Takes of Marine Mammals Incidental to Specified Activities; Marine Geophysical Survey in the Central-Western Bering Sea, August 2011, 33246-33266 [2011-14136]

Agencies

• DEPARTMENT OF COMMERCE
• National Oceanic and Atmospheric Administration

[Federal Register Volume 76, Number 110 (Wednesday, June 8, 2011)]
[Notices]
[Pages 33246-33266]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-14136]


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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

[RIN 0648-XA430]


Takes of Marine Mammals Incidental to Specified Activities; 
Marine Geophysical Survey in the Central-Western Bering Sea, August 
2011

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

ACTION: Notice; proposed Incidental Harassment Authorization; request 
for comments.

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SUMMARY: NMFS has received an application from the U.S. Geological 
Survey (USGS) for an Incidental Harassment Authorization (IHA) to take 
marine mammals, by harassment, incidental to conducting a marine 
geophysical survey in the central-western Bering Sea, August 2011. 
Pursuant to the Marine Mammal Protection Act (MMPA), NMFS requests 
comments on its proposal to issue an IHA to USGS to incidentally 
harass, by Level B harassment only, 12 species of marine mammals during 
the specified activity.

DATES: Comments and information must be received no later than July 8, 
2011.

ADDRESSES: Comments on the application should be addressed to P. 
Michael Payne, Chief, Permits, Conservation and Education Division, 
Office of Protected Resources, National Marine Fisheries Service, 1315 
East-West Highway, Silver Spring, MD 20910. The mailbox address for 
providing email comments is ITP.Hopper@noaa.gov. NMFS is not 
responsible for e-mail comments sent to addresses other than the one 
provided here. Comments sent via email, including all attachments, must 
not exceed a 10-megabyte file size.
    All comments received are a part of the public record and will 
generally be posted to https://www.nmfs.noaa.gov/pr/permits/incidental.htm#applications without change. All Personal Identifying 
Information (for example, name, address, etc.) voluntarily submitted by 
the commenter may be publicly accessible. Do not submit confidential 
business information or otherwise sensitive or protected information.
    A copy of the application containing a list of the references used 
in this document may be obtained by writing to the above address, 
telephoning the contact listed here (see FOR FURTHER INFORMATION 
CONTACT) or visiting the internet at: https://www.nmfs.noaa.gov/pr/permits/incidental.htm#applications.
    The U.S. Geological Survey (USGS), which is providing funding for 
the proposed action, has prepared a draft ``Environmental Assessment 
(EA) of a Marine Geophysical Survey by the R/V MARCUS G. LANGSETH in 
the Central-Western Bering Sea, August 2011,'' prepared by LGL Ltd., 
Environmental Research Associates (LGL), on behalf of USGS, which is 
also available at the same internet address. Documents cited in this 
notice may be viewed, by appointment, during regular business hours, at 
the aforementioned address.

FOR FURTHER INFORMATION CONTACT: Brian D. Hopper or Jolie Harrison, 
Office of Protected Resources, NMFS, (301) 713-2289.

SUPPLEMENTARY INFORMATION:

Background

    Section 101(a)(5)(D) of the MMPA (16 U.S.C. 1371 (a)(5)(D)) directs 
the Secretary of Commerce (Secretary) to authorize, upon request, the 
incidental, but not intentional, taking of small numbers of marine 
mammals of a species or population stock, by United States citizens who 
engage in a specified activity (other than commercial fishing) within a 
specified geographical region if certain findings are made and, if the 
taking is limited to harassment, a notice of a proposed authorization 
is provided to the public for review.
    Authorization for the incidental taking of small numbers of marine 
mammals 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 subsistence uses (where relevant). The authorization must 
set forth the permissible methods of taking, other means of effecting 
the least practicable adverse impact on the species or stock and its 
habitat, and requirements pertaining to the mitigation, monitoring and 
reporting of such takings. NMFS has defined ``negligible impact'' in 50 
CFR 216.103 as ``* * * an impact resulting from the specified activity 
that cannot be reasonably expected to, and is not reasonably likely to, 
adversely affect the species or stock through effects on annual rates 
of recruitment or survival.''
    Section 101(a)(5)(D) of the MMPA established an expedited process 
by which citizens of the United States can apply for an authorization 
to incidentally take small numbers of marine mammals by harassment. 
Section 101(a)(5)(D) of the MMPA establishes a 45-day time limit for 
NMFS' review of an application followed by a 30-day public notice and 
comment period on any proposed authorizations for the incidental 
harassment of small numbers of marine mammals. Within 45 days of the 
close of the public comment period, NMFS must either issue or deny the 
authorization.
    Except with respect to certain activities not pertinent here, the 
MMPA defines ``harassment'' as:

    any act of pursuit, torment, or annoyance which (i) Has the 
potential to injure a marine mammal or marine mammal stock in the 
wild [Level A harassment]; or (ii) has the potential to disturb a 
marine mammal or marine mammal stock in the wild by causing 
disruption of behavioral patterns, including, but not limited to, 
migration, breathing, nursing, breeding, feeding, or sheltering 
[Level B harassment].

Summary of Request

    NMFS received an application on April 8, 2011, from USGS for the 
taking by harassment, of marine mammals, incidental to conducting a 
marine geophysical survey in the central-western Bering Sea within the 
U.S. Exclusive Economic Zone (EEZ) and adjacent international waters in 
depths greater than 3,000 m (9,842 ft). USGS plans to conduct the 
proposed survey from approximately August 7 to September 1, 2011.
    USGS plans to use one source vessel, the R/V MARCUS G. LANGSETH 
(LANGSETH) and a seismic airgun array to collect seismic reflection and 
refraction profiles to be used to delineate the U.S. Extended 
Continental Shelf (ECS) in the Bering Sea. In addition to the proposed 
operations of the seismic airgun array, USGS intends to operate a 
multibeam echosounder (MBES) and a sub-bottom profiler (SBP) 
continuously throughout the survey.
    Acoustic stimuli (i.e., increased underwater sound) generated 
during the operation of the seismic airgun array may have the potential 
to cause a short-term behavioral disturbance for marine mammals in the 
survey area. This is the

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principal means of marine mammal taking associated with these 
activities and USGS has requested an authorization to take 12 species 
of marine mammals by Level B harassment. Take is not expected to result 
from the use of the MBES or SBP, for reasons discussed in this notice; 
nor is take expected to result from collision with the vessel because 
it is a single vessel moving at a relatively slow speed during seismic 
acquisition within the survey, for a relatively short period of time 
(approximately 25 days). It is likely that any marine mammal would be 
able to avoid the vessel.

Description of the Specified Activity

    USGS's proposed seismic survey in the central-western Bering Sea is 
between approximately 350 to 800 kilometers (km) (189 to 432 nautical 
miles [nmi]) offshore in the area 55 to 58.5[deg] North, 177[deg] West 
to 175[deg] East (see Figure 1 of the IHA application). Water depths in 
the survey area are greater than 3,000 m (9,842 ft). The project is 
scheduled to occur from approximately August 7 to September 1, 2011. 
Some minor deviation from these dates is possible, depending on 
logistics and weather.
    The proposed seismic survey will collect seismic reflection and 
refraction profiles to be used to delineate the U.S. ECS in the Bering 
Sea. The ECS is the region beyond 200 nmi where a nation can show that 
it satisfies the conditions of Article 76 of the United Nations 
Convention on the Law of the Sea. One of the conditions in Article 76 
is a function of sediment thickness. The seismic profiles are designed 
to identify the stratigraphic ``basement'' and to map the thickness of 
the overlying sediments. Acoustic velocities (required to convert 
measured travel times to true depth) will be measured directly using 
sonobuoys and ocean-bottom seismometers (OBSs), as well as by analysis 
of hydrophone streamer data. Acoustic velocity refers to the velocity 
of sound through sediments or crust.
    The survey will involve one source vessel, the LANGSETH. The 
LANGSETH will deploy an array of 36 airguns as an energy source. The 
receiving system will consist of one 8 km (4.3 nmi) long hydrophone 
streamer and/or five OBSs. As the airgun is towed along the survey 
lines, the hydrophone streamer will receive the returning acoustic 
signals and transfer the data to the on-board processing system. The 
OBSs record the returning acoustic signals internally for later 
analysis.
    The planned seismic survey (e.g., equipment testing, startup, line 
changes, repeat coverage of any areas, and equipment recovery) will 
consist of approximately 2,420 km (1,306.7 nmi) of transect lines in 
the central-western Bering Sea survey area (see Figure 1 of the IHA 
application). The array will be powered-down to one 40 in \3\ airgun 
during turns. All of the survey will take place in water deeper than 
1,000 m (3,280.8 ft). A multi-channel seismic (MCS) survey using the 
hydrophone streamer will take place along 14 MCS profile lines and 3 
OBS lines. Following the MCS survey, 18 OBSs will be deployed and a 
refraction survey will take place along three of the 14 lines. If time 
permits, an additional 525 km (283.5 nmi) contingency line will be 
added to the MCS survey. In addition to the operations of the airgun 
array, a Kongsberg EM 122 MBES and Knudsen 320B SBP will also be 
operated from the LANGSETH continuously throughout the cruise. There 
will be additional seismic operations associated with equipment 
testing, start-up, and possible line changes or repeat coverage of any 
areas where initial data quality is sub-standard. In USGS's 
calculations, 25% has been added for those additional operations.
    All planned geophysical data acquisition activities will be 
conducted by Lamont-Doherty Earth Observatory (L-DEO), the LANGSETH's 
operator, with on-board assistance by the scientists who have proposed 
the study. The Principal Investigators are Drs. Jonathan R. Childs and 
Ginger Barth of the USGS. The vessel will be self-contained, and the 
crew will live aboard the vessel for the entire cruise.

Vessel Specifications

    The LANGSETH, owned by the National Science Foundation, will tow 
the 36 airgun array, as well as the hydrophone streamer, along 
predetermined lines. The LANGSETH will also deploy and retrieve the 
OBSs. When the LANGSETH is towing the airgun array and the hydrophone 
streamer, the turning rate of the vessel is limited to five degrees per 
minute. Thus, the maneuverability of the vessel is limited during 
operations with the streamer.
    The vessel has a length of 71.5 m (235 ft); a beam of 17.0 m (56 
ft); a maximum draft of 5.9 m (19 ft); and a gross tonnage of 3,834. 
The LANGSETH was designed as a seismic research vessel with a 
propulsion system designed to be as quiet as possible to avoid 
interference with the seismic signals emanating from the airgun array. 
The ship is powered by two 3,550 horsepower (hp) Bergen BRG-6 diesel 
engines which drive two propellers directly. Each propeller has four 
blades and the shaft typically rotates at 750 revolutions per minute. 
The vessel also has an 800 hp bowthruster, which is not used during 
seismic acquisition. The LANGSETH's operation speed during seismic 
acquisition is typically 7.4 to 9.3 km per hour (hr) (km/hr) (4 to 5 
knots [kts]). When not towing seismic survey gear, the LANGSETH 
typically cruises at 18.5 km/hr (10 kts). The LANGSETH has a range of 
25,000 km (13,499 nmi) (the distance the vessel can travel without 
refueling).
    The vessel also has an observation tower from which protected 
species visual observers (PSVO) will watch for marine mammals before 
and during the proposed airgun operations. When stationed on the 
observation platform, the PSVO's eye level will be approximately 21.5 m 
(71 ft) above sea level providing the PSVO an unobstructed view around 
the entire vessel.

Acoustic Source Specifications

Seismic Airguns

    The LANGSETH will deploy a 36 airgun array, with a total volume of 
approximately 6,600 cubic inches (in \3\). The airgun array will 
consist of a mixture of Bolt 1500LL and Bolt 1900LLX airguns ranging in 
size from 40 to 360 in \3\, with a firing pressure of 1,900 pounds per 
square inch. The airguns will be configured as four identical linear 
arrays or ``strings.'' Each string will have 10 airguns, the first and 
last airguns in the strings are spaced 16 m (52 ft) apart. Of the 10 
airguns, nine airguns in each string will be fired simultaneously, 
whereas the tenth is kept in reserve as a spare, to be turned on in 
case of failure of another airgun. The four airgun strings will be 
distributed across an area of approximately 24x16 m (78.7x52.5 ft) 
behind the LANGSETH and will be towed approximately 100 m (328 ft) 
behind the vessel. The shot interval will be 50 m (164 ft) or 
approximately 22 seconds (s) for the MCS survey and 150 m (492.1 ft) or 
approximately 66 s for the OBS refraction survey. The firing pressure 
of the array is 1,900 pounds per square inch (psi). During firing, a 
brief (approximately 0.1 s) pulse sound is emitted. The airguns will be 
silent during the intervening periods. The dominant frequency 
components range from two to 188 Hertz (Hz).
    The tow depth of the array will be 9 m (29.5 ft) during OBS 
refraction and MCS surveys. Because the actual source is a distributed 
sound source (36 airguns) rather than a single point source, the 
highest sound measurable at

[[Page 33248]]

any location in the water will be less than the nominal source level. 
In addition, the effective source level for sound propagating in near-
horizontal directions will be substantially lower than the nominal 
source level applicable to downward propagation because of the 
directional nature of the sound from the airgun array.

Metrics Used in This Document

    This section includes a brief explanation of the sound measurements 
frequently used in the discussions of acoustic effects in this 
document. Sound pressure is the sound force per unit area, and is 
usually measured in micropascals ([mu]Pa), where 1 pascal (Pa) is the 
pressure resulting from a force of one newton exerted over an area of 
one square meter. Sound pressure level (SPL) is expressed as the ratio 
of a measured sound pressure and a reference level. The commonly used 
reference pressure level in underwater acoustics is 1 [mu]Pa, and the 
units for SPLs are dB re: 1 [mu]Pa. SPL (in decibels [dB]) = 20 log 
(pressure/reference pressure).
    SPL is an instantaneous measurement and can be expressed as the 
peak, the peak-peak (p-p), or the root mean square (rms). Root mean 
square, which is the square root of the arithmetic average of the 
squared instantaneous pressure values, is typically used in discussions 
of the effects of sounds on vertebrates and all references to SPL in 
this document refer to the root mean square unless otherwise noted. SPL 
does not take the duration of a sound into account.

Characteristics of the Airgun Pulses

    Airguns function by venting high-pressure air into the water which 
creates an air bubble. The pressure signature of an individual airgun 
consists of a sharp rise and then fall in pressure, followed by several 
positive and negative pressure excursions caused by the oscillation of 
the resulting air bubble. The oscillation of the air bubble transmits 
sounds downward through the seafloor and the amount of sound 
transmitted in the near horizontal directions is reduced. However, the 
airgun array also emits sounds that travel horizontally toward non-
target areas.
    The nominal source levels of the airgun arrays used by USGS on the 
LANGSETH are 236 to 265 dB re 1 [mu]Pa (p-p) and the rms value for a 
given airgun pulse is typically 16 dB re 1 [mu]Pa lower than the peak-
to-peak value. However, the difference between rms and peak or peak-to-
peak values for a given pulse depends on the frequency content and 
duration of the pulse, among other factors.
    Accordingly, L-DEO has predicted the received sound levels in 
relation to distance and direction from the 36 airgun array and the 
single Bolt 1900LL 40 in\3\ airgun, which will be used during power-
downs. A detailed description of L-DEO's modeling for marine seismic 
source arrays for species mitigation is provided in Appendix A of 
USGS's application. These are the nominal source levels applicable to 
downward propagation. The effective source levels for horizontal 
propagation are lower than those for downward propagation when the 
source consists of numerous airguns spaced apart from one another.
    Appendix B of USGS's EA discusses the characteristics of the airgun 
pulses and marine mammals. NMFS refers the reviewers to the application 
and EA documents for additional information.

Predicted Sound Levels for the Airguns

    Tolstoy et al., (2009) reported results for propagation 
measurements of pulses from the LANGSETH's 36 airgun, 6,600 in\3\ array 
in shallow-water (approximately 50 m [164 ft]) and deep-water depths 
(approximately 1,600 m [5,249 ft]) in the Gulf of Mexico in 2007 and 
2008. L-DEO has used these reported empirical values to determine 
exclusion zones (EZs) for the 36 airgun array and the single airgun; to 
designate mitigation zones, and to estimate take for marine mammals.
    Results of the Gulf of Mexico calibration study (Tolstoy et al., 
2009) showed that radii around the airguns for various received levels 
varied with water depth. The empirical data for deep water (greater 
than 1,000 m; 3,280 ft) indicated that the L-DEO model (as applied to 
the LANGSETH's 36 airgun array) overestimated the received sound levels 
at a given distance.
    Using the corrected measurements (array) or model (single airgun), 
Table 1 (below) shows the distances at which three rms sound levels are 
expected to be received from the 36 airgun array and a single airgun. 
The 180 and 190 dB re 1 [micro]Pa (rms) distances are the safety 
criteria as specified by NMFS (2000) and are applicable to cetaceans 
and pinnipeds, respectively. If marine mammals are detected within or 
about to enter the appropriate EZ, the airguns will be powered-down (or 
shut-down, if necessary) immediately.
    Table 1 (below) summarizes the predicted distances at which sound 
levels (160, 180, and 190 dB [rms]) are expected to be received from 
the 36 airgun array and a single airgun operating in deep water depths.

 Table 1--Measured (Array) or Predicted (Single Airgun) Distances to Which Sound Levels >= 190, 180, and 160 dB
    re: 1 [mu]Pa (rms) Could Be Received in Water Depths 1,000 m During the Proposed Survey in the
                            Central-Western Bering Sea, August 7 to September 1, 2011
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                                                                            Predicted RMS distances (m)
           Source and volume                   Water depth       -----------------------------------------------
                                                                      190 dB          180 dB          160 dB
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Single Bolt airgun (40 in \3\)........  Deep > 1,000 m..........              12              40             385
4 Strings 36 airguns (6,600 in \3\)...  Deep > 1,000 m..........             400             940           3,850
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    Along with the airgun operations, two additional acoustical data 
acquisition systems will be operated during the survey. The ocean floor 
will be mapped with the Kongsberg EM 122 MBES and a Knudsen 320B SBP. 
These sound sources will be operated continuously from the LANGSETH 
throughout the cruise.

MBES

    The LANGSETH will operate a Kongsberg EM 122 MBES concurrently 
during airgun operations to map characteristics of the ocean floor. The 
hull-mounted MBES emits brief pulses of sound (also called a ping) 
(10.5 to 13, usually 12 kHz) in a fan-shaped beam that extends downward 
and to the sides of the ship. The transmitting beamwidth is 1[deg] or 
2[deg] fore-aft and 150[deg] athwartship and the maximum source level 
is 242 dB re: 1 [mu]Pa.
    For deep-water operations, each ping consists of eight (in water 
greater than 1,000 m) or four (less than 1,000 m) successive, fan-
shaped transmissions, each ensonifying a sector that extends 1[deg] 
fore-aft. Continuous-wave pulses

[[Page 33249]]

increase from 2 to 15 milliseconds (ms) long in water depths up to 
2,600 m (8,530.2 ft), and FM chirp pulses up to 100 ms long are used in 
water greater than 2,600 m. The successive transmissions span an 
overall cross-track angular extent of about 150[deg], with 2 ms gaps 
between the pulses for successive sectors.

SBP

    The LANGSETH will also operate a Knudsen 320B SBP continuously 
throughout the cruise simultaneously with the MBES to map and provide 
information about the sedimentary features and bottom topography. The 
beam is transmitted as a 27[deg] cone, which is directed downward by a 
3.5 kHz transducer in the hull of the LANGSETH. The maximum output is 
1,000 watts (204 dB re 1 [mu]Pa), but in practice, the output varies 
with water depth. The pulse interval is one second, but a common mode 
of operation is to broadcast five pulses at one second intervals 
followed by a five second pause.
    NMFS expects that acoustic stimuli resulting from the proposed 
operation of the single airgun or the 36 airgun array has the potential 
to harass marine mammals, incidental to the conduct of the proposed 
seismic survey. NMFS expects these disturbances to be temporary and 
result, at worst, in a temporary modification in behavior and/or low-
level physiological effects (Level B harassment) of small numbers of 
certain species of marine mammals. NMFS does not expect that the 
movement of the LANGSETH, during the conduct of the seismic survey, has 
the potential to harass marine mammals because of the relatively slow 
operation speed of the vessel (4.6 knots [kts]; 8.5 km/hr; 5.3 mph) 
during seismic acquisition.

Description of the Proposed Dates, Duration, and Specified Geographic 
Region

    The survey will occur in the central-western Bering Sea, between 
approximately 350 and 800 km offshore, in the area 55 to 58.5[deg] 
North, 177[deg] West to 175[deg] East. The seismic survey will take 
place in water depths greater than 3,000 m. The exact dates of the 
activities depend on logistics and weather conditions. The LANGSETH 
will depart from Dutch Harbor, Alaska on August 7, 2011, and return 
there on September 1, 2011. Seismic operations will be carried out for 
an estimated 20 days.

Description of the Marine Mammals in the Area of the Proposed Specified 
Activity

    Twenty marine mammal species under NMFS jurisdiction (14 cetacean 
and 6 pinniped) are known to or could occur in the central-western 
Bering Sea. Several of these species are listed as endangered under the 
U.S. Endangered Species Act of 1973 (ESA; 16 U.S.C. 1531 et seq.), 
including the North Pacific right whale (Eubalaena japonica), bowhead 
(Balaena mysticetus), humpback (Megaptera novaeangliae), sei 
(Balaenoptera borealis), fin (Balaenoptera physalus), blue 
(Balaenoptera musculus), and sperm (Physeter macrocephalus) whales, as 
well as the western stock of Steller sea lions (Eumetopias jubatus). 
The eastern stock of Steller sea lions is listed as threatened.
    The marine mammals that occur in the proposed survey area belong to 
three taxonomic groups: Odontocetes (toothed cetaceans, such as 
dolphins), mysticetes (baleen whales), and pinnipeds (seals, sea lions, 
and walrus). Cetaceans and pinnipeds are the subject of the IHA 
application to NMFS. Walrus sightings are rare in the Bering Sea during 
the summer. The Pacific walrus is managed by the U.S. Fish and Wildlife 
Service (USFWS) and will not be considered further in this analysis; 
all others are managed by NMFS. Of the 20 species of marine mammals 
that could occur in the offshore waters of the central-western Bering 
Sea, six are seasonally common during summer (humpback, minke, fin, and 
killer whales, Dall's porpoises, and ribbon seals). The other 14 
species are uncommon to extremely rare. For example, the migratory 
patterns of bowhead whales from the Bering to the Beaufort Sea in 
spring make it unlikely that these whales would be encountered during 
the proposed seismic surveys. Because of their small population size, 
right whale sightings are rare and generally restricted to an area 
approximately 500 km from the proposed survey site. Blue whales are 
also low in abundance, and five NMFS vessel-based surveys between 1999 
and 2010 along the Bering shelf and slope have not reported a single 
blue whale sighting. Cuvier's beaked whales and Pacific white-sided 
dolphins are typically not found in high-latitude polar waters and 
would be considered very rare in the vicinity of the proposed seismic 
survey. Among the pinnipeds, the two species of ice seals (ringed and 
spotted seals) are not common in the Bering Sea in late summer. In 
addition, coastal cetacean species (gray whales) likely would not be 
encountered in the deep, offshore waters of the survey area. Although 
not considered common to the area, takes were requested for the 
remaining six species (sei whale, sperm whale, Baird's beaked whale, 
Stejneger's beaked whale, Steller sea lion, and northern fur seal) 
because they have been reported in deep water in the Bering Sea.
    Table 2 (below) presents information on the abundance, 
distribution, population status, conservation status, and density of 
the marine mammals that may occur in the proposed survey area during 
August 2011.
BILLING CODE 3510-10-P

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[GRAPHIC] [TIFF OMITTED] TN08JN11.000


[[Page 33251]]


[GRAPHIC] [TIFF OMITTED] TN08JN11.001

BILLING CODE 3510-22-C
    Refer to Section III of USGS's application for detailed information 
regarding the abundance and distribution, population status, and life

[[Page 33252]]

history and behavior of these species and their occurrence in the 
proposed project area. The application also presents how USGS 
calculated the estimated densities for the marine mammals in the 
proposed survey area. NMFS has reviewed these data and determined them 
to be the best available scientific information for the purposes of the 
proposed IHA.

Potential Effects on Marine Mammals

    Acoustic stimuli generated by the operation of the airguns, which 
introduce sound into the marine environment, may have the potential to 
cause Level B harassment of marine mammals in the proposed survey area. 
The effects of sounds from airgun operations might include one or more 
of the following: tolerance, masking of natural sounds, behavioral 
disturbance, temporary or permanent hearing impairment, or non-auditory 
physical or physiological effects (Richardson et al., 1995; Gordon et 
al., 2004; Nowacek et al., 2007; Southall et al., 2007).
    Permanent hearing impairment, in the unlikely event that it 
occurred, would constitute injury, but temporary threshold shift (TTS) 
is not an injury (Southall et al., 2007). Although the possibility 
cannot be entirely excluded, it is unlikely that the proposed project 
would result in any cases of temporary or permanent hearing impairment, 
or any significant non-auditory physical or physiological effects. 
Based on the available data and studies described here, some behavioral 
disturbance is expected, but NMFS expects the disturbance to be 
localized and short-term.

Tolerance to Sound

    Studies on marine mammals' tolerance to sound in the natural 
environment are relatively rare. Richardson et al. (1995) defines 
tolerance as the occurrence of marine mammals in areas where they are 
exposed to human activities or man-made noise. In many cases, tolerance 
develops by the animal habituating to the stimulus (i.e., the gradual 
waning of responses to a repeated or ongoing stimulus) (Richardson, et 
al., 1995; Thorpe, 1963), but because of ecological or physiological 
requirements, many marine animals may need to remain in areas where 
they are exposed to chronic stimuli (Richardson, et al., 1995).
    Numerous studies have shown that pulsed sounds from airguns are 
often readily detectable in the water at distances of many kilometers. 
Malme et al., (1985) studied the responses of humpback whales on their 
summer feeding grounds in southeast Alaska to seismic pulses from a 
airgun with a total volume of 100 in\3\. They noted that the whales did 
not exhibit persistent avoidance when exposed to the airgun and 
concluded that there was no clear evidence of avoidance, despite the 
possibility of subtle effects, at received levels up to 172 dB re 1 
[mu]Pa.
    Weir (2008) observed marine mammal responses to seismic pulses from 
a 24 airgun array firing a total volume of either 5,085 in \3\ or 3,147 
in \3\ in Angolan waters between August 2004 and May 2005. She recorded 
a total of 207 sightings of humpback whales (n = 66), sperm whales (n = 
124), and Atlantic spotted dolphins (n = 17) and reported that there 
were no significant differences in encounter rates (sightings/hr) for 
humpback and sperm whales according to the airgun array's operational 
status (i.e., active versus silent).

Masking of Natural Sounds

    The term masking refers to the inability of a subject to recognize 
the occurrence of an acoustic stimulus as a result of the interference 
of another acoustic stimulus (Clark et al., 2009). Introduced 
underwater sound may, through masking, reduce the effective 
communication distance of a marine mammal species if the frequency of 
the source is close to that used as a signal by the marine mammal, and 
if the anthropogenic sound is present for a significant fraction of the 
time (Richardson et al., 1995).
    Masking effects of pulsed sounds (even from large arrays of 
airguns) on marine mammal calls and other natural sounds are expected 
to be limited. Because of the intermittent nature and low duty cycle of 
seismic airgun pulses, animals can emit and receive sounds in the 
relatively quiet intervals between pulses. However, in some situations, 
reverberation occurs for much or the entire interval between pulses 
(e.g., Simard et al., 2005; Clark and Gagnon, 2006), which could mask 
calls. Some baleen and toothed whales are known to continue calling in 
the presence of seismic pulses, and their calls can usually be heard 
between the seismic pulses (e.g., Richardson et al., 1986; McDonald et 
al., 1995; Greene et al., 1999; Nieukirk et al., 2004; Smultea et al., 
2004; Holst et al., 2005a,b, 2006; and Dunn and Hernandez, 2009). 
However, Clark and Gagnon (2006) reported that fin whales in the 
northeast Pacific Ocean went silent for an extended period starting 
soon after the onset of a seismic survey in the area. Similarly, there 
has been one report that sperm whales ceased calling when exposed to 
pulses from a very distant seismic ship (Bowles et al., 1994). However, 
more recent studies found that they continued calling in the presence 
of seismic pulses (Madsen et al., 2002; Tyack et al., 2003; Smultea et 
al., 2004; Holst et al., 2006; and Jochens et al., 2008). Dolphins and 
porpoises commonly are heard calling while airguns are operating (e.g., 
Gordon et al., 2004; Smultea et al., 2004; Holst et al., 2005a, b; and 
Potter et al., 2007). 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, NMFS expects the masking effects of seismic pulses to 
be minor, given the normally intermittent nature of seismic pulses. 
Refer to Appendix B (4) of USGS's EA for a more detailed discussion of 
masking effects on marine mammals.

Behavioral Disturbance

    Disturbance includes a variety of effects, including subtle to 
conspicuous changes in behavior, movement, and displacement. Reactions 
to sound, if any, depend on species, state of maturity, experience, 
current activity, reproductive state, time of day, and many other 
factors (Richardson et al., 1995; Wartzok et al., 2004; Southall et 
al., 2007; Weilgart, 2007). If a marine mammal does react briefly to an 
underwater sound by changing its behavior or moving a small distance, 
the impacts of the change are unlikely to be significant to the 
individual, let alone the stock or population. However, if a sound 
source displaces marine mammals from an important feeding or breeding 
area for a prolonged period, impacts on individuals and populations 
could be significant (e.g., Lusseau and Bejder, 2007; Weilgart, 2007). 
Given the many uncertainties in predicting the quantity and types of 
impacts of noise on marine mammals, it is common practice to estimate 
how many mammals would be present within a particular distance of 
industrial activities and/or exposed to a particular level of 
industrial sound. In most cases, this approach likely overestimates the 
numbers of marine mammals that would be affected in some biologically-
important manner.
    The sound criteria used to estimate how many marine mammals might 
be disturbed to some biologically-important degree by a seismic program 
are based primarily on behavioral observations of a few species. 
Scientists have conducted detailed studies on humpback, gray, bowhead 
(Balaena mysticetus), and sperm whales. Less detailed data are 
available for some other species of baleen whales, small

[[Page 33253]]

toothed whales, and sea otters, but for many species there are no data 
on responses to marine seismic surveys.
    Baleen Whales--Baleen whales generally tend to avoid operating 
airguns, but avoidance radii are quite variable (reviewed in Richardson 
et al., 1995). Whales are often reported to show no overt reactions to 
pulses from large arrays of airguns at distances beyond a few kms, even 
though the airgun pulses remain well above ambient noise levels out to 
much longer distances. However, as reviewed in Appendix B (5) of USGS's 
EA, baleen whales exposed to strong noise pulses from airguns often 
react by deviating from their normal migration route and/or 
interrupting their feeding and moving away. In the cases of migrating 
gray and bowhead whales, the observed changes in behavior appeared to 
be of little or no biological consequence to the animals (Richardson, 
et al., 1995). They simply avoided the sound source by displacing their 
migration route to varying degrees, but within the natural boundaries 
of the migration corridors.
    Studies of gray, bowhead, and humpback whales have shown that 
seismic pulses with received levels of 160 to 170 dB re 1 [mu]Pa (rms) 
seem to cause obvious avoidance behavior in a substantial fraction of 
the animals exposed (Malme et al., 1986, 1988; Richardson et al., 
1995). In many areas, seismic pulses from large arrays of airguns 
diminish to those levels at distances ranging from four to 15 km from 
the source. A substantial proportion of the baleen whales within those 
distances may show avoidance or other strong behavioral reactions to 
the airgun array. Subtle behavioral changes sometimes become evident at 
somewhat lower received levels, and studies summarized in Appendix B 
(5) of USGS's EA have shown that some species of baleen whales, notably 
bowhead and humpback whales, at times, show strong avoidance at 
received levels lower than 160 to 170 dB re 1 [mu]Pa (rms).
    McCauley et al. (1998, 2000a) studied the responses of humpback 
whales off western Australia to a full-scale seismic survey with a 16 
airgun array (2,678 in \3\) and to a single airgun (20 in \3\) with 
source level of 227 dB re 1 [micro]Pa (p-p). In the 1998 study, they 
documented that avoidance reactions began at five to eight km from the 
array, and that those reactions kept most pods approximately three to 
four km from the operating seismic boat. In the 2000 study, they noted 
localized displacement during migration of four to five km by traveling 
pods and seven to 12 km by more sensitive resting pods of cow-calf 
pairs. Avoidance distances with respect to the single airgun were 
smaller but consistent with the results from the full array in terms of 
the received sound levels. The mean received level for initial 
avoidance of an approaching airgun was 140 dB re 1 [mu]Pa for humpback 
pods containing females, and at the mean closest point of approach 
distance the received level was 143 dB re 1 [mu]Pa. The initial 
avoidance response generally occurred at distances of five to eight km 
from the airgun array and two km from the single airgun. However, some 
individual humpback whales, especially males, approached within 
distances of 100 to 400 m (328 to 1,312 ft), where the maximum received 
level was 179 dB re 1 [mu]Pa.
    Humpback whales on their summer feeding grounds in southeast Alaska 
did not exhibit persistent avoidance when exposed to seismic pulses 
from a 1.64-L (100 in\3\) airgun (Malme et al., 1985). Some humpbacks 
seemed ``startled'' at received levels of 150 to 169 dB re 1 [mu]Pa. 
Malme et al. (1985) concluded that there was no clear evidence of 
avoidance, despite the possibility of subtle effects, at received 
levels up to 172 dB re 1 [mu]Pa (rms).
    Studies have suggested that south Atlantic humpback whales 
wintering off Brazil may be displaced or even strand upon exposure to 
seismic surveys (Engel et al., 2004). The evidence for this was 
circumstantial and subject to alternative explanations (IAGC, 2004). 
Also, the evidence was not consistent with subsequent results from the 
same area of Brazil (Parente et al., 2006), or with direct studies of 
humpbacks exposed to seismic surveys in other areas and seasons. After 
allowance for data from subsequent years, there was no observable 
direct correlation between strandings and seismic surveys (IWC, 
2007:236).
    There are no data on reactions of right whales to seismic surveys, 
but results from the closely-related bowhead whale show that their 
responsiveness can be quite variable depending on their activity 
(migrating versus feeding). Bowhead whales migrating west across the 
Alaskan Beaufort Sea in autumn, in particular, are unusually 
responsive, with substantial avoidance occurring out to distances of 20 
to 30 km from a medium-sized airgun source at received sound levels of 
around 120 to 130 dB re 1 [mu]Pa (Miller et al., 1999; Richardson et 
al., 1999; see Appendix B (5) of USGS's EA). However, more recent 
research on bowhead whales (Miller et al., 2005; Harris et al., 2007) 
corroborates earlier evidence that, during the summer feeding season, 
bowheads are not as sensitive to seismic sources. Nonetheless, subtle 
but statistically significant changes in surfacing-respiration-dive 
cycles were evident upon statistical analysis (Richardson et al., 
1986). In the summer, bowheads typically begin to show avoidance 
reactions at received levels of about 152 to 178 dB re 1 [mu]Pa 
(Richardson et al., 1986, 1995; Ljungblad et al., 1988; Miller et al., 
2005).
    Reactions of migrating and feeding (but not wintering) gray whales 
to seismic surveys have been studied. Malme et al. (1986, 1988) studied 
the responses of feeding eastern Pacific gray whales to pulses from a 
single 100 in\3\ airgun off St. Lawrence Island in the northern Bering 
Sea. They estimated, based on small sample sizes, that 50 percent of 
feeding gray whales stopped feeding at an average received pressure 
level of 173 dB re 1 [mu]Pa on an (approximate) rms basis, and that 10 
percent of feeding whales interrupted feeding at received levels of 163 
dB re 1 [micro]Pa. Those findings were generally consistent with the 
results of experiments conducted on larger numbers of gray whales that 
were migrating along the California coast (Malme et al., 1984; Malme 
and Miles, 1985), and western Pacific gray whales feeding off Sakhalin 
Island, Russia (Wursig et al., 1999; Gailey et al., 2007; Johnson et 
al., 2007; Yazvenko et al., 2007a, b), along with data on gray whales 
off British Columbia (Bain and Williams, 2006).
    Various species of Balaenoptera (blue, sei, fin, and minke whales) 
have occasionally been seen in areas ensonified by airgun pulses 
(Stone, 2003; MacLean and Haley, 2004; Stone and Tasker, 2006), and 
calls from blue and fin whales have been localized in areas with airgun 
operations (e.g., McDonald et al., 1995; Dunn and Hernandez, 2009). 
Sightings by observers on seismic vessels off the United Kingdom from 
1997 to 2000 suggest that, during times of good sightability, sighting 
rates for mysticetes (mainly fin and sei whales) were similar when 
large arrays of airguns were shooting vs. silent (Stone, 2003; Stone 
and Tasker, 2006). However, these whales tended to exhibit localized 
avoidance, remaining significantly further (on average) from the airgun 
array during seismic operations compared with non-seismic periods 
(Stone and Tasker, 2006). In a study off of Nova Scotia, Moulton and 
Miller (2005) found little difference in sighting rates (after 
accounting for water depth) and initial sighting distances of 
balaenopterid whales when airguns were operating vs. silent. However, 
there were indications that these whales

[[Page 33254]]

were more likely to be moving away when seen during airgun operations. 
Similarly, ship-based monitoring studies of blue, fin, sei and minke 
whales offshore of Newfoundland (Orphan Basin and Laurentian Sub-basin) 
found no more than small differences in sighting rates and swim 
directions during seismic versus non-seismic periods (Moulton et al., 
2005, 2006a,b).
    Data on short-term reactions by cetaceans to impulsive noises are 
not necessarily indicative of long-term or biologically significant 
effects. It is not known whether impulsive sounds affect reproductive 
rate or distribution and habitat use in subsequent days or years. 
However, gray whales have continued to migrate annually along the west 
coast of North America with substantial increases in the population 
over recent years, despite intermittent seismic exploration (and much 
ship traffic) in that area for decades (Appendix A in Malme et al., 
1984; Richardson et al., 1995; Allen and Angliss, 2010). The western 
Pacific gray whale population did not seem affected by a seismic survey 
in its feeding ground during a previous year (Johnson et al., 2007). 
Similarly, bowhead whales have continued to travel to the eastern 
Beaufort Sea each summer, and their numbers have increased notably, 
despite seismic exploration in their summer and autumn range for many 
years (Richardson et al., 1987; Allen and Angliss, 2010).
    Toothed Whales--Little systematic information is available about 
reactions of toothed whales to noise pulses. Few studies similar to the 
more extensive baleen whale/seismic pulse work summarized above and (in 
more detail) in Appendix B of USGS's EA have been reported for toothed 
whales. However, there are recent systematic studies on sperm whales 
(e.g., Gordon et al., 2006; Madsen et al., 2006; Winsor and Mate, 2006; 
Jochens et al., 2008; Miller et al., 2009). There is an increasing 
amount of information about responses of various odontocetes to seismic 
surveys based on monitoring studies (e.g., Stone, 2003; Smultea et al., 
2004; Moulton and Miller, 2005; Bain and Williams, 2006; Holst et al., 
2006; Stone and Tasker, 2006; Potter et al., 2007; Hauser et al., 2008; 
Holst and Smultea, 2008; Weir, 2008; Barkaszi et al., 2009; Richardson 
et al., 2009).
    Seismic operators and marine mammal observers on seismic vessels 
regularly see dolphins and other small toothed whales near operating 
airgun arrays, but in general there is a tendency for most delphinids 
to show some avoidance of operating seismic vessels (e.g., Goold, 
1996a,b,c; Calambokidis and Osmek, 1998; Stone, 2003; Moulton and 
Miller, 2005; Holst et al., 2006; Stone and Tasker, 2006; Weir, 2008; 
Richardson et al., 2009; see also Barkaszi et al., 2009). Some dolphins 
seem to be attracted to the seismic vessel and floats, and some ride 
the bow wave of the seismic vessel even when large arrays of airguns 
are firing (e.g., Moulton and Miller, 2005). Nonetheless, small toothed 
whales more often tend to head away, or to maintain a somewhat greater 
distance from the vessel, when a large array of airguns is operating 
than when it is silent (e.g., Stone and Tasker, 2006; Weir, 2008). In 
most cases, the avoidance radii for delphinids appear to be small, on 
the order of one km less, and some individuals show no apparent 
avoidance. The beluga whale (Delphinapterus leucas) is a species that 
(at least at times) shows long-distance avoidance of seismic vessels. 
Aerial surveys conducted in the southeastern Beaufort Sea during summer 
found that sighting rates of beluga whales were significantly lower at 
distances 10 to 20 km compared with 20 to 30 km from an operating 
airgun array, and observers on seismic boats in that area rarely see 
belugas (Miller et al., 2005; Harris et al., 2007).
    Captive bottlenose dolphins (Tursiops truncatus) and beluga whales 
exhibited changes in behavior when exposed to strong pulsed sounds 
similar in duration to those typically used in seismic surveys 
(Finneran et al., 2000, 2002, 2005). However, the animals tolerated 
high received levels of sound before exhibiting aversive behaviors.
    Results for porpoises depend on species. The limited available data 
suggest that harbor porpoises show stronger avoidance of seismic 
operations than do Dall's porpoises (Stone, 2003; MacLean and Koski, 
2005; Bain and Williams, 2006; Stone and Tasker, 2006). Dall's 
porpoises seem relatively tolerant of airgun operations (MacLean and 
Koski, 2005; Bain and Williams, 2006), although they too have been 
observed to avoid large arrays of operating airguns (Calambokidis and 
Osmek, 1998; Bain and Williams, 2006). This apparent difference in 
responsiveness of these two porpoise species is consistent with their 
relative responsiveness to boat traffic and some other acoustic sources 
(Richardson et al., 1995; Southall et al., 2007).
    Most studies of sperm whales exposed to airgun sounds indicate that 
the sperm whale shows considerable tolerance of airgun pulses (e.g., 
Stone, 2003; Moulton et al., 2005, 2006a; Stone and Tasker, 2006; Weir, 
2008). In most cases the whales do not show strong avoidance, and they 
continue to call (see Appendix B of USGS's EA for review). However, 
controlled exposure experiments in the Gulf of Mexico indicate that 
foraging behavior was altered upon exposure to airgun sound (Jochens et 
al., 2008; Miller et al., 2009; Tyack, 2009).
    There are almost no specific data on the behavioral reactions of 
beaked whales to seismic surveys. However, some northern bottlenose 
whales (Hyperoodon ampullatus) remained in the general area and 
continued to produce high-frequency clicks when exposed to sound pulses 
from distant seismic surveys (Gosselin and Lawson, 2004; Laurinolli and 
Cochrane, 2005; Simard et al., 2005). Most beaked whales tend to avoid 
approaching vessels of other types (e.g., Wursig et al., 1998). They 
may also dive for an extended period when approached by a vessel (e.g., 
Kasuya, 1986), although it is uncertain how much longer such dives may 
be as compared to dives by undisturbed beaked whales, which also are 
often quite long (Baird et al., 2006; Tyack et al., 2006). Based on a 
single observation, Aguilar-Soto et al. (2006) suggested that foraging 
efficiency of Cuvier's beaked whales may be reduced by close approach 
of vessels. In any event, it is likely that most beaked whales would 
also show strong avoidance of an approaching seismic vessel, although 
this has not been documented explicitly.
    There are increasing indications that some beaked whales tend to 
strand when naval exercises involving mid-frequency sonar operation are 
ongoing nearby (e.g., Simmonds and Lopez-Jurado, 1991; Frantzis, 1998; 
NOAA and USN, 2001; Jepson et al., 2003; Hildebrand, 2005; Barlow and 
Gisiner, 2006; see also the Stranding and Mortality section in this 
notice). These strandings are apparently a disturbance response, 
although auditory or other injuries or other physiological effects may 
also be involved. Whether beaked whales would ever react similarly to 
seismic surveys is unknown. Seismic survey sounds are quite different 
from those of the sonar in operation during the above-cited incidents.
    Odontocete reactions to large arrays of airguns are variable and, 
at least for delphinids and Dall's porpoises, seem to be confined to a 
smaller radius than has been observed for the more responsive of the 
mysticetes, belugas, and harbor porpoises (Appendix B of USGS's EA).
    Pinnipeds--Pinnipeds are not likely to show a strong avoidance 
reaction to the airgun array. Visual monitoring from seismic vessels 
has shown only slight (if any) avoidance of airguns by pinnipeds,

[[Page 33255]]

and only slight (if any) changes in behavior, see Appendix B of USGS's 
EA. In the Beaufort Sea, some ringed seals avoided an area of 100 m to 
(at most) a few hundred meters around seismic vessels, but many seals 
remained within 100 to 200 m (328 to 656 ft) of the trackline as the 
operating airgun array passed by (e.g., Harris et al., 2001; Moulton 
and Lawson, 2002; Miller et al., 2005). Ringed seal sightings averaged 
somewhat farther away from the seismic vessel when the airguns were 
operating than when they were not, but the difference was small 
(Moulton and Lawson, 2002). Similarly, in Puget Sound, sighting 
distances for harbor seals and California sea lions tended to be larger 
when airguns were operating (Calambokidis and Osmek, 1998). Previous 
telemetry work suggests that avoidance and other behavioral reactions 
may be stronger than evident to date from visual studies (Thompson et 
al., 1998).

Hearing Impairment and Other Physical Effects

    Exposure to high intensity sound for a sufficient duration may 
result in auditory effects such as a noise-induced threshold shift--an 
increase in the auditory threshold after exposure to noise (Finneran, 
Carder, Schlundt, and Ridgway, 2005). Factors that influence the amount 
of threshold shift include the amplitude, duration, frequency content, 
temporal pattern, and energy distribution of noise exposure. The 
magnitude of hearing threshold shift normally decreases over time 
following cessation of the noise exposure. The amount of threshold 
shift just after exposure is called the initial threshold shift. If the 
threshold shift eventually returns to zero (i.e., the threshold returns 
to the pre-exposure value), it is called temporary threshold shift 
(TTS) (Southall et al., 2007).
    Researchers have studied TTS in certain captive odontocetes and 
pinnipeds exposed to strong sounds (reviewed in Southall et al., 2007). 
However, there has been no specific documentation of TTS let alone 
permanent hearing damage, i.e., permanent threshold shift (PTS), in 
free-ranging marine mammals exposed to sequences of airgun pulses 
during realistic field conditions.
    Temporary Threshold Shift--TTS is the mildest form of hearing 
impairment that can occur during exposure to a strong sound (Kryter, 
1985). While experiencing TTS, the hearing threshold rises and a sound 
must be stronger in order to be heard. At least in terrestrial mammals, 
TTS can last from minutes or hours to (in cases of strong TTS) days. 
For sound exposures at or somewhat above the TTS threshold, hearing 
sensitivity in both terrestrial and marine mammals recovers rapidly 
after exposure to the noise ends. Few data on sound levels and 
durations necessary to elicit mild TTS have been obtained for marine 
mammals, and none of the published data concern TTS elicited by 
exposure to multiple pulses of sound. Available data on TTS in marine 
mammals are summarized in Southall et al. (2007). Table 1 (above) 
presents the distances from the LANGSETH's airguns at which the 
received energy level (per pulse, flat-weighted) would be expected to 
be greater than or equal to 180 dB re 1 [micro]Pa (rms).
    To avoid the potential for injury, NMFS (1995, 2000) concluded that 
cetaceans should not be exposed to pulsed underwater noise at received 
levels exceeding 180 dB re 1 [mu]Pa (rms). NMFS believes that to avoid 
the potential for permanent physiological damage (Level A harassment), 
cetaceans should not be exposed to pulsed underwater noise at received 
levels exceeding 180 dB re 1 [mu]Pa (rms). The 180 dB level is a 
shutdown criterion applicable to cetaceans, as specified by NMFS 
(2000); these levels were used to establish the EZs. NMFS also assumes 
that cetaceans exposed to levels exceeding 160 dB re 1 [mu]Pa (rms) may 
experience Level B harassment.
    Researchers have derived TTS information for odontocetes from 
studies on the bottlenose dolphin and beluga. For the one harbor 
porpoise tested, the received level of airgun sound that elicited onset 
of TTS was lower (Lucke et al., 2009). If these results from a single 
animal are representative, it is inappropriate to assume that onset of 
TTS occurs at similar received levels in all odontocetes (cf. Southall 
et al., 2007). Some cetaceans apparently can incur TTS at considerably 
lower sound exposures than are necessary to elicit TTS in the beluga or 
bottlenose dolphin.
    For baleen whales, there are no data, direct or indirect, on levels 
or properties of sound that are required to induce TTS. The frequencies 
to which baleen whales are most sensitive are assumed to be lower than 
those to which odontocetes are most sensitive, and natural background 
noise levels at those low frequencies tend to be higher. As a result, 
auditory thresholds of baleen whales within their frequency band of 
best hearing are believed to be higher (less sensitive) than are those 
of odontocetes at their best frequencies (Clark and Ellison, 2004). 
From this, it is suspected that received levels causing TTS onset may 
also be higher in baleen whales (Southall et al., 2007). For this 
proposed study, USGS expects no cases of TTS given: (1) The low 
abundance of baleen whales in the planned study area at the time of the 
survey; and (2) the strong likelihood that baleen whales would avoid 
the approaching airguns (or vessel) before being exposed to levels high 
enough for TTS to occur.
    In pinnipeds, TTS thresholds associated with exposure to brief 
pulses (single or multiple) of underwater sound have not been measured. 
Initial evidence from more prolonged (non-pulse) exposures suggested 
that some pinnipeds (harbor seals in particular) incur TTS at somewhat 
lower received levels than do small odontocetes exposed for similar 
durations (Kastak et al., 1999, 2005; Ketten et al., 2001). The TTS 
threshold for pulsed sounds has been indirectly estimated as being an 
SEL of approximately 171 dB re 1 [micro]Pa\2\[middot]s (Southall et 
al., 2007) which would be equivalent to a single pulse with received 
level approximately 181 to 186 dB re 1 [micro]Pa (rms), or a series of 
pulses for which the highest rms values are a few dB lower. 
Corresponding values for California sea lions and northern elephant 
seals are likely to be higher (Kastak et al., 2005).
    Permanent Threshold Shift--When PTS occurs, there is physical 
damage to the sound receptors in the ear. In severe cases, there can be 
total or partial deafness, whereas in other cases, the animal has an 
impaired ability to hear sounds in specific frequency ranges (Kryter, 
1985). There is no specific evidence that exposure to pulses of airgun 
sound can cause PTS in any marine mammal, even with large arrays of 
airguns. However, given the possibility that mammals close to an airgun 
array might incur at least mild TTS, there has been further speculation 
about the possibility that some individuals occurring very close to 
airguns might incur PTS (e.g., Richardson et al., 1995, p. 372ff; 
Gedamke et al., 2008). Single or occasional occurrences of mild TTS are 
not indicative of permanent auditory damage, but repeated or (in some 
cases) single exposures to a level well above that causing TTS onset 
might elicit PTS.
    Relationships between TTS and PTS thresholds have not been studied 
in marine mammals, but are assumed to be similar to those in humans and 
other terrestrial mammals. PTS might occur at a received sound level at 
least several dBs above that inducing mild TTS if the animal were 
exposed to strong sound pulses with rapid rise time--see Appendix B (6) 
of USGS's EA. Based on data from terrestrial mammals, a precautionary 
assumption is that the

[[Page 33256]]

PTS threshold for impulse sounds (such as airgun pulses as received 
close to the source) is at least 6 dB higher than the TTS threshold on 
a peak-pressure basis, and probably greater than six dB (Southall et 
al., 2007).
    Given the higher level of sound necessary to cause PTS as compared 
with TTS, it is considerably less likely that PTS would occur. Baleen 
whales generally avoid the immediate area around operating seismic 
vessels, as do some other marine mammals.
    Stranding and Mortality--Marine mammals close to underwater 
detonations of high explosives can be killed or severely injured, and 
the auditory organs are especially susceptible to injury (Ketten et 
al., 1993; Ketten, 1995). However, explosives are no longer used in 
marine waters for commercial seismic surveys or (with rare exceptions) 
for seismic research; they have been replaced entirely by airguns or 
related non-explosive pulse generators. Airgun pulses are less 
energetic and have slower rise times, and there is no specific evidence 
that they can cause serious injury, death, or stranding even in the 
case of large airgun arrays. However, the association of strandings of 
beaked whales with naval exercises involving mid-frequency active sonar 
and, in one case, an L-DEO seismic survey (Malakoff, 2002; Cox et al., 
2006), has raised the possibility that beaked whales exposed to strong 
``pulsed'' sounds may be especially susceptible to injury and/or 
behavioral reactions that can lead to stranding (e.g., Hildebrand, 
2005; Southall et al., 2007). Appendix B (6) of USGS's EA provides 
additional details.
    Specific sound-related processes that lead to strandings and 
mortality are not well documented, but may include:
    (1) Swimming in avoidance of a sound into shallow water;
    (2) A change in behavior (such as a change in diving behavior) that 
might contribute to tissue damage, gas bubble formation, hypoxia, 
cardiac arrhythmia, hypertensive hemorrhage or other forms of trauma;
    (3) A physiological change such as a vestibular response leading to 
a behavioral change or stress-induced hemorrhagic diathesis, leading in 
turn to tissue damage; and
    (4) Tissue damage directly from sound exposure, such as through 
acoustically-mediated bubble formation and growth or acoustic resonance 
of tissues. Some of these mechanisms are unlikely to apply in the case 
of impulse sounds. However, there are indications that gas-bubble 
disease (analogous to ``the bends''), induced in supersaturated tissue 
by a behavioral response to acoustic exposure, could be a pathologic 
mechanism for the strandings and mortality of some deep-diving 
cetaceans exposed to sonar. However, the evidence for this remains 
circumstantial and associated with exposure to naval mid-frequency 
sonar, not seismic surveys (Cox et al., 2006; Southall et al., 2007).
    Seismic pulses and mid-frequency sonar signals are quite different, 
and some mechanisms by which sonar sounds have been hypothesized to 
affect beaked whales are unlikely to apply to airgun pulses. Sounds 
produced by airgun arrays are broadband impulses with most of the 
energy below one kHz. Typical military mid-frequency sonar emits non-
impulse sounds at frequencies of two to 10 kHz, generally with a 
relatively narrow bandwidth at any one time. A further difference 
between seismic surveys and naval exercises is that naval exercises can 
involve sound sources on more than one vessel. Thus, it is not 
appropriate to assume that there is a direct connection between the 
effects of military sonar and seismic surveys on marine mammals. 
However, evidence that sonar signals can, in special circumstances, 
lead (at least indirectly) to physical damage and mortality (e.g., 
Balcomb and Claridge, 2001; NOAA and USN, 2001; Jepson et al., 2003; 
Fern[aacute]ndez et al., 2004, 2005; Hildebrand 2005; Cox et al., 2006) 
suggests that caution is warranted when dealing with exposure of marine 
mammals to any high-intensity ``pulsed'' sound.
    There is no conclusive evidence of cetacean strandings or deaths at 
sea as a result of exposure to seismic surveys, but a few cases of 
strandings in the general area where a seismic survey was ongoing have 
led to speculation concerning a possible link between seismic surveys 
and strandings. Suggestions that there was a link between seismic 
surveys and strandings of humpback whales in Brazil (Engel et al., 
2004) were not well founded (IAGC, 2004; IWC, 2007). In September 2002, 
there was a stranding of two Cuvier's beaked whales (Ziphius 
cavirostris) in the Gulf of California, Mexico, when the L-DEO vessel 
R/V Maurice Ewing was operating a 20 airgun (8,490 in\3\) array in the 
general area. The link between the stranding and the seismic surveys 
was inconclusive and not based on any physical evidence (Hogarth, 2002; 
Yoder, 2002). Nonetheless, the Gulf of California incident plus the 
beaked whale strandings near naval exercises involving use of mid-
frequency sonar suggests a need for caution in conducting seismic 
surveys in areas occupied by beaked whales until more is known about 
effects of seismic surveys on those species (Hildebrand, 2005). No 
injuries of beaked whales are anticipated during the proposed study 
because of:
    (1) The high likelihood that any beaked whales nearby would avoid 
the approaching vessel before being exposed to high sound levels, and
    (2) Differences between the sound sources operated by L-DEO and 
those involved in the naval exercises associated with strandings.
    Non-auditory Physiological Effects--Non-auditory physiological 
effects or injuries that theoretically might occur in marine mammals 
exposed to strong underwater sound include stress, neurological 
effects, bubble formation, resonance, and other types of organ or 
tissue damage (Cox et al., 2006; Southall et al., 2007). Studies 
examining such effects are limited. However, resonance effects (Gentry, 
2002) and direct noise-induced bubble formations (Crum et al., 2005) 
are implausible in the case of exposure to an impulsive broadband 
source like an airgun array. If seismic surveys disrupt diving patterns 
of deep-diving species, this might perhaps result in bubble formation 
and a form of the bends, as speculated to occur in beaked whales 
exposed to sonar. However, there is no specific evidence of this upon 
exposure to airgun pulses.
    In general, very little is known about the potential for seismic 
survey sounds (or other types of strong underwater sounds) to cause 
non-auditory physical effects in marine mammals. Such effects, if they 
occur at all, would presumably be limited to short distances and to 
activities that extend over a prolonged period. The available data do 
not allow identification of a specific exposure level above which non-
auditory effects can be expected (Southall et al., 2007), or any 
meaningful quantitative predictions of the numbers (if any) of marine 
mammals that might be affected in those ways. Marine mammals that show 
behavioral avoidance of seismic vessels, including most baleen whales 
and some odontocetes, are especially unlikely to incur non-auditor