Small Takes of Marine Mammals Incidental to Specified Activities; Low-Energy Seismic Survey on the Louisville Ridge, Southwest Pacific Ocean, 6041-6055 [06-1074]

Agencies

• DEPARTMENT OF COMMERCE
• National Oceanic and Atmospheric Administration

[Federal Register Volume 71, Number 24 (Monday, February 6, 2006)]
[Notices]
[Pages 6041-6055]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 06-1074]


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DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

[I.D. 080905A]


Small Takes of Marine Mammals Incidental to Specified Activities; 
Low-Energy Seismic Survey on the Louisville Ridge, Southwest Pacific 
Ocean

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

ACTION: Notice of issuance of an incidental harassment authorization.

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SUMMARY: In accordance with provisions of the Marine Mammal Protection 
Act (MMPA) as amended, notification is hereby given that an Incidental 
Harassment Authorization (IHA) to take small numbers of marine mammals, 
by harassment, incidental to conducting an oceanographic survey in the 
southwestern Pacific Ocean (SWPO) has been issued to the Scripps 
Institution of Oceanography (Scripps).

DATES: Effective from January 20, 2006, through January 19, 2007.

ADDRESSES: The authorization and application containing a list of the 
references used in this document may be obtained by writing to this 
address or by telephoning the contact listed here. The application is 
also available at: https://www.nmfs.noaa.gov/pr/permits/incidental.htm.

FOR FURTHER INFORMATION CONTACT: Kenneth Hollingshead, Office of 
Protected Resources, NMFS, (301) 713-2289, ext 128.

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 U.S. 
citizens who engage in a specified activity (other than commercial 
fishing) within a specified geographical region if certain findings are 
made and either regulations are issued or, if the taking is limited to 
harassment, a notice of a proposed authorization is provided to the 
public for review.
    An authorization may 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 and that the permissible methods of 
taking and requirements pertaining to the 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 [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].

    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 
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 June 29, 2005, NMFS received an application from Scripps for the 
taking, by harassment, of several species of marine mammals incidental 
to conducting a low-energy marine seismic survey program during early 
2006 in the SWPO. Scripps plans to conduct a seismic survey of several 
seamounts on the Louisville Ridge in the SWPO as part of the Integrated 
Ocean Drilling Program (IODP). As presently scheduled, the seismic 
survey will occur from about January 21 to February 26, 2006.
    The purpose of the research program is to conduct a planned 
scientific rock-dredging, magnetic, and seismic survey program of six 
seamounts of the Louisville seamount chain. The results will be used 
to: (1) Test hypotheses about the eruptive history of the submarine 
volcanoes, the subsequent formation (by subaerial erosion and 
submergence) of its many guyots, and motion of the hotspot plume; and 
(2) design an effective IODP cruise (not currently scheduled) to drill 
on carefully-selected seamounts. Included in the research planned for 
2006 is scientific rock dredging, extensive total-field and three-
component magnetic surveys, the use of multi-beam and Chirp techniques 
to map the seafloor, and high-resolution seismic methods to image the 
subsea floor. Following the cruise, chemical and geochronologic 
analyses will be conducted on rocks from 25 sites.

Description of the Activity

    The seismic surveys will involve one vessel. The source vessel, the 
R/V Roger Revelle, will deploy a pair of low-energy Generator-Injector 
(GI) airguns as an energy source (each with a discharge volume of 45 
in\3\), plus a 450-m (1476-ft) long, 48-channel, towed hydrophone 
streamer. As the airguns are towed along the survey lines, the 
receiving system will receive the returning acoustic signals.
    The program will consist of approximately 1840 km (994 nm) of 
surveys, including turns. Water depths within the seismic survey areas 
are 800-2300 m (2625-7456 ft). The GI guns will be operated on a small 
grid (see inset in Figure 1 in Scripps (2006)) for about 28 hours at 
each of 6 seamounts between approximately January 28 to February 19, 
2006. There will be additional seismic operations associated with 
equipment testing, start-up, and repeat coverage of any areas where 
initial data quality is sub-standard.
    The Revelle is scheduled to depart from Papeete, French Polynesia, 
on or about January 21, 2006, and to arrive at Wellington, New Zealand, 
on or about February 26, 2006. The GI guns will be used for about 28 
hours on each of 6 seamounts between about January 28th to February 
19th. The exact dates of the activities may vary by a few days

[[Page 6042]]

because of weather conditions, repositioning, streamer operations and 
adjustments, airgun deployment, or the need to repeat some lines if 
data quality is substandard. The overall area within which the seismic 
surveys will occur is located between approximately 25[deg] and 45[deg] 
S., and between 155[deg] and 175[deg] W. The surveys will be conducted 
entirely in International Waters.
    In addition to the operations of the GI guns, a 3.5-kHz sub-bottom 
profiler and passive geophysical sensors to conduct total-field and 
three-component magnetic surveys will be operated during seismic 
surveys. A Kongsberg-Simrad EM-120 multi-beam sonar will be used 
continuously throughout the cruise.
    The energy to the airguns is compressed air supplied by compressors 
on board the source vessel. Seismic pulses will be emitted at intervals 
of 6-10 seconds. At a speed of 7 knots (13 km/h), the 6-10 sec spacing 
corresponds to a shot interval of approximately 21.5-36 m (71-118 ft).
    The generator chamber of each GI gun, the one responsible for 
introducing the sound pulse into the ocean, is 45 in3. 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 two 45/105 in\3\ GI guns will be towed 8 m 
(26.2 ft) apart side by side, 21 m (68.9 ft) behind the Revelle, at a 
depth of 2 m (6.6 ft).

General-Injector Airguns

    Two GI-airguns will be used from the Revelle during the proposed 
program. These 2 GI-airguns have a zero to peak (peak) source output of 
230.7 dB re 1 microPascal-m (3.4 bar-m) and a peak-to-peak (pk-pk) 
level of 235.9B (6.2 bar-m). However, these downward-directed source 
levels do not represent actual sound levels that can be measured at any 
location in the water. Rather, they represent the level that would be 
found 1 m (3.3 ft) from a hypothetical point source emitting the same 
total amount of sound as is emitted by the combined airguns in the 
airgun array. The actual received level at any location in the water 
near the airguns will not exceed the source level of the strongest 
individual source and actual levels experienced by any organism more 
than 1 m (3.3 ft) from any GI gun will be significantly lower.
    Further, the root mean square (rms) received levels that are used 
as impact criteria for marine mammals (see Richardson et al., 1995) are 
not directly comparable to these peak or pk-pk values that are normally 
used to characterize source levels of airgun arrays. The measurement 
units used to describe airgun sources, peak or pk-pk decibels, are 
always higher than the rms decibels referred to in biological 
literature. For example, a measured received level of 160 dB rms in the 
far field would typically correspond to a peak measurement of about 170 
to 172 dB, and to a pk-pk measurement of about 176 to 178 decibels, 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 pk-pk 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 pk-pk level for an airgun-type source.
    The depth at which the sources are towed has a major impact on the 
maximum near-field output, because the energy output is constrained by 
ambient pressure. The normal tow depth of the sources to be used in 
this project is 2.0 m (6.6 ft), where the ambient pressure is 
approximately 3 decibars. This also limits output, as the 3 decibars of 
confining pressure cannot fully constrain the source output, with the 
result that there is loss of energy at the sea surface. Additional 
discussion of the characteristics of airgun pulses is provided in 
Scripps application and in previous Federal Register documents (see 69 
FR 31792 (June 7, 2004) or 69 FR 34996 (June 23, 2004)).
    Received sound levels have been modeled by Lamont-Doherty Earth 
Observatory (L-DEO) for a number of airgun configurations, including 
two 45-in\3\ Nucleus G-guns (G guns), in relation to distance and 
direction from the airguns. The L-DEO model does not allow for bottom 
interactions, and is therefore most directly applicable to deep water. 
Based on the modeling, estimates of the maximum distances from the GI 
guns where sound levels of 190, 180, 170, and 160 dB microPascal-m 
(rms) are predicted to be received are shown in Table 1. Because the 
model results are for the G guns, which have more energy than GI guns 
of the same size, those distances are overestimates of the distances 
for the 45 in\3\ GI guns.

 Table 1.--Distances to Which Sound Levels >=190, 180, 170, and 160 dB re 1 [mu]Pa (rms) Might Be Received From
Two 45-in \3\ G Guns, Similar to the Two 45-in \3\ GI Guns That Will Be Used During the Seismic Survey in the SW
       Pacific Ocean During January-February 2006. Distances Are Based on Model Results Provided By L-DEO.
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                                                                  Estimated distances at received levels (m)
                         Water depth                         ---------------------------------------------------
                                                                 190 dB       180 dB       170 dB       160 dB
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100-1000 m..................................................           15           60          188          525
>1000 m.....................................................           10           40          125          350
----------------------------------------------------------------------------------------------------------------

    Some empirical data concerning the 180- and 160-dB distances have 
been acquired based on measurements during an acoustic verification 
study conducted by L-DEO in the northern Gulf of Mexico between May 27 
and June 3, 2003 (Tolstoy et al., 2004). Although the results are 
limited, the data showed that water depth affected the radii around the 
airguns where the received level would be 180 dB re 1 microPa (rms), 
NMFS' current injury threshold safety criterion applicable to cetaceans 
(NMFS, 2000). Similar depth-related variation is likely in the 190-dB 
distances applicable to pinnipeds. Correction factors were developed 
for water depths 100-1000 m (328-3281 ft) and less than 100 m (328 ft). 
The proposed survey will occur in depths 800-2300 m (2625-7456 ft), so 
only the correction factor for intermediate water depths is relevant 
here.
    The empirical data indicate that for deep water (>1000 m (3281 
ft)), 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 airgun operations in deep water will 
be the values predicted by L-DEO's model (Table 1). Therefore, the 
assumed 180- and 190-dB radii are 40 m (131 ft) and 10 m (33 ft), 
respectively.

[[Page 6043]]

Bathymetric Sonar and Sub-bottom Profiler

    The Kongsberg-Simrad EM120 multi-beam sonar operates at 11.25-12.6 
kHz, and is mounted in the hull of the Revelle. It operates in several 
modes, depending on water depth. In the proposed survey, it will be 
used in deep (>800-m) water, and will operate in ``deep'' mode. The 
beamwidth is 1[deg] or 2[deg] fore-aft and a total of 150[deg] 
athwartship. Estimated maximum source levels are 239 and 233 dB at 
1[deg] and 2[deg] beam widths, respectively. Each ``ping'' consists of 
nine successive fan-shaped transmissions, each ensonifying a sector 
that extends 1[deg] or 2[deg] fore-aft. In the ``deep'' mode, the total 
duration of the transmission into each sector is 15 ms. The nine 
successive transmissions span an overall cross-track angular extent of 
about 150[deg], with 16-ms gaps between the pulses for successive 
sectors. A receiver in the overlap area between two sectors would 
receive two 15-ms pulses separated by a 16-ms gap. The ``ping'' 
interval varies with water depth, from approximately 5 sec at 1000 m 
(3281 ft) to 20 sec at 4000 m (13123 ft/2.2 nm).
    Sub-bottom Profiler--The sub-bottom profiler is normally operated 
to provide information about the sedimentary features and the bottom 
topography that is simultaneously being mapped by the multi-beam sonar. 
The energy from the sub-bottom profiler is directed downward by a 3.5-
kHz transducer mounted in the hull of the Revelle. The output varies 
with water depth from 50 watts in shallow water to 800 watts in deep 
water. Pulse interval is 1 second (sec) but a common mode of operation 
is to broadcast five pulses at 1-s intervals followed by a 5-sec pause. 
The beamwidth is approximately 30[deg] and is directed downward. 
Maximum source output is 204 dB re 1 microPa (800 watts) while normal 
source output is 200 dB re 1 microPa (500 watts). Pulse duration will 
be 4, 2, or 1 ms, and the bandwith of pulses will be 1.0 kHz, 0.5 kHz, 
or 0.25 kHz, respectively.
    Although the sound levels have not been measured directly for the 
sub-bottom profiler used by the Revelle, Burgess and Lawson (2000) 
measured sounds propagating more or less horizontally from a sub-bottom 
profiler similar to the Scripps unit with similar source output (i.e., 
205 dB re 1 microPa m). For that profiler, the 160- and 180-dB re 1 
microPa (rms) radii in the horizontal direction were estimated to be, 
respectively, near 20 m (66 ft) and 8 m (26 ft) from the source, as 
measured in 13 m (43 ft) water depth. The corresponding distances for 
an animal in the beam below the transducer would be greater, on the 
order of 180 m (591 ft) and 18 m (59 ft) respectively, assuming 
spherical spreading. Thus the received level for the Scripps sub-bottom 
profiler would be expected to decrease to 160 and 180 dB about 160 m 
(525 ft) and 16 m (52 ft) below the transducer, respectively, assuming 
spherical spreading. Corresponding distances in the horizontal plane 
would be lower, given the directionality of this source (30[deg] 
beamwidth) and the measurements of Burgess and Lawson (2000).

Characteristics of Airgun Pulses

    Discussion of the characteristics of airgun pulses was provided in 
several previous Federal Register documents (see 69 FR 31792 (June 7, 
2004) or 69 FR 34996 (June 23, 2004)) and is not repeated here. 
Reviewers are encouraged to read these earlier documents for additional 
information.

Comments and Responses

    A notice of receipt and request for 30-day public comment on the 
application and proposed authorization was published on October 17, 
2005 (70 FR 60287). During the 30-day public comment period, NMFS 
received comments only from the Marine Mammal Commission (Commission). 
It is the Commission's view that
    (1) Considerable uncertainty exists regarding the effects of sound 
on marine mammals;
    (2) Better understanding of those effects will require carefully 
designed studies, the results of which may not be available for years;
    (3) Important activities should not be postponed or delayed until 
such results become available; and
    (4) Until the results of needed studies become available and 
uncertainties are resolved or clarified, it is essential that agencies 
take a precautionary approach (as defined in the previous statements 
and publications) in authorizing and conducting activities.
    Comment 1: The Commission believes that NMFS' preliminary 
determinations are reasonable provided NMFS is satisfied that the 
proposed mitigation and monitoring activities are adequate to detect 
marine mammals in the vicinity of the proposed operations and to ensure 
that marine mammals are not being taken in unanticipated ways or 
numbers.
    Response: For this activity, the radius of the zone of potential 
impact ranges from 10 to 60 m (33 to 216.5 ft) depending upon water 
depth and whether the sighted mammal is a pinniped, a small cetacean, 
or a large cetacean (see Table 1). Considering the very small size of 
the conservative shutdown zones, the speed of the vessel when towing 
the airgun (7 kts), the length of daylight at this time of the year, 
and the marine mammal avoidance measures that are implemented by the 
vessel for animals on the vessel's track, it is very unlikely that any 
marine mammals would enter the safety zone undetected. If a marine 
mammal enters the small safety zone, operational shutdown will be 
implemented until the animal leaves the safety zone.
    Comment 2: The Commission notes that its April 2004 Beaked Whale 
Conference explored issues related to the vulnerability of beaked 
whales to anthropogenic sound. Discussions at the workshop appear to 
lend support to the hypothesis that beaked whales have unique 
characteristics that make them particularly vulnerable to certain 
anthropogenic sound sources (e.g., sonars). Preliminary research 
findings presented at the workshop suggest that at least some beaked 
whales exhibit a unique dive behavior that raises the possibility that 
they may live in a physiologic condition of chronic supersaturation 
that would increase their susceptibility to received sound levels less 
than 180 dB. Workshop participants theorized that the animals' 
behavioral response to anthropogenic sound, coupled with their 
susceptibility to gas bubble formation may lead to strandings (which in 
many cases are lethal). The Commission recognizes that the evidence 
with respect to this scenario is preliminary and that other 
explanations and scenarios exist. However, the uncertainties concerning 
the effects of sound on these species underscore the need for caution.
    Response: NMFS notes that the MMC's workshop summary report is 
available for reading or downloading at: https://www.mmc.gov/sound/
beakedwhalewrkshp/pdf/bwhale_wrkshpsummary.pdf.
    Comment 3: The Commission notes that although the proposed study is 
not expected to result in injuries or deaths to beaked whales or other 
species of marine mammals, observers will conduct monitoring for 
injured or dead animals along some recently run transect lines as the 
source vessel returns along parallel and perpendicular transect tracks. 
In this regard, the Commission would be interested in learning from 
NMFS and/or Scripps what the probability is that an injured or dead 
beaked whale, other small cetacean, or elephant seal would be sighted 
from a ship running transects through an area or retracing recently run 
transect lines.

[[Page 6044]]

    Response: NMFS is unaware of any scientific studies to demonstrate 
efficacy of conducting marine mammal sightings from a moving vessel for 
incapacitated or dead marine mammals. However, Scripps notes that the 
Revelle will spend approximately 28 hours at each of the 6 seamounts. 
As the inset to Figure 1 in the Scripp's application shows, parallel 
seismic lines are approximately 2.5 km (1.35 nm) apart, and the 
``perpendicular'' lines about twice that distance. Using big-eye 
binoculars, injured or dead mammals that are floating should be readily 
visible during daytime hours.
    Comment 4: The Commission notes that to obtain the best possible 
observations prior to initiating full-scale operations, NMFS should 
require Scripps not initiate ramp-up after dark and/or to maintain a 
low-level output from the airguns if full-scale operations may take 
place after dark.
    Response: The IHA to Scripps, similar to other seismic IHAs, 
requires that ramp-up not commence if the complete safety radii are not 
visible for at least 30 minutes prior to ramp-up in either daylight 
(rain/fog) or nighttime.
    Comment 5: The Commission notes that NMFS' discussion of Scripps' 
proposed shut-down procedures in the proposed IHA Federal Register 
notice states: ``The mammal has cleared the safety radius if it is 
visually observed to have left the safety radius, or if it has not been 
seen within the zone for 15 min. (small odontocetes and pinnipeds) or 
30 min. (mysticetes and large odontocetes)* * * .'' The Commission 
notes that elephant seals can dive for much longer than 15 minutes and, 
thus, could be directly below the sound source when it is reactivated.
    Response: For elephant seals and other pinnipeds, the safety radius 
around the 2-GI airgun seismic source (not the vessel itself) is 10-15 
m (33-49 ft) depending upon water depth. When towing seismic airguns, 
the Revelle's speed is about 7 knots (nm/hr or 13 km/hr). As a result, 
the likelihood of an elephant seal (or any other marine mammal) making 
a deep dive and returning to the immediate area of the vessel and its 
safety zone, which after 15 minutes of travel will be about 1.75 nm 
(3.2 km) away from the elephant seal sighting location, is considered 
remote.
    Comment 6: The Commission believes NMFS should require that 
operations be suspended immediately if a dead or seriously injured 
marine mammal is found in the vicinity of the operations, pending 
authorization to proceed or issuance of regulations authorizing such 
takes under section 101(a)(5)(A) of the MMPA.
    Response: A standard condition in all seismic IHAs is for an 
emergency shut-down. The IHA states that ``If observations are made or 
credible reports are received that one or more marine mammals or sea 
turtles are within the area of this activity in an injured or mortal 
state, or are indicating acute distress, the seismic airguns will be 
immediately shut down and the Chief of the Permits, Conservation and 
Education Division, Office of Protected Resources or a staff member 
contacted. The airgun array will not be restarted until review and 
approval has been given by the Director, Office of Protected Resources 
or his designee.'' However, this requirement pertains only to recently 
deceased marine mammals, not long-dead ``floaters.''

Description of Habitat and Marine Mammals Affected by the Activity

    Forty species of cetacean, including 31 odontocete (dolphin and 
small- and large-toothed whale) species and nine mysticete (baleen 
whales) species, are believed by scientists to occur in the southwest 
Pacific in the proposed seismic survey area. More detailed information 
on these species is contained in the Scripps application and the 
National Science Foundation (NSF) EA which are available at: https://
www.nmfs.noaa.gov/pr/permits/incidental.htm. Table 2 in both the 
Scripps application and NSF EA summarizes the habitat, occurrence, and 
regional population estimate for these species. The following species 
may be affected by this low-intensity seismic survey: Sperm whale, 
pygmy and dwarf sperm whales, southern bottlenose whale, Arnoux's 
beaked whale, Cuvier's beaked whale, Shepherd's beaked whale, 
mesoplodont beaked whales (Andrew's beaked whale, Blainville's beaked 
whale, gingko-toothed whale, Gray's beaked whale, Hector's beaked 
whale, spade-toothed whale, strap-toothed whale), melon-headed whale, 
pygmy killer whale, false killer whale, killer whale, long-finned pilot 
whale, short-finned pilot whale, rough-toothed dolphin, bottlenose 
dolphin, pantropical spotted dolphin, spinner dolphin, striped dolphin, 
short-beaked common dolphin, hourglass dolphin, Fraser's dolphin , 
Risso's dolphin, southern right whale dolphin, spectacled porpoise, 
humpback whale, southern right whale, pygmy right whale, common minke 
whale, Antarctic minke whale, Bryde's whale, sei whale, fin whale and 
blue whale. Because the proposed survey area spans a wide range of 
latitudes (25-45[deg] S), tropical, temperate, and possibly polar 
species are all likely to be found there. The survey area is all in 
deep-water habitat but is close to oceanic island (Kermadec Islands) 
habitats, so both coastal and oceanic species might be encountered. 
However, abundance and density estimates of cetaceans found there are 
provided for reference only, and are not necessarily the same as those 
that likely occur in the survey area.
    Five species of pinnipeds could potentially occur in the proposed 
seismic survey area: Southern elephant seal, leopard seal, crabeater 
seal, Antarctic fur seal, and the sub-Antarctic fur seal. All are 
likely to be rare, if they occur at all, as their normal distributions 
are south of the Scripps survey area. Outside the breeding season, 
however, they disperse widely in the open ocean (Boyd, 2002; King, 
1982; Rogers, 2002). Only three species of pinniped are known to wander 
regularly into the area (Reeves et al., 1999): the Antarctic fur seal, 
the sub-Antarctic fur seal, and the leopard seal. Leopard seals are 
seen as far north as the Cook Islands (Rogers, 2002).

Potential Effects on Marine Mammals

    As outlined in several previous NMFS documents, the effects of 
noise on marine mammals are highly variable, and can be categorized as 
follows (based on Richardson et al., 1995):
    (1) The noise may be too weak to be heard at the location of the 
animal (i.e., lower than the prevailing ambient noise level, the 
hearing threshold of the animal at relevant frequencies, or both);
    (2) The noise may be audible but not strong enough to elicit any 
overt behavioral response;
    (3) The noise may elicit reactions of variable conspicuousness and 
variable relevance to the well being of the marine mammal; these can 
range from temporary alert responses to active avoidance reactions such 
as vacating an area at least until the noise event ceases;
    (4) Upon repeated exposure, a marine mammal may exhibit diminishing 
responsiveness (habituation), or disturbance effects may persist; the 
latter is most likely with sounds that are highly variable in 
characteristics, infrequent and unpredictable in occurrence, and 
associated with situations that a marine mammal perceives as a threat;
    (5) Any anthropogenic noise that is strong enough to be heard has 
the potential to reduce (mask) the ability of a marine mammal to hear 
natural sounds at similar frequencies, including calls from 
conspecifics, and underwater environmental sounds such as surf noise;

[[Page 6045]]

    (6) If mammals remain in an area because it is important for 
feeding, breeding or some other biologically important purpose even 
though there is chronic exposure to noise, it is possible that there 
could be noise-induced physiological stress; this might in turn have 
negative effects on the well-being or reproduction of the animals 
involved; and
    (7) Very strong sounds have the potential to cause temporary or 
permanent reduction in hearing sensitivity. In terrestrial mammals, and 
presumably marine mammals, received sound levels must far exceed the 
animal's hearing threshold for there to be any temporary threshold 
shift (TTS) in its hearing ability. For transient sounds, the sound 
level necessary to cause TTS is inversely related to the duration of 
the sound. Received sound levels must be even higher for there to be 
risk of permanent hearing impairment. In addition, intense acoustic or 
explosive events may cause trauma to tissues associated with organs 
vital for hearing, sound production, respiration and other functions. 
This trauma may include minor to severe hemorrhage.

Effects of Seismic Surveys on Marine Mammals

    The Scripps' application provides the following information on what 
is known about the effects on marine mammals of the types of seismic 
operations planned by Scripps. The types of effects considered here are 
(1) tolerance, (2) masking of natural sounds, (2) behavioral 
disturbance, and (3) potential hearing impairment and other non-
auditory physical effects (Richardson et al., 1995). Given the 
relatively small size of the airguns planned for the present project, 
its effects are anticipated to be considerably less than would be the 
case with a large array of airguns. Scripps and NMFS believe it is very 
unlikely that there would be any cases of temporary or especially 
permanent hearing impairment, or non-auditory physical effects. Also, 
behavioral disturbance is expected to be limited to distances less than 
525 m (1722 ft) from the source, the zone calculated for 160 dB or the 
onset of Level B harassment. Additional discussion on species-specific 
effects can be found in the Scripps application.

Tolerance

    Numerous studies (referenced in Scripps, 2005) have shown that 
pulsed sounds from airguns are often readily detectable in the water at 
distances of many kilometers, but that marine mammals at distances more 
than a few kilometers from operating seismic vessels often show no 
apparent response. 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 that mammal group. 
However, most measurements of airgun sounds that have been reported 
concerned sounds from larger arrays of airguns, whose sounds would be 
detectable farther away than that planned for use in the proposed 
survey. Although various baleen whales, toothed whales, and 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 and small odontocetes seem to be 
more tolerant of exposure to airgun pulses than are baleen whales. 
Given the relatively small, low-energy airgun source planned for use in 
this project, mammals are expected to tolerate being closer to this 
source than would be the case for a larger airgun source typical of 
most seismic surveys.

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 (due in part to the small size of the GI airguns), 
although there are very few specific data on this. Given the small 
acoustic source planned for use in the SWPO, there is even less 
potential for masking of baleen or sperm whale calls during the present 
research than in most seismic surveys (Scripps, 2005). GI-airgun 
seismic sounds are short pulses generally occurring for less than 1 sec 
every 6-10 seconds or so. The 6-10 sec spacing corresponds to a shot 
interval of approximately 21.5-36 m (71-118 ft). Sounds from the multi-
beam sonar are very short pulses, occurring for 15 msec once every 5 to 
20 sec, depending on water depth.
    Some whales are known to continue calling in the presence of 
seismic pulses. Their calls can be heard between the seismic pulses 
(Richardson et al., 1986; McDonald et al., 1995, Greene et al., 1999). 
Although there has been one report that sperm whales cease calling when 
exposed to pulses from a very distant seismic ship (Bowles et al., 
1994), a recent study reports that sperm whales continued calling in 
the presence of seismic pulses (Madsen et al., 2002). Given the 
relatively small source planned for use during this survey, there is 
even less potential for masking of 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 smaller odontocete 
cetaceans, given the intermittent nature of seismic pulses and the 
relatively low source level of the airguns to be used in the SWPO. 
Also, the sounds important to small odontocetes are predominantly at 
much higher frequencies than are airgun sounds.
    Most of the energy in the sound pulses emitted by airgun arrays is 
at low frequencies, with strongest spectrum levels below 200 Hz and 
considerably lower spectrum levels above 1000 Hz. Among marine mammals, 
these low frequencies are mainly used by mysticetes, but generally not 
by odontocetes or pinnipeds. An industrial sound source will reduce the 
effective communication or echolocation distance only if its frequency 
is close to that of the marine mammal signal. If little or no overlap 
occurs between the industrial noise and the frequencies used, as in the 
case of many marine mammals relative to airgun sounds, communication 
and echolocation are not expected to be disrupted. Furthermore, the 
discontinuous nature of seismic pulses makes significant masking 
effects unlikely even for mysticetes.
    A few cetaceans are known to increase the source levels of their 
calls in the presence of elevated sound levels, or possibly to shift 
their peak frequencies in response to strong sound signals (Dahlheim, 
1987; Au, 1993; Lesage et al., 1999; Terhune, 1999; as reviewed in 
Richardson et al., 1995). These studies involved exposure to other 
types of anthropogenic sounds, not seismic pulses, and it is not known 
whether these types of responses ever occur upon exposure to seismic 
sounds. If so, these adaptations, along with directional hearing, pre-
adaptation to tolerate some masking by natural sounds (Richardson et 
al., 1995), and the relatively low-power acoustic sources being used in 
this survey, would all reduce the importance of masking marine mammal 
vocalizations.

Disturbance by Seismic Surveys

    Disturbance includes a variety of effects, including subtle changes 
in behavior, more conspicuous dramatic changes in behavioral 
activities, and displacement. However, there are difficulties in 
defining which marine mammals should be counted as ``taken by 
harassment''. For many species and situations, scientists do not have 
detailed information about their reactions to noise, including 
reactions to seismic (and sonar) pulses. Behavioral reactions of marine 
mammals to sound

[[Page 6046]]

are difficult to predict. 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 does 
react to an underwater sound by changing its behavior or moving a small 
distance, the impacts of the change may not rise to the level of a 
disruption of a behavioral pattern. However, if a sound source would 
displace marine mammals from an important feeding or breeding area, 
such a disturbance would likely constitute Level B harassment under the 
MMPA. Given the many uncertainties in predicting the quantity and types 
of impacts of noise on marine mammals, scientists often resort to 
estimating how many mammals may be present within a particular distance 
of industrial activities or exposed to a particular level of industrial 
sound. With the possible exception of beaked whales, NMFS believes that 
this is a conservative approach and likely overestimates the numbers of 
marine mammals that are affected in some biologically important manner.
    The sound exposure criteria used to estimate how many marine 
mammals might be harassed behaviorally by the seismic survey are based 
on behavioral observations during studies of several species. However, 
information is lacking for many species. Detailed information on 
potential disturbance effects on baleen whales, toothed whales, and 
pinnipeds can be found on pages 33-37 and Appendix A in Scripps's SWPO 
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 these effects for marine mammals exposed to 
airgun pulses. Current NMFS policy precautionarily sets impulsive 
sounds equal to or greater than 180 and 190 dB re 1 microPa (rms) as 
the exposure thresholds for onset of Level A harassment for cetaceans 
and pinnipeds, respectively (NMFS, 2000). Those criteria have been used 
in defining the safety (shut-down) radii for seismic surveys. However, 
those criteria were established before there were any data on the 
minimum received levels of sounds necessary to cause auditory 
impairment in marine mammals. As discussed in the Scripps application 
and summarized here,
    1. The 180-dB criterion for cetaceans is probably quite 
precautionary, i.e., lower than necessary to avoid TTS let alone 
permanent auditory injury, at least for delphinids.
    2. The minimum sound level necessary to cause permanent hearing 
impairment is higher, by a variable and generally unknown amount, than 
the level that induces barely-detectable TTS.
    3. The level associated with the onset of TTS is often considered 
to be a level below which there is no danger of permanent damage.
    Given the small size of the two 45 in 3 GI-airguns, 
along with the proposed monitoring and mitigation measures, there is 
little likelihood that any marine mammals will be exposed to sounds 
sufficiently strong to cause even the mildest (and reversible) form of 
hearing impairment. Several aspects of the planned monitoring and 
mitigation measures for this project are designed to detect marine 
mammals occurring near the 2 GI-airguns (and bathymetric sonar), and to 
avoid exposing them to sound pulses that might (at least in theory) 
cause hearing impairment. In addition, research and monitoring studies 
on gray whales, bowhead whales and other cetacean species indicate that 
many cetaceans are likely to show some avoidance of the area with 
ongoing seismic operations. In these cases, the avoidance responses of 
the animals themselves will reduce or avoid the 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, Scripps 
and NMFS believe that it is especially unlikely that any of these non-
auditory effects would occur during the survey given the small size of 
the acoustic sources, the brief duration of exposure of any given 
mammal, and the mitigation and monitoring measures. The following 
paragraphs discuss the possibility of TTS, permanent threshold shift 
(PTS), and non-auditory physical effects.

TTS

    TTS is the mildest form of hearing impairment that can occur during 
exposure to a strong sound (Kryter, 1985). When an animal experiences 
TTS, its hearing threshold rises and a sound must be stronger in order 
to be heard. TTS can last from minutes or hours to (in cases of strong 
TTS) days. Richardson et al. (1995) note that the magnitude of TTS 
depends on the level and duration of noise exposure, among other 
considerations. For sound exposures at or somewhat above the TTS 
threshold, hearing sensitivity recovers rapidly after exposure to the 
noise ends. Little data on sound levels and durations necessary to 
elicit mild TTS have been obtained for marine mammals.
    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). Given the 
available data, the received level of a single seismic pulse might need 
to be on the order of 210 dB re 1 microPa rms (approx. 221-226 dB pk-
pk) in order to produce brief, mild TTS. Exposure to several seismic 
pulses at received levels near 200-205 dB (rms) might result in slight 
TTS in a small odontocete, assuming the TTS threshold is (to a first 
approximation) a function of the total received pulse energy (Finneran 
et al., 2002). Seismic pulses with received levels of 200-205 dB or 
more are usually restricted to a zone of no more than 100 m (328 ft) 
around a seismic vessel operating a large array of airguns. Because of 
the small airgun source planned for use during this project, such sound 
levels would be limited to distances within a few meters directly 
astern of the Revelle.
    There are no data, direct or indirect, on levels or properties of 
sound that are required to induce TTS in any baleen whale. However, TTS 
is not expected to occur during this survey given the small size of the 
source limiting these sound pressure levels to the immediate proximity 
of the vessel, and the strong likelihood that baleen whales would avoid 
the approaching airguns (or vessel) before being exposed to levels high 
enough for there to be any possibility of TTS.
    TTS thresholds for pinnipeds exposed to brief pulses (single or 
multiple) have not been measured, although exposures up to 183 dB re 1 
microPa (rms) have been shown to be insufficient to induce TTS in 
California sea lions (Finneran et al., 2003). However, prolonged 
exposures show that some pinnipeds may incur TTS at somewhat lower 
received levels than do small odontocetes exposed for similar durations 
(Kastak et al., 1999; Ketten et al., 2001; Au et al., 2000). For this 
research cruise therefore, TTS is unlikely for pinnipeds.

[[Page 6047]]

    A marine mammal within a zone with a radius of <=100 m (<=328 ft) 
around a typical large array of operating airguns might be exposed to a 
few seismic pulses with levels of >=205 dB, and possibly more pulses if 
the mammal moved with the seismic vessel. Also, around smaller arrays, 
such as the 2 GI-airgun array proposed for use during this survey, a 
marine mammal would need to be even closer to the source to be exposed 
to levels greater than or equal to 205 dB. However, as noted 
previously, most cetacean species tend to avoid operating airguns, 
although not all individuals do so. In addition, ramping up airgun 
arrays, which is now standard operational protocol for many U.S. and 
some foreign seismic operations, should allow cetaceans to move away 
from 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 these 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. However, with a large airgun array, TTS would be possible in 
odontocetes that bow-ride or otherwise linger near the airguns. Bow-
riding odontocetes mostly would be at or above the surface, and thus 
not exposed to strong sound 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. 
During this project, the anticipated 180-dB radius is less than 60 m 
(197 ft), the array is towed about 21 m (69 ft) behind the Revelle, the 
bow of the Revelle will be about 104 m (341 ft) ahead of the airguns, 
and the 205-dB radius would be less than 50 m (165 ft). Thus, TTS would 
not be expected in the case of odontocetes bow riding during airgun 
operations, and if some cetaceans did incur TTS through exposure to 
airgun sounds, it would very likely be a temporary and reversible 
phenomenon.
    NMFS believes that, to avoid Level A harassment, cetaceans should 
not be exposed to pulsed underwater noise at received levels exceeding 
180 dB re 1 microPa (rms). The corresponding limit for pinnipeds has 
been set at 190 dB. The predicted 180- and 190-dB distances for the 
airgun arrays operated by Scripps during this activity are summarized 
in Table 1 in this document. These sound levels are not considered to 
be the levels at or above which TTS might occur. Rather, they are the 
received levels above which, in the view of a panel of bioacoustics 
specialists convened by NMFS (at a time before TTS measurements for 
marine mammals started to become available), one could not be certain 
that there would be no injurious effects, auditory or otherwise, to 
marine mammals. As noted here, TTS data that are now available imply 
that, at least for dolphins, TTS is unlikely to occur unless the 
dolphins are exposed to airgun pulses substantially stronger than 180 
dB re 1 microPa (rms).
    It has also been shown that most whales tend to avoid ships and 
associated seismic operations. Thus, whales will likely not be exposed 
to such high levels of airgun sounds. Because of the relatively slow 
ship speed, any whales close to the trackline could move away before 
the sounds become sufficiently strong for there to be any potential for 
hearing impairment. Therefore, there is little potential for whales 
being close enough to an array to experience TTS. In addition, ramping 
up the airgun array should allow cetaceans to move away from the 
seismic source and avoid being exposed to the full acoustic output of 
the GI airguns.

Permanent Threshold Shift (PTS)

    When PTS occurs there is physical damage to the sound receptors in 
the ear. In some 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. Although there is no specific evidence that 
exposure to pulses of airgun sounds can cause PTS in any marine 
mammals, even with the largest airgun arrays, physical damage to a 
mammal's hearing apparatus can potentially occur if it is exposed to 
sound impulses that have very high peak pressures, especially if they 
have very short rise times (time required for sound pulse to reach peak 
pressure from the baseline pressure). Such damage can result in a 
permanent decrease in functional sensitivity of the hearing system at 
some or all frequencies.
    Single or occasional occurrences of mild TTS are not indicative of 
permanent auditory damage in terrestrial mammals. However, very 
prolonged exposure to sound strong enough to elicit TTS, or shorter-
term exposure to sound levels well above the TTS threshold, can cause 
PTS, at least in terrestrial mammals (Kryter, 1985). 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. The low-to-moderate levels of TTS that have been induced in 
captive odontocetes and pinnipeds during recent controlled studies of 
TTS have been confirmed to be temporary, with no measurable residual 
PTS (Kastak et al., 1999; Schlundt et al., 2000; Finneran et al., 2002; 
Nachtigall et al., 2003). In terrestrial mammals, the received sound 
level from a single non-impulsive sound exposure must be far above the 
TTS threshold for any risk of permanent hearing damage (Kryter, 1994; 
Richardson et al., 1995). For impulse sounds with very rapid rise times 
(e.g., those associated with explosions or gunfire), a received level 
not greatly in excess of the TTS threshold may start to elicit PTS. 
Rise times for airgun pulses are rapid, but less rapid than for 
explosions.
    Some factors that contribute to onset of PTS are as follows: (1) 
Exposure to single very intense noises, (2) repetitive exposure to 
intense sounds that individually cause TTS but not PTS, and (3) 
recurrent ear infections or (in captive animals) exposure to certain 
drugs.
    Cavanagh (2000) reviewed the thresholds used to define TTS and PTS. 
Based on his review and SACLANT (1998), it is reasonable to assume that 
PTS might occur at a received sound level 20 dB or more above that 
which induces mild TTS. However, for PTS to occur at a received level 
only 20 dB above the TTS threshold, it is probable that the animal 
would have to be exposed to the strong sound for an extended period.
    Sound impulse duration, peak amplitude, rise time, and number of 
pulses are the main factors thought to determine the onset and extent 
of PTS. Ketten (1994) noted that the criteria for differentiating the 
sound pressure levels that result in PTS (or TTS) are location and 
species-specific. PTS effects may also be influenced strongly by the 
health of the receiver's ear.
    Given that marine mammals are unlikely to be exposed to received 
levels of seismic pulses that could cause TTS, it is highly unlikely 
that they would sustain permanent hearing impairment. If we assume that 
the TTS threshold for odontocetes for exposure to a series of seismic 
pulses may be on the order of 220 dB re 1 microPa (pk-pk) 
(approximately 204 dB re 1 microPa rms), then the PTS threshold might 
be about 240 dB re 1 microPa (pk-pk). In the units used by 
geophysicists, this is 10 bar-m. Such levels are found only in the 
immediate vicinity of the largest airguns (Richardson et al., 1995; 
Caldwell and Dragoset, 2000). However,

[[Page 6048]]

sea Gentryit is very unlikely that an odontocete would remain within a 
few meters of a large airgun for sufficiently long to incur PTS. The 
TTS (and thus PTS) thresholds of baleen whales and pinnipeds may be 
lower, and thus may extend to a somewhat greater distance from the 
source. However, baleen whales generally avoid the immediate area 
around operating seismic vessels, so it is unlikely that a baleen whale 
could incur PTS from exposure to airgun pulses. Some pinnipeds do not 
show strong avoidance of operating airguns. In summary, it is highly 
unlikely that marine mammals could receive sounds strong enough (and 
over a sufficient period of time) to cause permanent hearing impairment 
during this project. In this project marine mammals are unlikely to be 
exposed to received levels of seismic pulses strong enough to cause 
TTS, and because of the higher level of sound necessary to cause PTS, 
it is even less likely that PTS could occur. This is due to the fact 
that even levels immediately adjacent to the 2 GI-airguns may not be 
sufficient to induce PTS because the mammal would not be exposed to 
more than one strong pulse unless it swam alongside an airgun for a 
period of time.

Strandings 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). 
Airgun pulses are less energetic and have slower rise times. While 
there is no documented evidence that airgun arrays can cause serious 
injury, death, or stranding, the association of mass strandings of 
beaked whales with naval exercises and an L-DEO seismic survey in 2002 
have raised the possibility that beaked whales may be especially 
susceptible to injury and/or stranding when exposed to strong pulsed 
sounds. Information on recent beaked whale strandings may be found in 
Appendix A of the Scripps application and in several previous Federal 
Register documents (see 69 FR 31792 (June 7, 2004) or 69 FR 34996 (June 
23, 2004)). Reviewers are encouraged to read these documents for 
additional information.
    It is important to note that seismic pulses and mid-frequency sonar 
pulses are quite different. Sounds produced by the types of airgun 
arrays used to profile sub-sea geological structures are broadband with 
most of the energy below 1 kHz. Typical military mid-frequency sonars 
operate at frequencies of 2 to 10 kHz, generally with a relatively 
narrow bandwidth at any one time (though the center frequency may 
change over time). Because seismic and sonar sounds have considerably 
different characteristics and duty cycles, 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 to physical 
damage and, indirectly, mortality suggests that caution is warranted 
when dealing with exposure of marine mammals to any high-intensity 
pulsed sound.
    In addition to the sonar-related strandings, there was a September 
2002 stranding of two Cuvier's beaked whales in the Gulf of California 
(Mexico) when a seismic survey by the R/V Maurice Ewing was underway in 
the general area (Malakoff, 2002). The airgun array in use during that 
project was the Ewing's 20-gun 8490-in 3 array. This might 
be a first indication that seismic surveys can have effects, at least 
on beaked whales, similar to the suspected effects of naval sonars. 
However, the evidence linking the Gulf of California strandings to the 
seismic surveys is inconclusive, and to date, is not based on any 
physical evidence (Hogarth, 2002; Yoder, 2002). The ship was also 
operating its multi-beam bathymetric sonar at the same time but this 
sonar had much less potential than these naval sonars to affect beaked 
whales. Although the link between the Gulf of California strandings and 
the seismic (plus multi-beam sonar) survey is inconclusive, this event 
plus the various incidents involving beaked whale strandings associated 
with naval exercises suggests a need for caution in conducting seismic 
surveys in areas occupied by beaked whales. However, the present 
project will involve a much smaller sound source than used in typical 
seismic surveys. That, along with the monitoring and mitigation 
measures planned for this cruise are expected to eliminate any 
possibility for strandings and mortality.

Non-Auditory Physiological Effects

    Possible types of non-auditory physiological effects or injuries 
that might theoretically occur in marine mammals exposed to strong 
underwater sound might include stress, neurological effects, bubble 
formation, resonance effects, and other types of organ or tissue 
damage. There is no evidence that any of these effects occur in marine 
mammals exposed to sound from airgun arrays (even large ones). However, 
there have been no direct studies of the potential for airgun pulses to 
elicit any of these effects. If any such effects do occur, they would 
probably be limited to unusual situations when animals might be exposed 
at close range for unusually long periods.
    It is doubtful that any single marine mammal would be exposed to 
strong seismic sounds for sufficiently long that significant 
physiological stress would develop. That is especially so in the case 
of the present project where the airguns are small, the ship's speed is 
relatively fast (6 knots or approximately 11 km/h), and, except while 
on a seismic station, the survey lines are widely spaced with little or 
no overlap.
    Gas-filled structures in marine animals have an inherent 
fundamental resonance frequency. If stimulated at that frequency, the 
ensuing resonance could cause damage to the animal. There may also be a 
possibility that high sound levels could cause bubble formation in the 
blood of diving mammals that in turn could cause an air embolism, 
tissue separation, and high, localized pressure in nervous tissue 
(Gisner (ed), 1999; Houser et al., 2001).
    In April 2002, a workshop (Gentry [ed.] 2002) was held to discuss 
whether the stranding of beaked whales in the Bahamas in 2000 (Balcomb 
and Claridge, 2001; NOAA and USN, 2001) might have been related to air 
cavity resonance or bubble formation in tissues caused by exposure to 
noise from naval sonar. A panel of experts concluded that resonance in 
air-filled structures was not likely to have caused this stranding. 
Among other reasons, the air spaces in marine mammals are too large to 
be susceptible to resonant frequencies emitted by mid-or low-frequency 
sonar; lung tissue damage has not been observed in any mass, multi-
species stranding of beaked whales; and the duration of sonar pings is 
likely too short to induce vibrations that could damage tissues (Gentry 
[ed.], 2002). Opinions were less conclusive about the possible role of 
gas (nitrogen) bubble formation/growth in the Bahamas stranding of 
beaked whales.
    Until recently, it was assumed that diving marine mammals are not 
subject to decompression injury (the bends) or air embolism. However, a 
short paper concerning beaked whales stranded in the Canary Islands in 
2002 suggests that cetaceans might be subject to decompression injury 
in some situations (Jepson et al., 2003). If so, that might occur if 
they ascend unusually quickly when exposed to aversive sounds. However, 
the interpretation that strandings are related to decompression injury 
is unproven (Piantadosi and Thalmann, 2004; Fern[aacute]ndez et al., 
2004). Even if that effect can occur during exposure to mid-frequency

[[Page 6049]]

sonar, there is no evidence that this type of effect occurs in response 
to low-frequency airgun sounds. It is especially unlikely in the case 
of this project involving only two small, low-intensity GI-airguns.
    In summary, little is known about the potential for seismic survey 
sounds to cause either auditory impairment or other non-auditory 
physical effects in marine mammals. Available data suggest that such 
effects, if they occur at all, would be limited to short distances from 
the sound source. However, the available data do not allow for 
meaningful quantitative predictions of the numbers (if any) of marine 
mammals that might be affected in these ways. Marine mammals that show 
behavioral avoidance of seismic vessels, including most baleen whales, 
some odontocetes, and some pinnipeds, are unlikely to incur auditory 
impairment or other physical effects. Also, the planned mitigation and 
monitoring measures are expected to minimize any possibility of serious 
injury, mortality or strandings.

Possible Effects of Mid-frequency Sonar Signals

    A multi-beam bathymetric sonar (Simrad EM120, 11.25-12.6 kHz) and a 
sub-bottom profiler will be operated from the source vessel essentially 
continuously during much of the planned survey. Details about these 
sonars were provided previously in this document.
    Navy sonars that have been linked to avoidance reactions and 
stranding of cetaceans generally: (1) Are more powerful than the Simrad 
EM120 sonar; (2) have a longer pulse duration; and (3) are directed 
close to horizontally (vs. downward for the Simrad EM120). The area of 
possible influence of the Simrad EM120 is much smaller--a narrow band 
oriented in the cross-track direction below the source vessel. Marine 
mammals that encounter the Simrad EM120 at close range are unlikely to 
be subjected to repeated pulses because of the narrow fore-aft width of 
the beam, and will receive only limited amounts of pulse energy because 
of the short pulses and vessel speed. Therefore, as harassment or 
injury from pulsed sound is a function of total energy received, the 
actual harassment or injury threshold for the bathymetric sonar signals 
would be at a much higher dB level than that for longer duration pulses 
such as seismic signals. As a result, NMFS believes that marine mammals 
are unlikely to be harassed or injured from the multibeam sonar.

Masking by Mid-Frequency Sonar Signals

    Marine mammal communications will not be masked appreciably by the 
multibeam sonar signals or the sub-bottom profiler given the low duty 
cycle and directionality of the sonars and the brief period when an 
individual mammal is likely to be within its beam. Furthermore, in the 
case of baleen whales, the sonar signals from the Simrad EM120 do not 
overlap with the predominant frequencies of their calls, which would 
avoid significant masking.
    For the sub-bottom profiler, marine mammal communications will not 
be masked appreciably because of their relatively low power output, low 
duty cycle, directionality (for the profiler), and the brief period 
when an individual mammal may be within the sonar's beam. In the case 
of most odonotocetes, the sonar signals from the profiler do not 
overlap with the predominant frequencies in their calls. In the case of 
mysticetes, the pulses from the pinger do not overlap with their 
predominant frequencies.

Behavioral Responses Resulting From Mid-Frequency Sonar Signals

    Behavioral reactions of free-ranging marine mammals to military and 
other sonars appear to vary by species and circumstance. Observed 
reactions have included silencing and dispersal by sperm whales 
(Watkins et al., 1985), increased vocalizations and no dispersal by 
pilot whales (Rendell and Gordon, 1999), and the previously-mentioned 
strandings by beaked whales. Also, Navy personnel have described 
observations of dolphins bow-riding adjacent to bow-mounted mid-
frequency sonars during sonar transmissions. However, all of these 
observations are of limited relevance to the present situation. Pulse 
durations from these military tactical sonars were much longer than 
those of the Scripps multibeam sonar, and a given mammal would have 
received many pulses from the naval sonars. During Scripps' operations, 
the individual pulses will be very short, and a given mammal would not 
receive many of the downward-directed pulses as the vessel passes by.
    Captive bottlenose dolphins and a white whale exhibited changes in 
behavior when exposed to 1-sec pulsed sounds at frequencies similar to 
those that will be emitted by the multi-beam sonar used by Scripps and 
to shorter broadband pulsed signals. Behavioral changes typically 
involved what appeared to be deliberate attempts to avoid the sound 
exposure (Schlundt et al., 2000; Finneran et al., 2002). The relevance 
of these data to free-ranging odontocetes is uncertain and in any case 
the test sounds were quite different in either duration or bandwidth as 
compared to those from a bathymetric sonar.
    Scripps and NMFS are not aware of any data on the reactions of 
pinnipeds to sonar sounds at frequencies similar to those of the 12.0 
kHz frequency of the Revelle's multibeam sonar. Based on observed 
pinniped responses to other types of pulsed sounds, and the likely 
short duration of exposure to the bathymetric sonar sounds, pinniped 
reactions are expected to be limited to startle or otherwise brief 
responses of no lasting consequences to the individual animals. The 
pulsed signals from the sub-bottom profiler are much weaker than those 
from the multibeam sonar and somewhat weaker than those from the 2 GI-
airgun array. Therefore, significant behavioral responses are not 
expected.

Hearing Impairment and Other Physical Effects

    Given stranding events that have been associated with the operation 
of naval sonar, there is much concern that sonar noise can cause 
serious impacts to marine mammals (for discussion see Effects of 
Sei