
the proposed survey area from October through March (Kowarski et al.,
2022; PACM 2025), however there are no records in the Ocean
Biodiversity Information System (OBIS) database for the proposed survey
area (OBIS 2025).
Blue Whale--The blue whale has a cosmopolitan distribution and
tends to be pelagic, only coming nearshore to feed and possibly to
breed (Jefferson et al., 2015). The distribution of the species, at
least during times of the year when feeding is a major activity, occurs
in areas that provide large seasonal concentrations of euphausiids
(Yochem and Leatherwood 1985). Blue whales are most often found in
cool, productive waters where upwelling occurs (Reilly and Thayer
1990). Generally, blue whales are seasonal migrants between high
latitudes in summer, where they feed, and low latitudes in winter,
where they mate and give birth (Lockyer and Brown 1981). Their summer
range in the North Atlantic extends from Davis Strait, Denmark Strait,
and the waters north of Svalbard and the Barents Sea, south to the Gulf
of St. Lawrence and the Bay of Biscay (Rice 1998). Although the winter
range is mostly unknown, some occur near Cape Verde at that time of
year (Rice 1998).
In the western North Atlantic, blue whales have been detected
acoustically off North Carolina during all seasons, with the greatest
number of detections during fall and winter (Davis et al., 2020; Palka
et al., 2021; PACM 2025). South of North Carolina, no acoustic
detections have been recorded during May and June (Kowarski et al.,
2022; PACM 2025). However, blue whales have been detected acoustically
in the deep waters of Blake Plateau from summer through winter (Palka
et al., 2021; Kowarski et al., 2022; PACM 2025). There are no records
of blue whales in the OBIS database for the proposed survey area (OBIS
2025).
Sei Whale--Sei whales are found in all ocean basins (Horwood 2018)
but appear to prefer mid-latitude temperate waters (Jefferson et al.,
2015). Habitat suitability models indicate that sei whale distribution
is related to cool water with high chlorophyll levels (Palka et al.,
2017; Chavez-Rosales et al., 2019). They occur in deeper waters
characteristic of the continental shelf edge region (Hain et al., 1985)
and in other regions of steep bathymetric relief such as seamounts and
canyons (Kenney and Winn 1987; Gregr and Trites 2001).
In the North Atlantic, there are three sei whale populations: Nova
Scotia, Iceland-Denmark Strait, and Eastern (Donovan 1991). They
undertake seasonal migrations to feed in subpolar latitudes during
summer and return to lower latitudes during winter to calve (Gambell
1985; Horwood 2018). A small number of individuals have been sighted in
the eastern North Atlantic between October and December, indicating
that some animals may remain at higher latitudes during winter (Evans
1992). Sei whales have been seen from South Carolina south into the
Gulf of Mexico and the Caribbean during winter (Rice 1998); however,
the location of sei whale wintering grounds in the North Atlantic is
unknown (V[iacute]kingsson et al., 2010).
Sei whales have been acoustically detected off North Carolina and
Blake Plateau mainly during winter (Davis et al., 2020; Palka et al.,
2021; Kowarski et al., 2022; PACM 2025). Fewer detections were made off
North Carolina during summer and fall (Davis et al., 2020; Palka et
al., 2021; Kowarski et al., 2022). Acoustic detections have been made
just west of the proposed survey area during November through February
(Kowarski et al., 2022; PACM 2025). There are no sightings in the OBIS
database for the proposed survey area; the closest sightings are
located ~120

[[Page 18029]]

km to the west and were made during January (OBIS 2025).
Sperm Whale--The sperm whale is widely distributed, occurring from
the edge of the polar pack ice to the Equator in both hemispheres, with
the sexes occupying different distributions (Whitehead 2018). Their
distribution and relative abundance can vary in response to prey
availability, most notably squid (Jaquet and Gendron 2002). Females
generally inhabit waters >1,000 m deep at latitudes <40[deg] where sea
surface temperatures are <15[deg] C; adult males move to higher
latitudes as they grow older and larger in size, returning to warm-
water breeding grounds (Whitehead 2018).
In the Northwest Atlantic, the shelf edge, oceanic waters,
seamounts, and canyon shelf edges are predicted habitats of sperm
whales (Waring et al., 2001). Off the U.S. coast east coast, they are
also known to concentrate in regions with well-developed temperature
gradients, such as along the edges of the Gulf Stream and warm core
rings, which may aggregate their primary prey, squid (Jaquet 1996).
Based on modeling, sperm whales are expected to occur throughout the
deeper offshore waters of the western North Atlantic (Mannocci et al.,
2017; Palka et al., 2021; Robertson et al., 2023). Numerous sightings
of sperm whales have been made off North Carolina from winter through
spring (DoN 2008a, b), and off Florida from winter through summer (DoN
2008c). Acoustic detections have also been made off North Carolina and
the western edge of the Blake Plateau, as well as in deeper water
offshore during most of the year (Stanistreet et al., 2018; Krowaski et
al., 2022). In addition to whaling records, there are 14 sighting
records in the OBIS database for the proposed survey area; all were
made from April to July 2004-2005 (OBIS 2025).

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 (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). We note that the names of two
hearing groups and the generalized hearing ranges of all marine mammal
hearing groups have been recently updated (NMFS 2024) as reflected
below in table 2.

Table 2--Marine Mammal Hearing Groups
[NMFS 2024]
------------------------------------------------------------------------
Hearing group Generalized hearing range *
------------------------------------------------------------------------
Low-frequency (LF) cetaceans (baleen 7 Hz to 36 kHz.
whales).
High-frequency (HF) cetaceans 150 Hz to 160 kHz.
(dolphins, toothed whales, beaked
whales, bottlenose whales).
Very High-frequency (VHF) cetaceans 200 Hz to 165 kHz.
(true porpoises, Kogia, river
dolphins, Cephalorhynchid,
Lagenorhynchus cruciger & L.
australis).
Phocid pinnipeds (PW) (underwater) 40 Hz to 90 kHz.
(true seals).
Otariid pinnipeds (OW) (underwater) 60 Hz to 68 kHz.
(sea lions and fur seals).
------------------------------------------------------------------------
* Represents the generalized hearing range for the entire group as a
composite (i.e., all species within the group), where individual
species' hearing ranges may not be as broad. Generalized hearing range
chosen based on ~65 dB threshold from composite audiogram, previous
analysis in NMFS 2018, and/or data from Southall et al. (2007);
Southall et al. (2019). Additionally, animals are able to detect very
loud sounds above and below that ``generalized'' hearing range. Hz =
Hertz.

For more detail concerning these groups and associated frequency
ranges, please see NMFS (2024) for a review of available information.

Potential Effects of Specified Activities on Marine Mammals and Their
Habitat

This section provides a discussion of the ways in which components
of the specified activity may impact marine mammals and their habitat.
The Estimated Take of Marine Mammals section later in this document
includes a quantitative analysis of the number of individuals that are
expected to be taken by this activity. The Negligible Impact Analysis
and Determination section considers the content of this section, the
Estimated Take of Marine Mammals 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 hertz (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

[[Page 18030]]

distance of 1 m from the source (referenced to (re) 1 [mu]Pa) while the
received level is the SPL at the listener's position (re 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 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.
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 SLs (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 one 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 (ISO) 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 reverberated
environment.
Airgun arrays produce pulsed signals with energy in a frequency
range from about 10-2,000 Hz, with most energy

[[Page 18031]]

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 \1\--Anthropogenic sounds
cover a broad range of frequencies and sound levels and can have a
range of highly variable impacts on marine life, from none or minor to
potentially severe responses, depending on received levels, duration of
exposure, behavioral context, and various other factors. The potential
effects of underwater sound from active acoustic sources can
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.
---------------------------------------------------------------------------

\1\ Please refer to the information given previously Description
of Active Acoustic Sound Sources section 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
2013). 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 (AUD
INJ). 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 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, as 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

[[Page 18032]]

TTS measurements (see Southall et al., 2007, 2019), a TTS of 6 dB is
considered the minimum TS 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 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 TS approximates AUD INJ onset (Kryter et al., 1966; Miller
1974), while a 6-dB TS 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 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 sound exposure
level 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 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
193-195 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 exists 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

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that are highly motivated to remain in an area for feeding (Richardson
et al., 1995; 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; 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 species. 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.,
2007). 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, passive acoustic monitoring, 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 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,

[[Page 18034]]

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
2010). 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. (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 sound exposure level
(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 sonar (MFAS) 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 MFAS 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 states 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 arrays of large airguns
(considered to be 500

[[Page 18035]]

in\3\ (8,194 cc) 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.
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 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.

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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) 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 2010; 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 the Langseth 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 have 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; Erbe et al., 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 proposed 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, project 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

[[Page 18037]]

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).
The Langseth will travel at a speed of 4 kn (7.6 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, which is
much more common in both space and time than is geophysical survey
activity. Jensen and Silber (2004) summarized vessel strikes of large
whales worldwide from 1975 to 2003 and found that most collisions
occurred in the open ocean and involved large vessels (e.g., commercial
shipping). No such incidents were reported for geophysical survey
vessels during that time period.
It is possible for vessel strikes to occur while traveling at slow
speeds. For example, a hydrographic survey vessel traveling at low
speed (5.5 kn (10 kph)) while conducting mapping surveys off the
central California coast struck and killed a blue whale in 2009. The
State of California determined that the whale had suddenly and
unexpectedly surfaced beneath the hull, with the result that the
propeller severed the whale's vertebrae, and that this was an
unavoidable event. This strike represents the only such incident in
approximately 540,000 hours of similar coastal mapping activity (p =
1.9 x 10-6; 95 percent confidence interval = 0-5.5 x
10-6; NMFS 2013). In addition, an R/V reported a fatal
strike in 2011 of a dolphin in the Atlantic, demonstrating that it is
possible for strikes involving smaller cetaceans to occur. In that
case, the incident report indicated that an animal apparently was
struck by the vessel's propeller as it was intentionally swimming near
the vessel. While indicative of the type of unusual events that cannot
be ruled out, neither of these instances represents a circumstance that
would be considered reasonably foreseeable or that would be considered
preventable.
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 MFAS. MFAS 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 MFAS is
very different from airguns, and one should not assume that airguns
will cause the same effects as MFAS (including strandings).
To understand why military MFAS affects beaked whales differently
than airguns do, it is important to note the distinction between
behavioral sensitivity and susceptibility to AUD INJ. To understand the
potential for AUD INJ 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.,

[[Page 18038]]

these species have lower thresholds for these effects than other
hearing groups (NMFS 2018). 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 (AUD INJ), 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 mid-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 MFAS).
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 and 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 historical high-frequency hearing
group (beaked whales, sperm whales, and killer whales) are likely more
sensitive to lower frequencies 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 (Yoder 2002). 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 MFAS), 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 MFAS 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 MFAS 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, MFAS 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 mid-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 MFAS
and airgun signals and their relative potential to cause stranding
events or deaths at sea is the similarity between the MFAS 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., MFAS) 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

[[Page 18039]]

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 MFAS and the mid-frequency
sound component from airguns and the likelihood that MFAS 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 MFAS regarding the likely risk of
behaviorally-mediated injury. There is strong evidence for the
association of beaked whale stranding events with MFAS use, and
particularly detailed accounting of several events is available (e.g.,
a 2000 Bahamas stranding event for which investigators concluded that
MFAS use was responsible; Evans and England 2001). D'Amico et al.
(2009) reviewed 126 beaked whale mass stranding events over the period
from 1950 (from the time of development of modern MFAS 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 MFAS
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 MFAS 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 MFAS 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 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 MFAS (International Council for the Exploration of 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 MFAS 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 MFAS
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 this stranding event, which did
not involve beaked whales, concluded that use of a high-frequency
mapping system (12-kHz MBES) 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 Cuvier's
beaked whales, was contemporaneous with and reasonably associated
spatially with a 2002 seismic survey in the Gulf of California
conducted by L-DEO, as was the case for the 2007 Gulf of Cadiz seismic
survey discussed by Castellote and Llorens (also involving two Cuvier's
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 the authors that
this lack of evidence 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 mid-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.

Entanglement

Entanglements occur when marine mammals become wrapped around
cables, lines, nets, or other objects suspended in the water column.
During seismic operations, numerous cables, lines, and other objects
primarily associated with the airgun array and hydrophone streamers
will be towed behind the Langseth near the water's surface. However, we
are not aware of any cases of entanglement of marine mammals in seismic
survey equipment. No incidents of entanglement of marine mammals with
seismic survey gear have been documented in over 54,000 nautical miles
(100,000 km) of previous NSF-funded seismic surveys when observers were
aboard (e.g., Smultea and Holst 2003; Haley and Koski 2004; Holst 2004;
Smultea et al., 2004; Holst et al., 2005; Haley and Ireland 2006;
Scripps Institution of Oceanography and NSF 2006; Hauser et al., 2008;
Holst and Smultea 2008). Although entanglement with the streamer is
theoretically possible, it has not been documented during tens of
thousands of miles of NSF-sponsored seismic cruises or, to our
knowledge, during hundreds of thousands of miles of industrial seismic
cruises. There are relatively few deployed devices, and no interaction
between marine mammals and any such device has been recorded during
prior NSF surveys using the devices. There are no meaningful
entanglement risks posed by the proposed survey, and entanglement risks
are not discussed further in this document.

[[Page 18040]]

Anticipated Effects on Marine Mammal Habitat

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; Pena et al., 2013; Chapman and Hawkins 1969; Wardle et
al., 2001; Sara et al., 2007; 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 (Engas et al., 1996; Engas 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 this
survey, 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 (Andre et al., 2011; Sole et al.,
2013). Behavioral responses, such as inking and jetting, have also been
reported upon exposure to low-frequency sound (McCauley et al., 2000b;
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 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.

[[Page 18041]]

Notably, a more recently described 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 blast 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.,
2000b). 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 this survey 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 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 of Marine Mammals

This section provides an estimate of the number of incidental takes
proposed for authorization through the IHA, which informs NMFS'
consideration of ``small numbers,'' and the negligible impact
determinations.
Harassment is the only type of take expected to result from these
activities. Except with respect to certain activities not pertinent
here, section 3(18) of the

[[Page 18042]]

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).
Authorized takes would primarily be by Level B harassment, as use
of the acoustic source (i.e., airguns) has the potential to result in
disruption of behavioral patterns for individual marine mammals. There
is also some potential for AUD INJ (Level A harassment) to result,
primarily for low- and very high-frequency species because predicted
AUD INJ zones are larger than the zones for high-frequency species. AUD
INJ is unlikely to occur for high-frequency species. As described
previously, no serious injury or mortality is anticipated or proposed
to be authorized for this activity. Below we describe how the proposed
take numbers are estimated.
For acoustic impacts, generally speaking, we estimate take by
considering: (1) acoustic criteria above which NMFS believes there is
some reasonable potential for marine mammals to be behaviorally
harassed or incur some degree of AUD INJ; (2) the area or volume of
water that will be ensonified above these levels in a day; (3) the
density or occurrence of marine mammals within these ensonified areas;
and, (4) the number of days of activities. We note that while these
factors can contribute to a basic calculation to provide an initial
prediction of potential takes, additional information that can
qualitatively inform take estimates is also sometimes available (e.g.,
previous monitoring results or average group size). Below, we describe
the factors considered here in more detail and present the proposed
take estimates.

Acoustic Criteria

NMFS recommends the use of acoustic criteria that identify the
received level of underwater sound above which exposed marine mammals
would be reasonably expected to be behaviorally harassed (equated to
Level B harassment) or to incur AUD INJ of some degree (equated to
Level A harassment). We note that the criteria for AUD INJ, as well as
the names of two hearing groups, have been recently updated (NMFS 2024)
as reflected below in the Level A Harassment section.
Level B Harassment--Though significantly driven by received level,
the onset of behavioral disturbance from anthropogenic noise exposure
is also informed to varying degrees by other factors related to the
source or exposure context (e.g., frequency, predictability, duty
cycle, duration of the exposure, signal-to-noise ratio, distance to the
source), the environment (e.g., bathymetry, other noises in the area,
predators in the area), and the receiving animals (hearing, motivation,
experience, demography, life stage, depth) and can be difficult to
predict (e.g., Southall et al., 2007, 2021, Ellison et al., 2012).
Based on what the available science indicates and the practical need to
use a threshold based on a metric 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
behavioral harassment. NMFS generally predicts that marine mammals are
likely to be behaviorally harassed in a manner considered to be Level B
harassment when exposed to underwater anthropogenic noise above root-
mean-squared pressure received levels (RMS SPL) of 120 dB (referenced
to 1 micropascal (re 1 [mu]Pa)) for continuous (e.g., vibratory pile
driving, drilling) and above RMS SPL 160 dB re 1 [mu]Pa for non-
explosive impulsive (e.g., seismic airguns) or intermittent (e.g.,
scientific sonar) sources. Generally speaking, Level B harassment take
estimates based on these behavioral harassment thresholds are expected
to include any likely takes by TTS as, in most cases, the likelihood of
TTS occurs at distances from the source less than those at which
behavioral harassment is likely. TTS of a sufficient degree can
manifest as behavioral harassment, as reduced hearing sensitivity and
the potential reduced opportunities to detect important signals
(conspecific communication, predators, prey) may result in changes in
behavior patterns that would not otherwise occur. L-DEO's proposed
activity includes the use of impulsive seismic sources (i.e., airguns),
and therefore the RMS SPL threshold of 160 dB re 1 [mu]Pa is
applicable.
Level A harassment--NMFS' Updated Technical Guidance for Assessing
the Effects of Anthropogenic Sound on Marine Mammal Hearing (Version
3.0) (NMFS, 2024) identifies dual criteria to assess AUD INJ (Level A
harassment) to five different underwater marine mammal groups (based on
hearing sensitivity) as a result of exposure to noise from two
different types of sources (impulsive or non-impulsive). L-DEO's
proposed activity includes the use of impulsive seismic sources (i.e.,
airguns).
The 2024 Updated Technical Guidance criteria include both updated
thresholds and updated weighting functions for each hearing group. The
thresholds are provided in table 3. The references, analysis, and
methodology used in the development of the criteria are described in
NMFS' 2024 Updated Technical Guidance, which may be accessed at:
<a href="https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance-other-acoustic-tools">https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance-other-acoustic-tools</a>.

Table 3--Thresholds Identifying the Onset of Auditory Injury
----------------------------------------------------------------------------------------------------------------
AUD INJ onset acoustic thresholds * (received level)
Hearing group ------------------------------------------------------------------------
Impulsive Non-impulsive
----------------------------------------------------------------------------------------------------------------
Low-Frequency (LF) Cetaceans........... Cell 1: Lpk,flat: 222 dB; Cell 2: LE,LF,24h: 197 dB.
LE,LF,24h: 183 dB.
High-Frequency (HF) Cetaceans.......... Cell 3: Lpk,flat: 230 dB; Cell 4: LE,HF,24h: 201 dB.
LE,HF,24h: 193 dB.
Very High-Frequency (VHF) Cetaceans.... Cell 5: Lpk,flat: 202 dB; Cell 6: LE,VHF,24h: 181 dB.
LE,VHF,24h: 159 dB.
Phocid Pinnipeds (PW) (Underwater)..... Cell 7: Lpk,flat: 223 dB; Cell 8: LE,PW,24h: 195 dB.
LE,PW,24h: 183 dB.
Otariid Pinnipeds (OW) (Underwater).... Cell 9: Lpk,flat: 230 dB; Cell 10:LE,OW,24h: 199 dB.
LE,OW,24h: 185 dB.
----------------------------------------------------------------------------------------------------------------
* Dual metric criteria for impulsive sounds: Use whichever criteria results in the larger isopleth for
calculating AUD INJ onset. If a non-impulsive sound has the potential of exceeding the peak SPL criteria
associated with impulsive sounds, the PK SPL criteria are recommended for consideration for non-impulsive
sources.

[[Page 18043]]

Note: Peak SPL (Lp,0-pk) has a reference value of 1 [micro]Pa, and weighted cumulative sound exposure level
(LE,p) has a reference value of 1 [micro]Pa\2\s. In this table, criteria are abbreviated to be more reflective
of International Organization for Standardization standards (ISO 2017; ISO 2020). The subscript ``flat'' is
being included to indicate peak sound pressure are flat weighted or unweighted within the generalized hearing
range of marine mammals underwater (i.e., 7 Hz to 165 kHz). The subscript associated with cumulative sound
exposure level criteria indicates the designated marine mammal auditory weighting function (LF, HF, and VHF
cetaceans, and PW and OW pinnipeds) and that the recommended accumulation period is 24 hours. The LE,p
criteria could be exceeded in a multitude of ways (i.e., varying exposure levels and durations, duty cycle).
When possible, it is valuable for action proponents to indicate the conditions under which these criteria will
be exceeded.

Ensonified Area

Here, we describe operational and environmental parameters of the
activity that are used in estimating the area ensonified above the
acoustic thresholds, including SLs and transmission loss coefficient.
The ensonified area associated with Level A harassment is more
technically challenging to predict due to the need to account for a
duration component. Therefore, NMFS developed an optional User
Spreadsheet tool to accompany the 2024 Updated Technical Guidance that
can be used to relatively simply predict an isopleth distance for use
in conjunction with marine mammal density or occurrence to help predict
potential takes. We note that because of some of the assumptions
included in the methods underlying this optional tool, we anticipate
that the resulting isopleth estimates are typically going to be
overestimates of some degree, which may result in an overestimate of
potential take by Level A harassment. However, this optional tool
offers the best way to estimate isopleth distances when more
sophisticated modeling methods are not available or practical.
The proposed survey would entail the use of a 36-airgun array with
a total discharge volume of 6,600 in\3\ (108,155 cc) at a tow depth of
12 m. L-DEO's model results are used to determine the 160
dB<INF>RMS</INF> radius for the airgun source down to a maximum depth
of 2,000 m. Received sound levels have been predicted by L-DEO's model
(Diebold et al., 2010) as a function of distance from the 36-airgun
array. This modeling approach uses ray tracing for the direct wave
traveling from the array to the receiver and its associated source
ghost (reflection at the air-water interface in the vicinity of the
array), in a constant-velocity half-space (infinite homogeneous ocean
layer, unbounded by a seafloor). In addition, propagation measurements
of pulses from the 36-airgun array at a tow depth of 6 m have been
reported in deep water (~1,600 m), intermediate water depth on the
slope (~600-1,100 m), and shallow water (~50 m) in the Gulf of America
(Tolstoy et al., 2009; Diebold et al., 2010).
For deep and intermediate water cases, the field measurements
cannot be used readily to derive the harassment isopleths, as at those
sites the calibration hydrophone was located at a roughly constant
depth of 350-550 m, which may not intersect all the SPL isopleths at
their widest point from the sea surface down to the assumed maximum
relevant water depth (~2,000 m) for marine mammals. At short ranges,
where the direct arrivals dominate and the effects of seafloor
interactions are minimal, the data at the deep sites are suitable for
comparison with modeled levels at the depth of the calibration
hydrophone. At longer ranges, the comparison with the model--
constructed from the maximum SPL through the entire water column at
varying distances from the airgun array--is the most relevant.
In deep and intermediate water depths at short ranges, sound levels
for direct arrivals recorded by the calibration hydrophone and L-DEO
model results for the same array tow depth are in good alignment (see
figures 12 and 14 in Diebold et al., 2010). Consequently, isopleths
falling within this domain can be predicted reliably by the L-DEO
model, although they may be imperfectly sampled by measurements
recorded at a single depth. At greater distances, the calibration data
show that seafloor-reflected and sub-seafloor-refracted arrivals
dominate, whereas the direct arrivals become weak and/or incoherent
(see figures 11, 12, and 16 in Diebold et al., 2010). Aside from local
topography effects, the region around the critical distance is where
the observed levels rise closest to the model curve. However, the
observed sound levels are found to fall almost entirely below the model
curve. Thus, analysis of the Gulf of America calibration measurements
demonstrates that although simple, the L-DEO model is a robust tool for
conservatively estimating isopleths.
The proposed geophysical survey would acquire data with the 36-
airgun array at a tow depth of 12 m. For deep water (>1,000 m), we use
the deep-water radii obtained from L-DEO model results down to a
maximum water depth of 2,000 m for the airgun array.
L-DEO's modeling methodology is described in greater detail in L-
DEO's application. The estimated distances to the Level B harassment
isopleth for the proposed airgun configuration are shown in table 4.

Table 4--Predicted Radial Distances From the R/V Langseth Seismic Source to Isopleth Corresponding to Level B
Harassment Threshold
----------------------------------------------------------------------------------------------------------------
Predicted distances (in
Airgun configuration Tow depth Water depth m) to the Level B
(m) (m) harassment threshold
----------------------------------------------------------------------------------------------------------------
4 strings, 36 airguns, 6,600 in\3\ (108,155 cc)............ 12 >1,000 6,733
----------------------------------------------------------------------------------------------------------------

Table 5--Modeled Radial Distance to Isopleths Corresponding to Level A Harassment Thresholds *
----------------------------------------------------------------------------------------------------------------
Low frequency High frequency Very high frequency
cetaceans cetaceans cetaceans
----------------------------------------------------------------------------------------------------------------
MCS Survey
----------------------------------------------------------------------------------------------------------------
PTS SEL............................................... 468.7 0.2 0.9
PTS Peak.............................................. 28.3 13.6 268.3
----------------------------------------------------------------------------------------------------------------

[[Page 18044]]

OBS Survey
----------------------------------------------------------------------------------------------------------------
PTS SEL............................................... 117.2 0 0.2
PTS Peak.............................................. 28.3 13.6 268.3
----------------------------------------------------------------------------------------------------------------
* The largest distance (in bold) of the dual criteria (SELcum or Peak) was used to estimate threshold distances
and potential takes by Level A harassment.

Table 5 presents the modeled Level A harassment isopleths for each
marine mammal hearing group based on L-DEO modeling incorporated in the
companion user spreadsheet, for the low-energy surveys with the
shortest shot interval (i.e., greatest potential to cause PTS based on
accumulated sound energy) (NMFS 2018, 2024).
Predicted distances to Level A harassment isopleths, which vary
based on marine mammal hearing groups, were calculated based on
modeling performed by L-DEO using the Nucleus software program and the
NMFS user spreadsheet, described below. The acoustic thresholds for
impulsive sounds contained in the NMFS Technical Guidance were
presented as dual metric acoustic thresholds using both
SEL<INF>cum</INF> and peak sound pressure metrics (NMFS 2016). As dual
metrics, NMFS considers onset of PTS (Level A harassment) to have
occurred when either one of the two metrics is exceeded (i.e., metric
resulting in the largest isopleth). The SEL<INF>cum</INF> metric
considers both level and duration of exposure, as well as auditory
weighting functions by marine mammal hearing group.
The SEL<INF>cum</INF> for the 36-airgun array is derived from
calculating the modified farfield signature. The farfield signature is
often used as a theoretical representation of the SL. To compute the
farfield signature, the SL is estimated at a large distance (right)
below the array (e.g., 9 km), and this level is back projected
mathematically to a notional distance of 1 m from the array's
geometrical center. However, it has been recognized that the SL from
the theoretical farfield signature is never physically achieved at the
source when the source is an array of multiple airguns separated in
space (Tolstoy et al., 2009). Near the source (at short ranges,
distances <1 km), the pulses of sound pressure from each individual
airgun in the source array do not stack constructively as they do for
the theoretical farfield signature. The pulses from the different
airguns spread out in time such that the SLs observed or modeled are
the result of the summation of pulses from a few airguns, not the full
array (Tolstoy et al., 2009). At larger distances, away from the source
array center, sound pressure of all the airguns in the array stack
coherently, but not within one time sample, resulting in smaller SLs (a
few dB) than the SL derived from the far-field signature. Because the
far-field signature does not take into account the large array effect
near the source and is calculated as a point source, the far-field
signature is not an appropriate measure of the sound SL for large
arrays. See L-DEO's application for further detail on acoustic
modeling.
AUD INJ is unlikely to occur for high-frequency cetaceans, given
the very small modeled zones of injury for those species in the context
of distributed source dynamics. The estimated zone is less than 15 m
for high-frequency species. In consideration of the received sound
levels in the near-field as described above, we expect the potential
for Level A harassment of high-frequency cetaceans to be de minimis,
even before the likely moderating effects of aversion and/or other
compensatory behaviors (e.g., Nachtigall et al., 2018) are considered.
We do not anticipate that Level A harassment is a likely outcome for
any high-frequency cetacean and do not propose to authorize any take by
Level A harassment for these species.
The Level A and Level B harassment estimates are based on a
consideration of the number of marine mammals that could be within the
area around the operating airgun array where received levels of sound
>=160 dB re 1 [micro]Pa RMS are predicted to occur. The estimated
numbers are based on the densities (numbers per unit area) of marine
mammals expected to occur in the area in the absence of seismic
surveys. To the extent that marine mammals tend to move away from
seismic sources before the sound level reaches the criterion level and
tend not to approach an operating airgun array, these estimates likely
overestimate the numbers actually exposed to the specified level of
sound.

Marine Mammal Occurrence

In this section we provide information about the occurrence of
marine mammals, including density or other relevant information which
will inform the take calculations.
L-DEO used habitat-based stratified marine mammal densities for the
North Atlantic from the US Navy Atlantic Fleet Training and Testing
Area Marine Mammal Density (Roberts et al., 2023; Mannocci et al.,
2017), which represent the best available information regarding marine
mammal densities in the region. This density information incorporates
visual line-transect surveys of marine mammals for over 35 years,
resulting in various studies that estimated the abundance, density, and
distributions of marine mammal populations. The habitat-based density
models consisted of 10 km x 10 km grid cells. Densities in the grid
cells for the AFTT study area overlapping with the proposed survey area
were averaged. More information is available online at <a href="https://seamap.env.duke.edu/models/Duke/AFTT/">https://seamap.env.duke.edu/models/Duke/AFTT/</a>. The range of most populations
extends past the coverage of the model.
For most species, only annual densities were available. For some
species, seasonal or monthly densities were available; thus, densities
that overlapped the timing of the proposed survey (i.e., July through
September) or the highest mean monthly density during the proposed
survey months were used.

Take Estimation

Here we describe how the information provided above is synthesized
to produce a quantitative estimate of the take that is reasonably
likely to occur and proposed for authorization.
In order to estimate the number of marine mammals predicted to be
exposed to sound levels that would result in Level A or Level B
harassment, radial distances from the airgun array to the predicted
isopleth corresponding to the Level A harassment and Level B harassment
thresholds are calculated, as described above. Those radial distances
were then used to calculate the area(s)

[[Page 18045]]

around the airgun array predicted to be ensonified to sound levels that
exceed the harassment thresholds. The distance for the 160-dB Level B
harassment threshold and AUD INJ (Level A harassment) thresholds (based
on L-DEO model results) was used to draw a buffer around the area
expected to be ensonified (i.e., the survey area). The ensonified areas
were then increased by 25 percent to account for potential delays,
which is equivalent to adding 25 percent to the proposed line distance
to be surveyed. The density for each species was then multiplied by the
daily ensonified areas (increased as described above) and then
multiplied by the number of survey days (4 days for MCS survey and 16
days for OBS survey) to estimate potential takes (see appendix B of L-
DEO's application for more information).
L-DEO assumed that their estimates of marine mammal exposures above
harassment thresholds equate to take and requested authorization of
those takes. Those estimates in turn form the basis for our proposed
take authorization numbers. Based on the nature of the activity and due
to the unlikelihood of the calculated Level A harassment exposures for
high-frequency species because of the small Level A harassment zones
and the need for individuals to stay in the Level A harassment zone for
24-hours to incur AUD INJ, Level A harassment is neither anticipated
nor proposed to be authorized. For some species, we have added L-DEO's
estimated exposures above Level A harassment thresholds to their
estimated exposures above the Level B harassment threshold to produce a
total number of incidents of take by Level B harassment that is
proposed for authorization. Estimated exposures and proposed take
numbers for authorization are shown in table 6.

Table 6--Estimated Take Proposed for Authorization
----------------------------------------------------------------------------------------------------------------
Estimated take Proposed authorized take Population
Species ---------------------------------------------------- abundance Percent of
A B A B \1\ population \2\
----------------------------------------------------------------------------------------------------------------
Fin whale..................... 0 1 0 1 11,672 0.01
Blue whale.................... 0 2 0 2 191 0.80
Bryde's whale................. 1 6 1 6 536 1.24
Sei whale..................... 1 23 1 23 19,530 0.12
Minke whale................... 2 57 2 57 13,784 0.43
Humpback whale................ 0 0 0 \3\ 2 3,569 0.05
Sperm whale................... 1 273 0 274 64,015 0.43
Kogia spp.\4\................. 8 193 8 193 20,043 0.77
Beaked whales \5\............. 1 284 0 285 65,069 0.44
Rough-toothed dolphin......... 1 259 0 260 32,848 0.79
Bottlenose dolphin............ 2 904 0 906 418,151 0.22
Atlantic white-sided dolphin.. 0 0 0 \3\ 13 175,299 <0.01
Pantropical spotted dolphin... 5 2,298 0 2,303 321,740 0.72
Atlantic spotted dolphin...... 4 2,204 0 2,208 259,519 0.85
Spinner dolphin............... 3 1,263 0 1,266 152,511 0.83
Clymene dolphin............... 3 1,633 0 1,636 181,209 0.90
Striped dolphin............... 3 1,389 0 1,392 412,729 0.34
Fraser's dolphin.............. 0 223 0 223 19,585 1.77
Risso's dolphin............... 0 102 0 102 78,205 0.13
Common dolphin................ 1 578 0 579 473,260 0.12
Melon-headed whale............ 1 664 0 665 64,114 1.04
Pygmy killer whale............ 0 93 0 93 9,001 1.04
False killer whale............ 0 131 0 131 12,682 1.04
Killer whale.................. 0 2 0 \3\ 4 191 2.09
Pilot whales \6\.............. 2 934 0 936 264,907 0.35
Harbor porpoise............... 0 1 0 \3\ 3 94,583 <0.01
----------------------------------------------------------------------------------------------------------------
\1\ Modeled abundance (Roberts et al. 2023).
\2\ Requested take authorization is expressed as percent of population for the AFTT study area (Roberts et al.,
2023).
\3\ Proposed take increased to mean group size from AMAPPS (Palka et al., 2017 and 2021).
\4\ Includes pygmy sperm whale and dwarf sperm whale.
\5\ Includes goose-beaked whale, Gervais's beaked whale, Blainville's beaked whale, and True's beaked whale.
\6\ Includes short-finned pilot whale and long-finned pilot whale.

Proposed Mitigation

In order to issue an IHA under section 101(a)(5)(D) of the MMPA,
NMFS must set forth the permissible methods of taking pursuant to the
activity and other means of effecting the least practicable impact on
the species or stock and its habitat, paying particular attention to
rookeries, mating grounds, and areas of similar significance, and on
the availability of the species or stock for taking for certain
subsistence uses (the latter not applicable for this action). NMFS
regulations require applicants for incidental take authorizations to
include information about the availability and feasibility (economic
and technological) of equipment, methods, and manner of conducting the
activity or other means of effecting the least practicable adverse
impact upon the affected species or stocks, and their habitat (50 CFR
216.104(a)(11)).
In evaluating how mitigation may or may not be appropriate to
ensure the least practicable adverse impact on species or stocks and
their habitat, as well as subsistence uses where applicable, NMFS
considers two primary factors:
(1) The manner in which, and the degree to which, the successful
implementation of the measure(s) is expected to reduce impacts to
marine mammals, marine mammal species or stocks, and their habitat.
This considers the nature of the potential adverse impact being
mitigated (likelihood, scope, range). It further considers the
likelihood that the measure will be effective if implemented
(probability of accomplishing the mitigating result if implemented as
planned), the likelihood of effective implementation

[[Page 18046]]

(probability implemented as planned); and
(2) The practicability of the measures for applicant
implementation, which may consider such things as cost, and impact on
operations.
The proposed mitigation requirements described in the following
were proposed by L-DEO in its adequate and complete application or are
the result of subsequent coordination between NMFS and L-DEO. L-DEO has
agreed that all of the mitigation measures are practicable. NMFS has
fully reviewed the specified activities and the mitigation measures to
determine if the mitigation measures would result in the least
practicable adverse impact on marine mammals and their habitat, as
required by the MMPA, and has determined the proposed measures are
appropriate. NMFS describes these below as proposed mitigation
requirements, and has included them in the proposed IHA.

Vessel-Based Visual Mitigation Monitoring

Visual monitoring requires the use of trained observers (herein
referred to as visual protected species observers (PSOs)) to scan the
ocean surface for the presence of marine mammals. The area to be
scanned visually includes primarily the shutdown zone (SZ), within
which observation of certain marine mammals requires shutdown of the
acoustic source, a buffer zone, and to the extent possible depending on
conditions, the surrounding waters. The buffer zone means an area
beyond the SZ to be monitored for the presence of marine mammals that
may enter the SZ. During pre-start clearance monitoring (i.e., before
ramp-up begins), the buffer zone also acts as an extension of the SZ in
that observations of marine mammals within the buffer zone would also
prevent airgun operations from beginning (i.e., ramp-up). The buffer
zone encompasses the area at and below the sea surface from the edge of
the 0-500 m SZ, out to a radius of 1,000 m from the edges of the airgun
array (500-1,000 m). This 1,000-m zone (SZ plus buffer) represents the
pre-start clearance zone. Visual monitoring of the SZ and adjacent
waters (buffer plus surrounding waters) is intended to establish and,
when visual conditions allow, maintain zones around the sound source
that are clear of marine mammals, thereby reducing or eliminating the
potential for injury and minimizing the potential for more severe
behavioral reactions for animals occurring closer to the vessel. Visual
monitoring of the buffer zone is intended to (1) provide additional
protection to marine mammals that may be in the vicinity of the vessel
during pre-start clearance, and (2) during airgun use, aid in
establishing and maintaining the SZ by alerting the other visual
observer and crew of marine mammals that are outside of, but may
approach and enter, the SZ.
During survey operations (e.g., any day on which use of the airgun
array is planned to occur and whenever the airgun array is in the
water, whether activated or not), a minimum of two visual PSOs must be
on duty and conducting visual observations at all times during daylight
hours (i.e., from 30 minutes prior to sunrise through 30 minutes
following sunset). Visual monitoring of the pre-start clearance zone
must begin no less than 30 minutes prior to ramp-up and monitoring must
continue until 1 hour after use of the airgun array ceases or until 30
minutes past sunset. Visual PSOs shall coordinate to ensure 360[deg]
visual coverage around the vessel from the most appropriate observation
posts and shall conduct visual observations using binoculars and the
naked eye while free from distractions and in a consistent, systematic,
and diligent manner.
PSOs shall establish and monitor the SZ and buffer zone. These
zones shall be based upon the radial distance from the edges of the
airgun array (rather than being based on the center of the array or
around the vessel itself). During use of the airgun array (i.e.,
anytime airguns are active, including ramp-up), detections of marine
mammals within the buffer zone (but outside the SZ) shall be
communicated to the operator to prepare for the potential shutdown of
the airgun array. Visual PSOs will immediately communicate all
observations to the on duty acoustic PSO(s), including any
determination by the PSO regarding species identification, distance,
and bearing and the degree of confidence in the determination. Any
observations of marine mammals by crew members shall be relayed to the
PSO team. During good conditions (e.g., daylight hours; Beaufort sea
state (BSS) 3 or less), visual PSOs shall conduct observations when the
airgun array is not operating for comparison of sighting rates and
behavior with and without use of the airgun array and between
acquisition periods, to the maximum extent practicable.
Visual PSOs may be on watch for a maximum of 4 consecutive hours
followed by a break of at least 1 hour between watches and may conduct
a maximum of 12 hours of observation per 24-hour period. Combined
observational duties (visual and acoustic but not at same time) may not
exceed 12 hours per 24-hour period for any individual PSO.

Passive Acoustic Monitoring (PAM)

PAM means the use of trained personnel (sometimes referred to as
PAM operators, herein referred to as acoustic PSOs) to operate PAM
equipment to acoustically detect the presence of marine mammals.
Acoustic monitoring involves acoustically detecting marine mammals
regardless of distance from the source, as localization of animals may
not always be possible. Acoustic monitoring is intended to further
support visual monitoring (during daylight hours) in maintaining a SZ
around the sound source that is clear of marine mammals. In cases where
visual monitoring is not effective (e.g., due to weather, nighttime),
acoustic monitoring may be used to allow certain activities to occur,
as further detailed below.
PAM would take place in addition to the visual monitoring program.
Visual monitoring typically is not effective during periods of poor
visibility or at night and even with good visibility, is unable to
detect marine mammals when they are below the surface or beyond visual
range. Acoustic monitoring can be used in addition to visual
observations to improve detection, identification, and localization of
cetaceans. The acoustic monitoring would serve to alert visual PSOs (if
on duty) when vocalizing cetaceans are detected. It is only useful when
marine mammals vocalize, but it can be effective either by day or by
night and does not depend on good visibility. It would be monitored in
real time so that the visual observers can be advised when cetaceans
are detected.
The Langseth will use a towed PAM system, which must be monitored
by at a minimum one on duty acoustic PSO beginning at least 30 minutes
prior to ramp-up and at all times during use of the airgun array.
Acoustic PSOs may be on watch for a maximum of 4 consecutive hours
followed by a break of at least 1 hour between watches and may conduct
a maximum of 12 hours of observation per 24-hour period. Combined
observational duties (acoustic and visual but not at same time) may not
exceed 12 hours per 24-hour period for any individual PSO.
Survey activity may continue for 30 minutes when the PAM system
malfunctions or is damaged, while the PAM operator diagnoses the issue.
If the diagnosis indicates that the PAM system must be repaired to
solve the problem, operations may continue for an additional 10 hours
without acoustic

[[Page 18047]]

monitoring during daylight hours only under the following conditions:
<bullet> Sea state is less than or equal to BSS 4;
<bullet> No marine mammals (excluding delphinids) detected solely
by PAM in the SZ in the previous 2 hours;
<bullet> NMFS is notified via email as soon as practicable with the
time and location in which operations began occurring without an active
PAM system; and
<bullet> Operations with an active airgun array, but without an
operating PAM system, do not exceed a cumulative total of 10 hours in
any 24-hour period.

Establishment of Shutdown and Pre-Start Clearance Zones

A SZ is a defined area within which occurrence of a marine mammal
triggers mitigation action intended to reduce the potential for certain
outcomes (e.g., AUD INJ, disruption of critical behaviors). The PSOs
would establish a minimum SZ with a 500-m radius. The 500-m SZ would be
based on radial distance from the edge of the airgun array (rather than
being based on the center of the array or around the vessel itself).
With certain exceptions (described below), if a marine mammal appears
within or enters this zone, the airgun array would be shut down.
The pre-start clearance zone is defined as the area that must be
clear of marine mammals prior to beginning ramp-up of the airgun array
and includes the SZ plus the buffer zone. Detections of marine mammals
within the pre-start clearance zone would prevent airgun operations
from beginning (i.e., ramp-up).
The 500-m SZ is intended to be precautionary in the sense that it
would be expected to contain sound exceeding the injury criteria for
all cetacean hearing groups, (based on the dual criteria of
SEL<INF>cum</INF> and peak SPL), while also providing a consistent,
reasonably observable zone within which PSOs would typically be able to
conduct effective observational effort. Additionally, a 500-m SZ is
expected to minimize the likelihood that marine mammals will be exposed
to levels likely to result in more severe behavioral responses.
Although significantly greater distances may be observed from an
elevated platform under good conditions, we expect that 500 m is likely
regularly attainable for PSOs using the naked eye during typical
conditions. The pre-start clearance zone simply represents the addition
of a buffer to the SZ, doubling the SZ size during pre-clearance.
An extended SZ of 1,500 m must be enforced for all beaked whales, a
large whale with a calf, and groups of six or more large whales. No
buffer of this extended SZ is required, as NMFS concludes that this
extended SZ is sufficiently protective to mitigate harassment to these
groups.

Pre-Start Clearance and Ramp-Up

Ramp-up (sometimes referred to as ``soft start'') means the gradual
and systematic increase of emitted sound levels from an airgun array.
Ramp-up begins by first activating a single airgun of the smallest
volume, followed by doubling the number of active elements in stages
until the full complement of an array's airguns are active. Each stage
should be approximately the same duration, and the total duration
should not be less than approximately 20 minutes. The intent of pre-
start clearance observation (30 minutes) is to ensure no marine mammals
are observed within the pre-start clearance zone (or extended SZ, for
beaked whales, a large whale with a calf, and groups of six or more
large whales) prior to the beginning of ramp-up. During the pre-start
clearance period is the only time observations of marine mammals in the
buffer zone would prevent operations (i.e., the beginning of ramp-up).
The intent of ramp-up is to warn marine mammals of pending seismic
survey operations and to allow sufficient time for those animals to
leave the immediate vicinity prior to the sound source reaching full
intensity. A ramp-up procedure, involving a step-wise increase in the
number of airguns firing and total array volume until all operational
airguns are activated and the full volume is achieved, is required at
all times as part of the activation of the airgun array. All operators
must adhere to the following pre-start clearance and ramp-up
requirements:
<bullet> The operator must notify a designated PSO of the planned
start of ramp-up as agreed upon with the lead PSO; the notification
time should not be less than 60 minutes prior to the planned ramp-up in
order to allow the PSOs time to monitor the pre-start clearance zone
(and extended SZ) for 30 minutes prior to the initiation of ramp-up
(pre-start clearance);
<bullet> Ramp-ups shall be scheduled so as to minimize the time
spent with the source activated prior to reaching the designated run-
in;
<bullet> One of the PSOs conducting pre-start clearance
observations must be notified again immediately prior to initiating
ramp-up procedures and the operator must receive confirmation from the
PSO to proceed;
<bullet> Ramp-up may not be initiated if any marine mammal is
within the applicable shutdown or buffer zone. If a marine mammal is
observed within the pre-start clearance zone (or extended SZ, for
beaked whales, a large whale with a calf, and groups of six or more
large whales) during the 30 minute pre-start clearance period, ramp-up
may not begin until the animal(s) has been observed exiting the zones
or until an additional time period has elapsed with no further
sightings (15 minutes for small odontocetes, and 30 minutes for all
mysticetes and all other odontocetes, including sperm whales, beaked
whales, and large delphinids, such as pilot whales);
<bullet> Ramp-up shall begin by activating a single airgun of the
smallest volume in the array and shall continue in stages by doubling
the number of active elements at the commencement of each stage, with
each stage of approximately the same duration. Duration shall not be
less than 20 minutes. The operator must provide information to the PSO
documenting that appropriate procedures were followed;
<bullet> PSOs must monitor the pre-start clearance zone and
extended SZ during ramp-up, and ramp-up must cease and the source must
be shut down upon detection of a marine mammal within the applicable
zone. Once ramp-up has begun, detections of marine mammals within the
buffer zone do not require shutdown, but such observation shall be
communicated to the operator to prepare for the potential shutdown;
<bullet> Ramp-up may occur at times of poor visibility, including
nighttime, if appropriate acoustic monitoring has occurred with no
detections in the 30 minutes prior to beginning ramp-up. Airgun array
activation may only occur at times of poor visibility where operational
planning cannot reasonably avoid such circumstances;
<bullet> If the airgun array is shut down for brief periods (i.e.,
less than 30 minutes) for reasons other than implementation of
prescribed mitigation (e.g., mechanical difficulty), it may be
activated again without ramp-up if PSOs have maintained constant visual
and/or acoustic observation and no visual or acoustic detections of
marine mammals have occurred within the pre-start clearance zone (or
extended SZ, where applicable). For any longer shutdown, pre-start
clearance observation and ramp-up are required; and
<bullet> Testing of the airgun array involving all elements
requires ramp-up. Testing limited to individual source elements or
strings does not require ramp-up but does require pre-start clearance
of 30 minutes.

[[Page 18048]]

Shutdown

The shutdown of an airgun array requires the immediate de-
activation of all individual airgun elements of the array. Any PSO on
duty will have the authority to call for shutdown of the airgun array
if a marine mammal is detected within the applicable SZ. The operator
must also establish and maintain clear lines of communication directly
between PSOs on duty and crew controlling the airgun array to ensure
that shutdown commands are conveyed swiftly while allowing PSOs to
maintain watch. When both visual and acoustic PSOs are on duty, all
detections will be immediately communicated to the remainder of the on-
duty PSO team for potential verification of visual observations by the
acoustic PSO or of acoustic detections by visual PSOs. When the airgun
array is active (i.e., anytime one or more airguns is active, including
during ramp-up) and (1) a marine mammal appears within or enters the
applicable SZ and/or (2) a marine mammal (other than delphinids, see
below) is detected acoustically and localized within the applicable SZ,
the airgun array will be shut down. When shutdown is called for by a
PSO, the airgun array will be immediately deactivated and any dispute
resolved only following deactivation. Additionally, shutdown will occur
whenever PAM alone (without visual sighting), confirms presence of
marine mammal(s) in the SZ. If the acoustic PSO cannot confirm presence
within the SZ, visual PSOs will be notified but shutdown is not
required.
Following a shutdown, airgun activity would not resume until the
marine mammal has cleared the SZ. The animal would be considered to
have cleared the SZ if it is visually observed to have departed the SZ
(i.e., animal is not required to fully exit the buffer zone where
applicable), or it has not been seen within the SZ for 15 minutes for
small odontocetes or 30 minutes for all mysticetes and all other
odontocetes, including sperm whales, beaked whales, and large
delphinids, such as pilot whales.
The shutdown requirement is waived for specific genera of small
dolphins if an individual is detected within the SZ. The small dolphin
group is intended to encompass those members of the Family Delphinidae
most likely to voluntarily approach the source vessel for purposes of
interacting with the vessel and/or airgun array (e.g., bow riding).
This exception to the shutdown requirement applies solely the specific
genera of small dolphins (Delphinus, Lagenodelphis, Stenella, Steno,
and Tursiops).
We include this small dolphin exception because shutdown
requirements for these species under all circumstances represent
practicability concerns without likely commensurate benefits for the
animals in question. Small dolphins are generally the most commonly
observed marine mammals in the specific geographic region and would
typically be the only marine mammals likely to intentionally approach
the vessel. As described above, AUD INJ is extremely unlikely to occur
for high-frequency cetaceans (e.g., delphinids), as this group is
relatively insensitive to sound produced at the predominant frequencies
in an airgun pulse while also having a relatively high threshold for
the onset of AUD INJ (i.e., PTS).
A large body of anecdotal evidence indicates that small dolphins
commonly approach vessels and/or towed arrays during active sound
production for purposes of bow riding with no apparent effect observed
(e.g., Barkaszi et al., 2012, Barkaszi and Kelly 2018). The potential
for increased shutdowns resulting from such a measure would require the
Langseth to revisit the missed track line to reacquire data, resulting
in an overall increase in the total sound energy input to the marine
environment and an increase in the total duration over which the survey
is active in a given area. Although other high-frequency hearing
specialists (e.g., large delphinids) are no more likely to incur AUD
INJ than are small dolphins, they are much less likely to approach
vessels. Therefore, retaining a shutdown requirement for large
delphinids would not have similar impacts in terms of either
practicability for the applicant or corollary increase in sound energy
output and time on the water. We do anticipate some benefit for a
shutdown requirement for large delphinids in that it simplifies
somewhat the total range of decision-making for PSOs and may preclude
any potential for physiological effects other than to the auditory
system as well as some more severe behavioral reactions for any such
animals in close proximity to the Langseth.
Visual PSOs shall use best professional judgment in making the
decision to call for a shutdown if there is uncertainty regarding
identification (i.e., whether the observed marine mammal(s) belongs to
one of the delphinid genera for which shutdown is waived or one of the
species with a larger SZ).
L-DEO must implement shutdown if a marine mammal species for which
take was not authorized or a species for which authorization was
granted but the authorized takes have been met approaches the Level A
or Level B harassment zones. L-DEO must also implement the extended
1,500 m shutdown if any large whale (defined as a sperm whale or any
mysticete species) with a calf (defined as an animal less than two-
thirds the body size of an adult observed to be in close association
with an adult) and/or an aggregation of six or more large whales are
observed.

Vessel Strike Avoidance Mitigation Measures

Vessel personnel should use an appropriate reference guide that
includes identifying information on all marine mammals that may be
encountered. Vessel operators must comply with the below measures
except under extraordinary circumstances when the safety of the vessel
or crew is in doubt or the safety of life at sea is in question. These
requirements do not apply in any case where compliance would create an
imminent and serious threat to a person or vessel or to the extent that
a vessel is restricted in its ability to maneuver and, because of the
restriction, cannot comply.
Vessel operators and crews must maintain a vigilant watch for all
marine mammals and slow down, stop their vessel, or alter course, as
appropriate and regardless of vessel size, to avoid striking any marine
mammal. A single marine mammal at the surface may indicate the presence
of submerged animals in the vicinity of the vessel; therefore,
precautionary measures should always be exercised. A visual observer
aboard the vessel must monitor a vessel strike avoidance zone around
the vessel (separation distances stated below). Visual observers
monitoring the vessel strike avoidance zone may be third-party
observers (i.e., PSOs) or crew members, but crew members responsible
for these duties must be provided sufficient training to (1)
distinguish marine mammals from other phenomena and (2) broadly to
identify a marine mammal as a right whale, other whale (defined in this
context as sperm whales or baleen whales other than right whales), or
other marine mammals.
Vessel speeds must be reduced to 10 kn (18.5 kph) or less when
mother/calf pairs, pods, or large assemblages of cetaceans are observed
near a vessel. All vessels must maintain a minimum separation distance
of 100 m from sperm whales and all other baleen whales. All vessels
must, to the maximum extent practicable, attempt to maintain a minimum
separation distance of 50 m from all other marine

[[Page 18049]]

mammals, with an understanding that at times this may not be possible
(e.g., for animals that approach the vessel).
When marine mammals are sighted while a vessel is underway, the
vessel shall take action as necessary to avoid violating the relevant
separation distance (e.g., attempt to remain parallel to the animal's
course, avoid excessive speed or abrupt changes in direction until the
animal has left the area). If marine mammals are sighted within the
relevant separation distance, the vessel must reduce speed and shift
the engine to neutral, not engaging the engines until animals are clear
of the area. This does not apply to any vessel towing gear or any
vessel that is navigationally constrained.
NMFS conducted an independent evaluation of the proposed measures,
and has preliminarily determined that the proposed mitigation measures
provide the means of effecting the least practicable impact on the
affected species or stocks and their habitat, paying particular
attention to rookeries, mating grounds, and areas of similar
significance.

Proposed Monitoring and Reporting

In order to issue an IHA for an activity, section 101(a)(5)(D) of
the MMPA states that NMFS must set forth requirements pertaining to the
monitoring and reporting of such taking. The MMPA implementing
regulations at 50 CFR 216.104(a)(13) indicate that requests for
authorizations must include the suggested means of accomplishing the
necessary monitoring and reporting that will result in increased
knowledge of the species and of the level of taking or impacts on
populations of marine mammals that are expected to be present while
conducting the activities. Effective reporting is critical both to
compliance as well as ensuring that the most value is obtained from the
required monitoring.
Monitoring and reporting requirements prescribed by NMFS should
contribute to improved understanding of one or more of the following:
<bullet> Occurrence of marine mammal species or stocks in the area
in which take is anticipated (e.g., presence, abundance, distribution,
density);
<bullet> Nature, scope, or context of likely marine mammal exposure
to potential stressors/impacts (individual or cumulative, acute or
chronic), through better understanding of: (1) action or environment
(e.g., source characterization, propagation, ambient noise); (2)
affected species (e.g., life history, dive patterns); (3) co-occurrence
of marine mammal species with the activity; or (4) biological or
behavioral context of exposure (e.g., age, calving or feeding areas);
<bullet> Individual marine mammal responses (behavioral or
physiological) to acoustic stressors (acute, chronic, or cumulative),
other stressors, or cumulative impacts from multiple stressors;
<bullet> How anticipated responses to stressors impact either: (1)
long-term fitness and survival of individual marine mammals; or (2)
populations, species, or stocks;
<bullet> Effects on marine mammal habitat (e.g., marine mammal prey
species, acoustic habitat, or other important physical components of
marine mammal habitat); and,
<bullet> Mitigation and monitoring effectiveness.
The proposed monitoring and reporting requirements described in the
following were proposed by L-DEO in its adequate and complete
application and/or are the result of subsequent coordination between
NMFS and L-DEO. L-DEO has agreed to the requirements. NMFS describes
these below as requirements and has included them in the proposed IHA.
L-DEO must use dedicated, trained, and NMFS-approved PSOs. The PSOs
must have no tasks other than to conduct observational effort, record
observational data, and communicate with and instruct relevant vessel
crew with regard to the presence of marine mammals and mitigation
requirements. PSO resumes shall be provided to NMFS for approval.
At least one of the visual and two of the acoustic PSOs (discussed
below) aboard the vessel must have a minimum of 90 days at-sea
experience working in those roles, respectively, with no more than 18
months elapsed since the conclusion of the at-sea experience. One
visual PSO with such experience shall be designated as the lead for the
entire protected species observation team. The lead PSO shall serve as
primary point of contact for the vessel operator and ensure all PSO
requirements per the IHA are met. To the maximum extent practicable,
the experienced PSOs should be scheduled to be on duty with those PSOs
with appropriate training but who have not yet gained relevant
experience.

Vessel-Based Visual Monitoring

As described above, PSO observations would take place during
daytime airgun operations. During seismic survey operations, at least
five visual PSOs would be based aboard the Langseth. Two visual PSOs
would be on duty at all times during daytime hours. Monitoring shall be
conducted in accordance with the following requirements:
<bullet> The operator shall provide PSOs with bigeye binoculars
(e.g., 25 x 150; 2.7 view angle; individual ocular focus; height
control) of appropriate quality solely for PSO use. These shall be
pedestal-mounted on the deck at the most appropriate vantage point that
provides for optimal sea surface observation, PSO safety, and safe
operation of the vessel; and
<bullet> The operator will work with the selected third-party
observer provider to ensure PSOs have all equipment (including backup
equipment) needed to adequately perform necessary tasks, including
accurate determination of distance and bearing to observed marine
mammals.
PSOs must have the following requirements and qualifications:
<bullet> PSOs shall be independent, dedicated, trained visual and
acoustic PSOs and must be employed by a third-party observer provider;
<bullet> PSOs shall have no tasks other than to conduct
observational effort (visual or acoustic), collect data, and
communicate with and instruct relevant vessel crew with regard to the
presence of protected species and mitigation requirements (including
brief alerts regarding maritime hazards);
<bullet> PSOs shall have successfully completed an approved PSO
training course appropriate for their designated task (visual or
acoustic). Acoustic PSOs are required to complete specialized training
for operating PAM systems and are encouraged to have familiarity with
the vessel with which they will be working;
<bullet> PSOs can act as acoustic or visual observers (but not at
the same time) as long as they demonstrate that their training and
experience are sufficient to perform the task at hand;
<bullet> NMFS must review and approve PSO resumes accompanied by a
relevant training course information packet that includes the name and
qualifications (i.e., experience, training completed, or educational
background) of the instructor(s), the course outline or syllabus, and
course reference material as well as a document stating successful
completion of the course;
<bullet> PSOs must successfully complete relevant training,
including completion of all required coursework and passing (80 percent
or greater) a written and/or oral examination developed for the
training program;
<bullet> PSOs must have successfully attained a bachelor's degree
from an accredited college or university with a major in one of the
natural sciences, a

[[Page 18050]]

minimum of 30 semester hours or equivalent in the biological sciences,
and at least one undergraduate course in math or statistics; and
<bullet> The educational requirements may be waived if the PSO has
acquired the relevant skills through alternate experience. Requests for
such a waiver shall be submitted to NMFS and must include written
justification. Requests shall be granted or denied (with justification)
by NMFS within 1 week of receipt of submitted information. Alternate
experience that may be considered includes, but is not limited to (1)
secondary education and/or experience comparable to PSO duties; (2)
previous work experience conducting academic, commercial, or
government-sponsored protected species surveys; or (3) previous work
experience as a PSO; the PSO should demonstrate good standing and
consistently good performance of PSO duties.
<bullet> For data collection purposes, PSOs shall use standardized
electronic data collection forms. PSOs shall record detailed
information about any implementation of mitigation requirements,
including the distance of animals to the airgun array and description
of specific actions that ensued, the behavior of the animal(s), any
observed changes in behavior before and after implementation of
mitigation, and if shutdown was implemented, the length of time before
any subsequent ramp-up of the airgun array. If required mitigation was
not implemented, PSOs should record a description of the circumstances.
At a minimum, the following information must be recorded:
[cir] Vessel name, vessel size and type, maximum speed capability
of vessel;
[cir] Dates (MM/DD/YYYY) of departures and returns to port with
port name;
[cir] PSO names and affiliations, PSO ID (initials or other
identifier);
[cir] Date (MM/DD/YYYY) and participants of PSO briefings;
[cir] Visual monitoring equipment used (description);
[cir] PSO location on vessel and height (meters) of observation
location above water surface;
[cir] Watch status (description);
[cir] Dates (MM/DD/YYYY) and times (Greenwich Mean Time/UTC) of
survey on/off effort and times (GMC/UTC) corresponding with PSO on/off
effort;
[cir] Vessel location (decimal degrees) when survey effort began
and ended and vessel location at beginning and end of visual PSO duty
shifts;
[cir] Vessel location (decimal degrees) at 30-second intervals if
obtainable from data collection software, otherwise at practical
regular interval;
[cir] Vessel heading (compass heading) and speed (knots) at
beginning and end of visual PSO duty shifts and upon any change;

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