Incidental Takes of Marine Mammals During Specified Activities; Low-Energy Marine Seismic Survey in the Northeast Pacific Ocean, July 2009, 24799-24818 [E9-12149]

Agencies

• DEPARTMENT OF COMMERCE
• National Oceanic and Atmospheric Administration

[Federal Register Volume 74, Number 99 (Tuesday, May 26, 2009)]
[Notices]
[Pages 24799-24818]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: E9-12149]


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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

RIN 0648-XO71


Incidental Takes of Marine Mammals During Specified Activities; 
Low-Energy Marine Seismic Survey in the Northeast Pacific Ocean, July 
2009

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

ACTION:  Notice; proposed incidental take authorization; request for 
comments.

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SUMMARY:  NMFS has received an application from the Scripps Institution 
of Oceanography (SIO), a part of the University of California San Diego 
(UCSD), for an Incidental Harassment Authorization (IHA) to take small 
numbers of marine mammals, by harassment, incidental to conducting a 
marine seismic survey in the Northeast Pacific Ocean during July 2009. 
Pursuant to the Marine Mammal Protection Act (MMPA), NMFS requests 
comments on its proposal to authorize SIO to incidentally take, by 
Level B harassment only, small numbers of marine mammals during the 
aforementioned activity.

DATES:  Comments and information must be received no later than June 
25, 2009.

ADDRESSES:  Comments on the application should be addressed to Michael 
Payne, Chief, Permits, Conservation and Education Division, Office of 
Protected Resources, National Marine Fisheries Service, 1315 East-West 
Highway, Silver Spring, MD 20910-3225. The mailbox address for 
providing email comments is PR1.0648-XO71@noaa.gov. Comments sent via 
e-mail, including all attachments, must not exceed a 10-megabyte file 
size.
    A copy of the application containing a list of the references used 
in this document may be obtained by writing to the address specified 
above, telephoning the contact listed below (see FOR FURTHER 
INFORMATION CONTACT), or visiting the internet at: https://www.nmfs.noaa.gov/pr/permits/incidental.htm.

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    Documents cited in this notice may be viewed, by appointment, 
during regular business hours, at the aforementioned address.

FOR FURTHER INFORMATION CONTACT:  Howard Goldstein or Ken Hollingshead, 
Office of Protected Resources, NMFS, 301-713-2289.

SUPPLEMENTARY INFORMATION:

Background

    Sections 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361 et seq.) 
direct the Secretary of Commerce to allow, upon request, the 
incidental, but not intentional, taking of marine mammals 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 either regulations are issued or, if the taking 
is limited to harassment, a notice of a proposed authorization is 
provided to the public for review.
    Authorization for incidental taking shall be granted if NMFS finds 
that the taking will have a negligible impact on the species or 
stock(s), will not have an unmitigable adverse impact on the 
availability of the species or stock(s) for subsistence uses, and if 
the permissible methods of taking and requirements pertaining to the 
mitigation, monitoring and reporting of such takings are set forth. 
NMFS has defined ``negligible impact'' in 50 CFR 216.103 as ''...an 
impact resulting from the specified activity that cannot be reasonably 
expected to, and is not reasonably likely to, adversely affect the 
species or stock through effects on annual rates of recruitment or 
survival.''
    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. 
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 [ALevel 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 
[ALevel B harassment@].
    Section 101(a)(5)(D) 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 
comment period, NMFS must either issue or deny issuance of the 
authorization.

Summary of Request

    On March 9, 2009, NMFS received an application from SIO for the 
taking, by Level B harassment only, of small numbers of marine mammals 
incidental to conducting, under cooperative agreement with the National 
Science Foundation (NSF), a low-energy marine seismic survey in the 
Northeast Pacific Ocean. The funding for the survey is provided by the 
NSF. The proposed survey will occur in an overall area between 
approximately 44[deg] and 45[deg] N. and 124.5[deg] and 126[deg] W. 
within the Exclusive Economic Zone (EEZ) of the U.S.A., and is 
scheduled to occur from July 14-20, 2009. The survey will use a single 
Generator Injector (GI) airgun with a discharge volume of 45 in\3\. 
Some minor deviation from these dates is possible, depending on 
logistics and weather.
    The proposed survey is virtually identical to one conducted by SIO 
in 2007 under an IHA issued in September 2007 (NMFS 2007). The proposed 
SIO 2009 IHA application contains minor updates to the project 
description, updated marine mammal population sizes based on the most 
recent NMFS annual stock assessment, an assessment of the relevance of 
the marine mammal density and distribution data contained in the SIO 
2007 IHA application based on cruise reports from the NMFS SWFSC 
ORCAWHALE 2008 cruise, and updated information on effects of airguns on 
marine mammals (see Appendix A of SIO's application).

Description of the Specified Activity

    SIO plans to conduct an ocean bottom seismograph (OBS) deployment 
and a magnetic, bathymetric, and seismic survey. The planned survey 
will involve one source vessel, the R/V Wecoma (Wecoma), and will occur 
in the Northeast Pacific Ocean off the coast of Oregon.
    The purpose of the research program is to record micro-earthquakes 
in the forearc to determine whether seismicity on the plate boundary is 
characteristic of a locked or a freely slipping fault plane. Several 
earthquakes large enough to be recorded on land-based seismic nets have 
occurred along this segment in the past several years. The occurrence 
of ``repeating earthquakes'' (earthquakes with identical waveforms 
indicating repeated rupture of almost the same fault patch) suggests 
that this region is at a boundary between a freely slipping and a 
locked portion of the fault. Some models suggest that the forearc basin 
north of the seismically active zone may be locked; others suggest that 
portion of the fault is slipping freely. OBSs have been deployed for a 
year, and a seismic survey will be used to characterize the shallow 
sediment structure around the instruments. Also, included in the 
research is the use of a magnetometer and sub-bottom profiler.
    The source vessel, the Wecoma, will deploy a single low-energy GI 
airgun as an energy source (with a discharge volume of 45 in\3\) and a 
300 m (984 ft), 16 channel, towed hydrophone streamer. Sixteen OBSs 
were deployed in July and September 2008. They will continue to acquire 
data during this cruise, and will be recovered before returning to 
port. The energy to the GI airgun is compressed air supplied by 
compressors onboard the source vessel. As the GI airgun is towed along 
the survey lines, the receiving systems will receive the returning 
acoustic signals.
    The seismic program will consist of approximately 21 km (13 mi) of 
surveys over each of the 16 OBSs (see Figure 1 of SIO's application). 
Water depths at the seismic survey locations rang from just less than 
100 m (328 ft) to almost 3,000 m (9,842 ft) (see Figure 1 of SIO's 
application). The GI airgun will be operated on a small grid for 
approximately two hours at each of the 16 OBS sites. There will be 
additional seismic operations associated with equipment testing, start-
ups, and repeat coverage of any areas where initial data quality is 
substandard.
    All planned geophysical data acquisition activities will be 
conducted by SIO with on-board assistance by the scientists who have 
proposed the study. The Chief Scientist is Dr. Anne Trehu of Oregon 
State University. The vessel will be self-contained, and the crew will 
live aboard the vessel for the entire cruise.
    In addition to the seismic operations of the single GI airgun, a 
3.5 and 12 kHz sub-bottom profiler will be used continuously throughout 
the cruise, and a magnetometer may be run on the transit between OBS 
locations.

Vessel Specifications

    The Wecoma has a length of 56.4 m (185 ft), a beam of 10.1 m (33.1 
ft), and a maximum draft of 5.6 m (18.4 ft). The ship is powered by a 
single 3,000-hp EMD diesel engine driving a single, controllable-pitch 
propeller through a clutch and reduction gear, and an electric 350-hp 
azimuthing bow thruster. An operations speed of 11.1 km/hour (6 knots) 
will be used during seismic acquisition. When not towing seismic survey 
gear, the Wecoma cruises at 22.2 km/hour (12 knots) and has a maximum 
speed of 26 km/hour (14

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knots). It has a normal operating range of approximately 13,300 km. The 
Wecoma will also serve as the platform from which vessel-based Marine 
Mammal Visual Observers (MMVO) will watch for animals before and during 
GI airgun operations.

Acoustic Source Specifications

Seismic Airguns
    During the proposed survey, the Wecoma will tow a single GI airgun, 
with a volume of 45 in\3\, and a 300 m long streamer containing 
hydrophones along predetermined lines. Seismic pulses will be emitted 
at intervals of 10 seconds. At a speed of 6 knots (11.1 km/hour), the 
10 second shot spacing corresponds to a shot interval of approximately 
31 m (101.7 ft).
    The generator chamber of the GI airgun, the one responsible for 
introducing the sound pulse into the ocean, is 45 in\3\. The larger 
(105 in\3\) injector chamber injects air into the previously-generated 
bubble to maintain its shape, and does not introduce more sound into 
the water. The 45 in\3\ GI airgun will be towed 21 m (68.9 ft) behind 
the Wecoma at a depth of 4 m (13.1ft). The sound pressure field of that 
GI airgun variation at a tow depth of 2.5 m has been modeled by Lamont-
Doherty Earth Observatory (L-DEO) in relation to distance and direction 
for the GI airgun.
    As the GI airgun is towed along the survey line, the towed 
hydrophone array in the 300 m streamer receives the reflected signals 
and transfers the data on the on-board processing system. Given the 
relatively short streamer length behind the vessel, the turning rate of 
the vessel while the gear is deployed is much higher than the limit of 
five degrees per minute for a seismic vessel towing a streamer of more 
typical length (much greater than 1 km). Thus, the maneuverability of 
the vessel is not limited much during operations.
    The root mean square (rms) received levels that are used as impact 
criteria for marine mammals are not directly comparable to the peak (pk 
or 0-pk) or peak-to-peak (pk - pk) values normally used to characterize 
source levels of airgun arrays. The measurement units used to describe 
airgun sources, peak or peak-to-peak decibels, are always higher than 
the ``root mean square'' (rms) decibels referred to in biological 
literature. A measured received level of 160 dB re 1 microPa (rms) in 
the far field would typically correspond to a peak measurement of 
approximately 170 to 172 dB, and to a peak-to-peak measurement of 
approximately 176 to 178 dB, as measured for the same pulse received at 
the same location (Greene, 1997; McCauley et al., 1998, 2000). The 
precise difference between rms and peak or peak-to-peak values depends 
on the frequency content and duration of the pulse, among other 
factors. However, the rms level is always lower than the peak or peak-
to-peak level for an airgun-type source.
    Received sound levels have been modeled by L-DEO for a number of 
airgun configurations, including one 45 in\3\ GI airgun, in relation to 
distance from the airgun(s) (see Figure 2 of SIO's application). The 
model does not allow for bottom interactions, and is most directly 
applicable to deep water. Based on modeling, estimates of the maximum 
distances from the GI airgun where sound levels of 190, 180, and 160 dB 
re 1 microPa (rms) are predicted to be received in deep (>1,000 m) 
water are shown in Table 1 of SIO's application. Because the model 
results are for a 2.5 m tow depth, the distances in Table 1 slightly 
underestimate the distances for the 45 in\3\ GI airgun towed at 4 m 
depth.
Sub-bottom Profiler
    Along with the GI airgun operations, one additional acoustical data 
acquisition system will be operated throughout the cruise. The ocean 
floor will be mapped with a Knudsen Engineering Model 320BR 12 kHz and 
3.5 kHz sub-bottom profiler (SBP). Multi-beam sonar will not be used.
    The Knudsen Engineering Model 320BR SBP is a dual-frequency 
transceiver designed to operate at 3.5 and/or 12 kHz. It is used to 
provide data about the sedimentary features that occur below the sea 
floor. The energy from the sub-bottom profiler is directed downward via 
a 12 kHz transducer (EDO 323B) or a 3.5 kHz array of 16 ORE 137D 
transducers in a 4x4 arrangement. The maximum power output of the 320BR 
is 10 kilowatts for the 3.5 kHz section and 2 kilowatts for the 12 kHz 
section.
    The pulse length for the 3.5 kHz section of the 320 BR is 0.8-24 
ms, controlled by the system operator in regards to water depth and 
reflectivity of the bottom sediments, and will usually be 12 or 24 ms 
in this survey. The system produces one sound pulse and then waits for 
its return before transmitting again. Thus, the pulse interval is 
directly dependent upon water depth, and in this survey is 4.5-8 
seconds. Using the Sonar Equations and assuming 100 percent efficiency 
in the system (impractical in real world applications), the source 
level for the 320BR is calculated to be 211 dB re 1 Pam. In practical 
operation, the 3.5 kHz array is seldom driven at more than 80 percent 
of maximum, usually less than 50 percent.
Safety Radii
    NMFS has determined that for acoustic effects, using acoustic 
thresholds in combination with corresponding safety radii is an 
effective way to consistently apply measures to avoid or minimize the 
impacts of an action, and to quantitatively estimate the effects of an 
action. Thresholds are used in two ways: (1) to establish a mitigation 
shut-down or power-down zone, i.e., if an animal enters an area 
calculated to be ensonified above the level of an established 
threshold, a sound source is powered down or shut down; and (2) to 
calculate take, in that a model may be used to calculate the area 
around the sound source that will be ensonified to that level or above, 
then, based on the estimated density of animals and the distance that 
the sound source moves, NMFS can estimate the number of marine mammals 
that may be ``taken.''
    As a matter of past practice and based on the best available 
information at the time regarding the effects of marine sound compiled 
over the past decade, NMFS has used conservative numerical estimates to 
approximate where Level A harassment from acoustic sources begins: 180 
dB re 1 microPa (rms) level for cetaceans and 190 dB re 1 microPa (rms) 
for pinnipeds. A review of the available scientific data using an 
application of science-based extrapolation procedures (Southall et al., 
2007) strongly suggests that Level A harassment (as well as TTS) from 
single sound exposure impulse events may occur at much higher levels 
than the levels previously estimated using very limited data. However, 
for purposes of this proposed action, SIO's application sets forth, and 
NMFS is using, the more conservative 180 and 190 dB re 1 microPa (rms) 
criteria. NMFS also considers 160 dB re 1 microPa (rms) as the 
criterion for estimating the onset of Level B harassment from acoustic 
sources like impulse sounds used in the seismic survey.
    Emperical data concerning the 180 and 160 dB distances have been 
acquired based on measurements during the acoustic verification study 
conducted by L-DEO in the northern Gulf of Mexico from May 27 to June 
3, 2003 (Tolstoy et al., 2004). Although the results are limited the 
data showed that radii around the airguns where the received level 
would be 180 dB re 1 microPa (rms), the safety criterion applicable to 
cetaceans (NMFS, 2000), vary with water depth. Similar depth-related 
variation is likely in the 190 dB distances applicable to pinnipeds.

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Correction factors were developed for water depths 100-1,000 m and <100 
m.
    The empirical data indicate that, for deep water (>1,000 m), the L-
DEO model tends to overestimate the received sound levels at a given 
distance (Tolstoy et al., 2004). However, to be precautionary pending 
acquisition of additional empirical data, it is proposed that safety 
radii during GI airgun operations in deep water will be values 
predicted by L-DEO's model (see Table 1 below). Therefore, the assumed 
180 and 190 dB radii are 23 m (75.5 ft) and 8 m (26 ft) respectively.
    Empirical measurements indicated that in shallow water (<100 m), 
the L-DEO model under estimates actual levels. In previous L-DEO 
projects, the exclusion zones were typically based on measured values 
and ranged from 1.3 to 15x higher than the modeled values depending on 
the size of the airgun array and the sound level measured (Tolstoy et 
al., 2004). During the proposed cruise, similar factors will be applied 
to derive appropriate shallow water radii from the modeled deep water 
radii for the GI airgun (see Table 1 below).
    Empirical measurements were not conducted for intermediate depths 
(100-1,000 m). On the expectation that results will be intermediate 
between those from shallow and deep water, a 1.5x correction factor is 
applied to the estimates provided by the model for deep water 
situations. This is the same factor that was applied to the model 
estimates during L-DEO cruises in 2003. The assumed 180 and 190 dB 
radii in intermediate depth water are 35 m (115 ft) and 12 m (39.4 ft), 
respectively (see Table 1 below).

Table 1. Predicted distances to which sound levels [gteqt]190, 180, and 160 dB re 1 microPa might be received in
shallow (<100 m; 328 ft), intermediate (100-1,000 m; 328-3,280 ft), and deep (>1,000 m; 3,280 ft) water from the
  single 45 in\3\ GI airgun used during the seismic surveys in the northeastern Pacific Ocean during July 2009.
                             Distances are based on model results provided by L-DEO.
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                                                                                 Predicted RMS Distances (m)
          Source and Volume            Tow Depth (m)       Water Depth     -------------------------------------
                                                                             190 dB      180 dB        160 dB
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Single GI airgun 45 in\3\                          4      Deep (>1,000 m)         8            23           220
                                                      ----------------------------------------------
                                      ...............  Intermediate (100-        12            35           330
                                                                 1,000 m)
                                                      ----------------------------------------------
                                      ...............   Shallow (< 100 m)        95           150           570
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Proposed Dates, Duration, and Region of Activity

    The Wecoma is scheduled to depart from Newport, Oregon, on July 14, 
2009 and to return on July 20, 2009. The GI airgun will be used for 
approximately two hours at each of 16 OBS locations. The program will 
consist of approximately 7 days of seismic acquisition. The exact dates 
of the activities may vary by a few days because of weather conditions, 
repositioning, streamer operations, and adjustments, GI airgun 
deployment, or the need to repeat some lines if data quality is 
substandard. The seismic surveys will take place off the Oregon coast 
in the northeastern Pacific Ocean (see Figure 1 of SIO's application). 
The overall area within which the seismic surveys will occur is located 
between approximately 44[deg] and 45[deg] N and 124.5[deg] and 126[deg] 
W (see Figure 1 of SIO's application). The surveys will take place in 
water depths just less than 100 m and to almost 3,000 m, entirely 
within the Exclusive Economic Zone (EEZ) of the U.S.A.

Description of Marine Mammals in the Proposed Activity Area

    A total of 32 marine mammal species may occur or have been 
documented to occur in the marine waters off Oregon and Washington, 
excluding extralimital sightings or strandings (Fiscus and Niggol, 
1965; Green et al., 1992, 1993; Barlow, 1997, 2003; Mangels and 
Gerrodette, 1994; Von Saunder and Barlow, 1999; Barlow and Taylor, 
2001; Buchanan et al., 2001; Calambokidis et al., 2004; Calambokidis 
and Barlow, 2004). The species include 19 odontocetes (toothed 
cetaceans, such as dolphins), 7 mysticetes (baleen whales), 5 
pinnipeds, and sea otters. Six of the species that may occur in the 
project area are listed under the Endangered Species Act (ESA) as 
Endangered, including sperm, humpback, sei, fin, blue, and North 
Pacific right whales. Another species, the Steller sea lion, is listed 
as Threatened and may occur in the project area.
    The study area is located approximately 25 to 110 km (15.5 to 68.4 
mi) offshore from Oregon over water depths from just less than 100 m to 
almost 3,000 m. Two of the 32 species, gray whales and sea otters, are 
not expected in the project area because their occurrence off Oregon is 
limited to very shallow, coastal waters. Three other species, 
California sea lions, Steller sea lions, and harbor seals, are mainly 
coastal, and would be rare at most at the OBS locations. Information on 
the habitat, abundance, and conservation status of the species that may 
occur in the study area are given in Table 2 (below, see Table 2 of 
SIO's application). Vagrant ringed seals, hooded seals, and ribbon 
seals have been sighted or stranded on the coast of California (see 
Mead, 1981; Reeves et al., 2002) and presumably passed through Oregon 
waters. A vagrant beluga whale was seen off the coast of Washington 
(Reeves et al., 2002). Those seven species are not addressed in detail 
in the summaries in SIO's application.
    The six species of marine mammals expected to be most common in the 
deep pelagic or slope waters of the project area, where most of the 
survey sites are located, include the Pacific white-sided dolphin, 
northern right whale dolphin, Risso's dolphin, short beaked common 
dolphin, Dall's porpoise, and northern fur seal (Green et al., 1992, 
1993; Buchanan et al., 2001; Barlow, 2003; Barlow and Forney, 2007; 
Carretta et al., 2007). The fin whale, Dall's porpoise, and the 
northern elephant seal were the species sighted most often off Oregon 
and Washington during the ORCAWALE 2008 surveys (NMFS, 2008).
    Table 2 below outlines the marine mammal species, their habitat, 
abundance, density, and conservation status in the proposed project 
area. Additional information regarding the distribution of these 
species expected to be found in the project area and how the estimated 
densities were calculated may be found in SIO's application.
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Potential Effects on Marine Mammals

Potential Effects of Airguns

    The effects of sounds from airguns might result in one or more of 
the following: tolerance, masking of natural sounds, behavioral 
disturbances, temporary or permanent hearing impairment, and 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). With the possible exception of some 
cases of temporary threshold shift in harbor seals, it is unlikely that 
the project would result in any cases of temporary or especially 
permanent hearing impairment, or any significant non-auditory physical 
or physiological effects.
Tolerance
    Numerous studies have shown that pulsed sounds from airguns are 
often readily detectable in the water at distances of many kilometers. 
For a brief summary of the characteristics of airgun pulses, see 
Appendix A(3) of SIO's application. However, it should be noted that 
most of the measurements are for airguns that would be detectable 
considerably farther away than the GI airgun planned for use in the 
present project.
    Several studies have shown that marine mammals at distances more 
than a few kilometers from operating seismic vessels often show no 
apparent response-see Appendix A(5) of SIO's application. That is often 
true even in cases when the pulsed sounds must be readily audible to 
the animals based on measured received levels and the hearing 
sensitivity of the mammal group. Although various baleen whales, 
toothed whales, and (less frequently) pinnipeds have been shown to 
react behaviorally to airgun pulses under some conditions, at other 
times, mammals of all three types have shown no overt reactions. In 
general, pinnipeds usually seem to be more tolerant of exposure to 
airgun pulses than are cetaceans, with relative responsiveness of 
baleen and toothed whales being variable. Given the relatively small 
and low-energy GI airgun source planned for use in this project, 
mammals are expected to be tolerate being closer to this source than 
would be the case for a larger airgun source typical of most seismic 
surveys.
Masking
    Obscuring of sounds of interest by interfering sounds, generally at 
similar frequencies, is known as masking. Masking effects of pulsed 
sounds (even from large arrays of airguns) on marine mammal calls and 
other natural sounds are expected to be limited, although there are few 
specific data of relevance. 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 some 
situations, multi-path arrivals and reverberation cause airgun sound to 
arrive for much or all of the interval between pulses (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. The airgun sounds are pulsed, with quiet periods between the 
pulses, and whale calls often can be heard between the seismic pulses 
(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). In the northeast Pacific Ocean, blue whale calls have been 
recorded during a seismic survey off Oregon (McDonald et al., 1995). 
Among odontocetes, there has been one report that sperm whales cease 
calling when exposed to pulses from a very distant seismic ship (Bowles 
et al., 1994). However, more recent studies found that sperm whales 
continued calling in the presence of seismic pulses (Madsen et al., 
2002; Tyack et al., 2003; Smultea et al., 2004; Holst et al., 2006; 
Jochens et al., 2006, 2008). Given the small source planned for use 
during the proposed survey, there is even less potential for masking of 
baleen or sperm whale calls during the present study than in most 
seismic surveys. Masking effects of seismic pulses are expected to be 
negligible in the case of the small odontocetes given the intermittent 
nature of seismic pulses. Dolphins and porpoises commonly are heard 
calling while airguns are operating (Gordon et al., 2004; Smultea et 
al., 2004; Holst et al., 2005a,b; Potter et al., 2007). Also, the 
sounds important to small odontocetes are predominantly at much higher 
frequencies than the airgun sounds, thus further 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. 
Masking effects on marine mammals are discussed further in Appendix A 
(4) of SIO's application.
Disturbance Reactions
    Disturbance includes a variety of effects, including subtle changes 
in behavior, more conspicuous changes in activities, 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. If a marine mammal responds to an underwater sound by 
changing its behavior or moving a small distance, the response may or 
may not rise to the level of ``harassment,'' or affect the stock or the 
species as a whole. However, if a sound

[[Page 24806]]

source displaces marine mammals from an important feeding or breeding 
area for a prolonged period, impacts on animals or on the stock or 
species could potentially be significant (Lusseau and Bejder, 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 are likely to be present within a particular distance 
of industrial activities, or exposed to a particular level of 
industrial sound. This practice potentially overestimates the numbers 
of marine mammals that are affected in some biologically-important 
manner.
    The sound exposure thresholds that are used to estimate how many 
marine mammals might be harassed by a seismic survey are based on 
behavioral observations during studies of several species. However, 
information is lacking for many species. Detailed studies have been 
done on humpback, gray, bowhead, and on ringed seals. Less detailed 
data are available for some other species of baleen whales, sperm 
whales, small toothed whales, and sea otters, but for many species 
there are no data on responses to marine seismic surveys. Most of those 
studies have concerned reactions to much larger airgun sources than 
planned for use in the proposed project. Thus, effects are expected to 
be limited to considerably smaller distances and shorter periods of 
exposure in the present project than in most of the previous work 
concerning marine mammal reactions to airguns.
    Baleen Whales - Baleen whales generally tend to avoid operating 
airguns, but avoidance radii are quite variable. Whales are often 
reported to show no overt reactions to pulses from large arrays of 
airguns at distances beyond a few kilometers, even though the airgun 
pulses remain well above ambient noise levels out to much longer 
distances. However, as reviewed in Appendix A(5) of SIO's application, 
baleen whales exposed to strong noise pulses from airguns often react 
by deviating from their normal migration route and/or interrupting 
their feeding activities and moving away from the sound source. In the 
case of the migrating gray and bowhead whales, the observed changes in 
behavior appeared to be of little or no biological consequence to the 
animals. 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 demonstrated 
that received levels of pulses in the 160-170 dB re 1 microPa rms range 
seem to cause obvious avoidance behavior in a substantial fraction of 
the animals exposed. In many areas, seismic pulses from large arrays of 
airguns diminish to those levels at distances ranging from 4.5-14.5 km 
(2.8-9 mi) from the source. A substantial proportion of the baleen 
whales within those distances may show avoidance or other strong 
disturbance reactions to the airgun array. Subtle behavioral changes 
sometimes become evident at somewhat lower received levels, and studies 
summarized in Appendix A(5) of SIO's application have shown that some 
species of baleen whales, notably bowhead and humpback whales, at times 
show strong avoidance at received levels lower than 160-170 dB re 1 
microPa (rms). Reaction distances would be considerably smaller during 
the proposed project, for which the 160 dB radius is predicted to be 
220 to 570 m (722 to 1,870 ft) (see Table 1 above), as compared with 
several km when a large array of airguns is operating.
    Responses of humpback whales to seismic surveys have been studied 
during migration, on the summer feeding grounds, and on Angolan winter 
breeding grounds; there has also been discussion of effects on the 
Brazilian wintering grounds. McCauley et al. (1998, 2000a) studied the 
responses of humpback whales off Western Australia to a full-scale 
seismic survey with a 16-airgun, 2,678 in\3\ array, and to a single 20 
in\3\ airgun with a source level of 227 dB re 1 microPa m peak-to-peak. 
McCauley et al. (1998) documented that initial avoidance reactions 
began at 5 to 8 km (3.1 to 5 mi) from the array, and that those 
reactions kept most pods approximately 3 to 4 km (1.9 to 2.5 mi) from 
the operating seismic boat. McCauley et al. (2000) noted localized 
displacement during migration of 4 to 5 km (2.5 to 3.1 mi) by traveling 
pods and 7 to12 km (4.3 to 7.5 mi) by cow-calf pairs. Avoidance 
distances with respect to the single airgun were smaller (2 km (1.2 
mi)) but consistent with the results from the full array in terms of 
received sound levels. The mean received level for initial avoidance 
reactions of an approaching airgun was a sound level of 140 dB re 1 
microPa (rms) for humpback whale pods containing females. The standoff 
range, i.e., the closest point of approach (CPA) of the whales to the 
airgun, corresponded to a received level of 143 dB re 1 microPa (rms). 
The initial avoidance response generally occurred at distances of 5 to 
8 km (3.1 to 5 mi) from the airgun array and 2 km (1.2 mi) 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 microPa (rms).
    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 150-169 dB re 1 microPa on an 
approximate rms basis. 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 re 1 microPa on an approximate rms basis.
    Among wintering humpback whales off Angola (n = 52 useable groups), 
there were no significant differences in encounter rates (sightings/hr) 
when a 24 airgun array (3,147 in\3\ or 5,805 in\3\) was operating vs. 
silent (Weir, 2008). There was also no significant difference in the 
mean CPA distance of the humpback whale sightings when airguns were on 
vs. off (3,050 m vs. 2,700 m or 10,007 vs. 8,858 ft, respectively).
    It has been 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 results from 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, 2007b: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 the activity (e.g., 
migrating vs. 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 
20-30 km (12.4-18.6 mi) from a medium-sized airgun source at received 
sound levels of around 120-130 dB re 1 microPa (rms) (Miller et al., 
1999; Richardson et al., 1999; see Appendix B (5) of L-DEO's 
application). However, more recent research on bowhead whales (Miller 
et al., 2005a; 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

[[Page 24807]]

upon statistical analysis (Richardson et al., 1986). In summer, 
bowheads typically begin to show avoidance reactions at a received 
level of about 160-170 dB re 1 microPa (rms) (Richardson et al., 1986; 
Ljungblad et al., 1988; Miller et al., 2005a).
    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. Malme et al. (1986, 1988) estimated, based on small sample sizes, 
that 50 percent of feeding gray whales ceased feeding at an average 
received pressure level of 173 dB re 1 microPa on an (approximate) rms 
basis, and that 10 percent of feeding whales interrupted feeding at 
received levels of 163 dB. 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 with observations of Western Pacific 
gray whales feeding off Sakhalin Island, Russia, when a seismic survey 
was underway just offshore of their feeding area (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). Gray whales 
typically show no conspicuous responses to airgun pulses with received 
levels up to 150 to 160 dB re 1 microPa (rms), but are increasingly 
likely to show avoidance as received levels increase above that range.
    Various species of Balaenoptera (blue, sei, fin, Bryde's, and minke 
whales) have occasionally been reported in areas ensonified by airgun 
pulses (Stone, 2003; MacLean and Haley, 2004; Stone and Tasker, 2006). 
Sightings by observers on seismic vessels off the United Kingdom from 
1997 to 2000 suggest that, at times of good sightability, sighting 
rates for mysticetes (mainly fin and sei whales) were similar when 
large arrays of airguns were shooting and not shooting (Stone, 2003; 
Stone and Tasker, 2006). However, these whales tended to exhibit 
localized avoidance, remaining significantly (on average) from the 
airgun array during seismic operations compared with non-seismic 
periods (Stone and Tasker, 2006). In a study off 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 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 direction during seismic 
vs. non-seismic periods (Moulton et al., 2005, 2006a,b).
    Data on short-term reactions (or lack of reactions) of cetaceans to 
impulsive noises do not necessarily provide information about long-term 
effects. It is not known whether impulsive noises affect reproductive 
rate or distribution and habitat use in subsequent days or years. 
However, gray whales 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 (see Appendix A in Malme et al., 1984; 
Richardson et al., 1995; Angliss and Outlaw, 2008). The Western Pacific 
gray whale population did not seem affected by a seismic survey in its 
feeding ground during a prior year (Johnson et al., 2007). Bowhead 
whales 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). 
In any event, brief exposures to sound pulses from the proposed airgun 
source are highly unlikely to result in prolonged effects.
    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 have 
been reported for toothed whales. However, systematic studies on sperm 
whales have been done (Jochens and Biggs, 2003; Tyack et al., 2003; 
Jochens et al., 2006; Miller et al., 2006), and there is an increasing 
amount of information about responses of various odontocetes to seismic 
surveys based on monitoring studies (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; Weir, 2008).
    Seismic operators and MMOs on seismic vessels regularly see 
dolphins and other small toothed whales near operating airgun arrays, 
but in general there seems to be a tendency for most delphinids to show 
some avoidance of operating seismic vessels (Goold, 1996a,b,c; 
Calambokidis and Osmek, 1998; Stone, 2003; Moulton and Miller, 2005; 
Holst et al., 2006; Stone and Tasker, 2006; Weir, 2008). 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 airgun arrays are 
firing (Moulton and Miller, 2005). Nonetheless, there have been 
indications that small toothed whales sometimes 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 (Stone and Tasker, 
2006; Weir, 2008). In most cases, the avoidance radii for delphinids 
appear to be small, on the order of 1 km (0.62 mi) or less, and some 
individuals show no apparent avoidance. The beluga is a species that 
(at least at times) shows long-distance avoidance of seismic vessels. 
Aerial surveys during seismic operations in the southeastern Beaufort 
Sea during summer recorded much lower sighting rates of beluga whales 
within 10-20 km (6.2-12.4 mi) compared with 20-30 km (mi) from an 
operating airgun array, and observers on seismic boats in that area 
rarely see belugas (Miller et al., 2005a; Harris et al., 2007).
    Captive bottlenose dolphins 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; Finneran and Schlundt, 2004). The animals tolerated high received 
levels of sound (pk-pk level >200 dB re 1 microPa) before exhibiting 
aversive behaviors. For pooled data at 3, 10, and 20 kHz, sound 
exposure levels during sessions with 25, 50, and 75 percent altered 
behavior were 180, 190, and 199 dB re 1 microPa\2\, respectively 
(Finneran and Schlundt, 2004).
    Results for porpoises depend on species. Dall's porpoises seem 
relatively tolerant of airgun operations (MacLean and Koski, 2005) and, 
during a survey with a large airgun array, tolerated higher noise 
levels than did harbor porpoises and gray whales (Bain and Williams, 
2006). However, Dall's porpoises do respond to the approach of large 
airgun arrays by moving away (Calambokidis and Osmek, 1998; Bain and 
Williams, 2006). The limited available data suggest that harbor 
porpoises show stronger avoidance (Stone, 2003; Bain and Williams, 
2006; Stone and Tasker, 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 
in general (Richardson et al., 1995; Southall et al. 2007).

[[Page 24808]]

    Most studies of sperm whales exposed to airgun sounds indicate that 
this species shows considerable tolerance of airgun pulses (Stone, 
2003; Moulton et al., 2005, 2006a; Stone and Tasker, 2006; Weir, 2008). 
In most cases, the whales do not show strong avoidance and continue to 
call (see Appendix A in SIO's application). However, controlled 
exposure experiments in the Gulf of Mexico indicate that foraging 
effort is somewhat altered upon exposure to airgun sounds (Jochens et 
al., 2006, 2008). In the SWSS study, D-tags (Johnson and Tyack, 2003) 
were used to record the movement and acoustic exposure of eight 
foraging sperm whales before, during, and after controlled sound 
exposures of airgun arrays in the Gulf of Mexico (Jochens et al., 
2008). Whales were exposed to maximum received sound levels between 111 
and 147 dB re 1 microPa (rms) (131 to 164 dB re 1 microPa pk-pk) at 
ranges of approximately 1.4 to 12. 6 km (0.9 to 7.8 mi) from the sound 
source. Although the tagged whales showed no horizontal avoidance, some 
whales changed foraging behavior during full array exposure (Jochens et 
al., 2008).
    There are almost no specific data on the behavioral reactions of 
beaked whales to seismic surveys. However, northern bottlenose whales 
(Hyperodon ampullatus) continued to produce high-frequency clicks when 
exposed to sound pulses from distant seismic surveys (Laurinolli and 
Cochrane, 2005; Simard et al., 2005). Most beaked whales tend to avoid 
approaching vessels of other types (Wursig et al., 1998). They may also 
dive for an extended period when approached by a vessel (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 quire long 
(Baird et al., 2006; Tyack et al., 2006). In any event, it is likely 
that these beaked whales would normally show strong avoidance of an 
approaching seismic vessel, but this has not been documented 
explicitly.
    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 A of SIO's 
application).
    Additional details on the behavioral reactions (or the lack 
thereof) by all types of marine mammals to seismic vessels can be found 
in Appendix A(5) of SIO's application.
Hearing Impairment and Other Physical Effects
    Temporary or permanent hearing impairment is a possibility when 
marine mammals are exposed to very strong sounds, but there has been no 
specific documentation of this for marine mammals exposed to sequences 
of airgun pulses.
    NMFS will be developing new noise exposure criteria for marine 
mammals that take account of the now-available scientific data on 
temporary threshold shift (TTS), the expected offset between the TTS 
and permanent threshold shift (PTS) thresholds, differences in the 
acoustic frequencies to which different marine mammal groups are 
sensitive, and other relevant factors. Detailed recommendations for new 
science-based noise exposure criteria were published in late 2007 
(Southall et al., 2007).
    Several aspects of the planned monitoring and mitigation measures 
for this project (see below) are designed to detect marine mammals 
occurring near the airguns to avoid exposing them to sound pulses that 
might, at least in theory, cause hearing impairment. In addition, many 
cetaceans and (to a limited degree) pinnipeds are likely to show some 
avoidance of the area where received levels of airgun sound are high 
enough such that hearing impairment could potentially occur. In those 
cases, the avoidance responses of the animals themselves will reduce or 
(most likely) avoid any possibility of hearing impairment.
    Non-auditory physical effects may also occur in marine mammals 
exposed to strong underwater pulsed sound. Possible types of non-
auditory physiological effects or injuries that theoretically might 
occur in mammals close to a strong sound source include stress, 
neurological effects, bubble formation, resonance effects, and other 
types of organ or tissue damage. It is possible that some marine mammal 
species (i.e., beaked whales) may be especially susceptible to injury 
and/or stranding when exposed to strong pulsed sounds. However, as 
discussed below, there is no definitive evidence that any of these 
effects occur even for marine mammals in close proximity to large 
arrays of airguns. It is especially unlikely that any effects of these 
types would occur during the present project given the brief duration 
of exposure of any given mammal and the proposed monitoring and 
mitigation measures (see below). The following subsections discuss in 
somewhat more detail the possibilities of TTS, PTS, and non-auditory 
physical effects.
    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).
    For toothed whales exposed to single short pulses, the TTS 
threshold appears to be, to a first approximation, a function of the 
energy content of the pulse (Finneran et al., 2002, 2005). Given the 
available data, the received level of a single seismic pulse (with no 
frequency weighting) might need to be approximately 186 dB re 1 
microPa\2\\.\s (i.e., 186 dB SEL or approximately 221-226 dB pk-pk) in 
order to produce brief, mild TTS. Exposure to several strong seismic 
pulses that each have received levels near 190 dB re 1 microPa (rms) 
(175-180 dB SEL) might result in cumulative exposure of approximately 
186 dB SEL and thus slight TTS in a small odontocete, assuming the TTS 
threshold is (to a first approximation) a function of the total 
received pulse energy. Levels [gteqt] 190 dB 1 microPa (rms) are 
expected to be restricted to radii no more than 95 m (312 ft) from the 
Wecoma's GI airgun. For an odontocete closer to the surface, the 
maximum radius with [gteqt]190 dB 1 microPa (rms) would be smaller.
    The above TTS information for odontocetes is derived from studies 
on the bottlenose dolphin and beluga. There is not published TTS 
information for other species of cetaceans. However, preliminary 
evidence from harbor porpoise exposed to airgun sound suggests that its 
TTS threshold may have been lower (Lucke et al., 2007).
    For baleen whales, there are no data, direct or indirect, on levels 
or properties of sound required to induce TTS. The frequencies to which 
baleen whales are most sensitive are lower than those for odontocetes, 
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)

[[Page 24809]]

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. In any event, no cases 
of TTS are expected given three considerations:
    (1) Small size of the GI airgun source;
    (2) The strong likelihood that baleen whales would avoid the 
approaching airguns (or vessel) before being exposed to levels high 
enough for TTS to possibly occur; and
    (3) The mitigation measures that are planned.
    In pinnipeds, TTS thresholds associated with exposure to brief 
pulses (single or multiple) of underwater sound have not been measured. 
Initial evidence from prolonged (non-pulse) exposures suggested that 
some pinnipeds may incur TTS at somewhat lower received levels than do 
small odontocetes exposed for similar durations (Kastak et al., 1999, 
2005; Ketten et al., 2001; Au et al., 2000). The TTS threshold for 
pulsed sounds has been indirectly estimated as being an SEL of 
approximately 171 dB re 1 microPa2.s (Southall et al., 
2007), which would be equivalent to a single pulse with received level 
approximately 181-186 re 1 microPa (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).
    A marine mammal within a radius of less than 100 m (328 ft) around 
a typical large array of operating airguns might be exposed to a few 
seismic pulses with levels of greater than or equal to 205 dB, and 
possibly more pulses if the mammal moved with the seismic vessel. (As 
noted above, most cetacean species tend to avoid operating airguns, 
although not all individuals do so.) In addition, ramping up airgun 
arrays, which is standard operational protocol for large airgun arrays 
and proposed for this action, should allow cetaceans to move away form 
the seismic source and avoid being exposed to the full acoustic output 
of the airgun array. Even with a large airgun array, it is unlikely 
that the cetaceans would be exposed to airgun pulses at a sufficiently 
high level for a sufficiently long period to cause more than mild TTS, 
given the relative movement of the vessel and the marine mammal. The 
potential for TTS is much lower in this project. With a large array of 
airguns, TTS would be most likely in any odontocetes that bow-ride or 
otherwise linger near the airguns. While bow-riding, odontocetes would 
be at or above the surface, and thus not exposed to strong pulses given 
the pressure-release effect at the surface. However, bow-riding animals 
generally dive below the surface intermittently. If they did so while 
bow-riding near airguns, they would be exposed to strong sound pulses, 
possibly repeatedly. If some cetaceans did incur TTS through exposure 
to airgun sounds, this would very likely be mild, temporary, and 
reversible.
    To avoid the potential for injury, NMFS has determined that 
cetaceans and pinnipeds should not be exposed to pulsed underwater 
noise at received levels exceeding, respectively, 180 and 190 dB re 1 
microPa (rms). As summarized above, data that are now available imply 
that TTS is unlikely to occur unless odontocetes (and probably 
mysticetes as well) are exposed to airgun pulses stronger than 180 dB 
re 1 microPa (rms).
    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, while 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 TTS, there has been further speculation about the 
possibility that some individuals occurring very close to airguns might 
incur PTS (Richardson et al., 1995). Single or occasional occurrences 
of mild TTS are not indicative of permanent auditory damage.
    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 decibels above that inducing mild TTS if the animal were 
exposed to strong sound pulses with rapid rise time (see Appendix A(5) 
of SIO's application). Based on data from terrestrial mammals, a 
precautionary assumption is that the 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 
>6 dB (Southall et al., 2007). On an SEL basis, Southall et al. (2007) 
estimated that received levels would need to exceed the TTS threshold 
by at least 15 dB for there to be risk of PTS. Thus, for cetaceans they 
estimate that the PTS threshold might be an M-weighted SEL (for the 
sequence of received pulses) of approximately 198 dB re 1 
microPa\2\micros (15 dB higher than the TTS threshold for an impulse). 
Additional assumptions had to be made to derive a corresponding 
estimate for pinnipeds, as the only available data on TTS thresholds in 
pinnipeds pertain to non-impulse sound. Southall et al. (2007) estimate 
that the PTS threshold could be a cumulative Mpw-weighted SEL of 
approximately 186 dB 1 microPa\2\\.\s in the harbor seal to impulse 
sound. The PTS threshold for the California sea lion and northern 
elephant seal the PTS threshold would probably be higher, given the 
higher TTS thresholds in those species.
    Southall et al. (2007) also note that, regardless of the SEL, there 
is concern about the possibility of PTS if a cetacean or pinniped 
receives one or more pulses with peak pressure exceeding 230 or 218 dB 
re 1 microPa (3.2 bar\.\ m, 0-pk), which would only be found within a 
few meters of the largest (600-in\3\) airguns in the planned airgun 
array (Caldwell and Dragoset, 2000). A peak pressure of 218 dB re 1 
microPa could be received somewhat farther away; to estimate that 
specific distance, one would need to apply a model that accurately 
calculates peak pressures in the near-field around an array of airguns.
    Given the higher level of sound necessary to cause PTS as compared 
with TTS, it is considerably less likely that PTS could occur. Baleen 
whales generally avoid the immediate area around operating seismic 
vessels, as do some other marine mammals. The planned monitoring and 
mitigation measures, including visual monitoring and shut downs of the 
airguns when mammals are seen about to enter or within the proposed 
exclusion zone (EZ), will further reduce the probability of exposure of 
marine mammals to sounds strong enough to induce PTS, see the section 
below on Proposed Mitigation and Monitoring.
    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 effects, and other types of organ 
or tissue damage (Cox et al., 2006; Southall et al., 2007). Studies 
examining such effects are limited. However, resonance (Gentry, 2002) 
and direct noise-induced bubble formation (Crum et al., 2005) are not 
expected in the case of an impulsive 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

[[Page 24810]]

specific evidence of this upon exposure to airgun pulses.
    In general, little is known about the potential for seismic survey 
sounds to cause auditory impairment or other physical effects in marine 
mammals. Available data suggest that such effects, if they occur at 
all, would presumably be limited to short distances from the sound 
source 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, some odontocetes, and some pinnipeds, are 
especially unlikely to incur auditory impairment or non-auditory 
physical effects. Also, the planned mitigation measures, including shut 
downs of the airgun, would reduce any such effects that might otherwise 
occur.
Strandings and Mortality
    Marine mammals close to underwater detonations of high explosives 
can be killed or severely injured, and their auditory organs are 
especially susceptible to injury (Ketten et al., 1993; Ketten, 1995). 
However, explosives are no longer used for marine seismic research or 
commercial seismic surveys, and 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 injury, death, or stranding even in the case of 
large airgun arrays. However, the association of mass strandings of 
beaked whales with naval exercises 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 (Hildebrand, 2005; Southall et al., 2007). Appendix 
A(5) of SIO's application 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 hemorrahage or other forms of trauma;
    (3) A physiological change such as a vestibular response leading to 
a behavioral change or stress-induced hemorrahagic 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.
    As noted in SIO's application, some of these mechanisms are 
unlikely to apply in the case of impulse sounds. However, there are 
increasing indications that gas-bubble disease (analogous to ``the 
bends''), induced in super-saturated tissue by a behavioral response to 
acoustic exposure, could be pathologic mechanism for the strandings and 
mortality of some deep diving cetaceans exposed to sonar. 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 pulses 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 with most of the energy below 1 
kHz. Typical military mid-frequency sonars operate at frequencies of 2-
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 pulses can, in special 
circumstances, lead (at least indirectly) to physical damage and 
mortality (Balcomb and Claridge, 200