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OCR for page 41
Assessment of
Continuing Research Needs
In its 1994 report the NRC recommended future research that would provide
a better understanding of the effects of low-frequency sounds on marine mammals
and their prey. This chapter assesses the progress made in addressing the targeted
issues since publication of that report. The major aims are presented as described
in that 1994 report; specific goals are described in boldface type.
Since 1994 the Office of Naval Research (ONR) has invested significant
funds into attempts to address a number of the issues raised in the 1994 report.
These studies were described to the Committee by Robert Gisiner at the April
1999 meeting, and their results are cited below as appropriate. Although an
excellent start has been made in addressing the many questions, there is still a
dearth of data on marine mammal bioacoustics. Some of these research needs fall
within the purview of ONR, but it is a mission agency and its goals are highly
oriented toward the missions of the Navy. Thus, it cannot be expected to deal
with all of the important issues raised in the 1994 NRC report, implying that
additional sources of funding for marine mammal bioacoustic research are
required if better knowledge of marine mammal hearing is deemed by policy
makers to be desirable.
Since publication of the 1994 NRC report, the anatomy of the inner ear of
several additional marine mammal species has been studied. Computational
models based on the anatomical parameters of marine mammal cochleae have
been developed, and predictions from such models have correlated well with
behaviorally determined audiograms in several species of toothed whales (Ketten,
1997~. This kind of modeling provides an important new tool for assessing the
auditory sensitivity and frequency range in whales that are not amenable to
experimental measurements. There have been significant advances in our knowl-
41
OCR for page 42
42
MARINE MAMMALS AND LOW-FREQUENCY SOUND
edge of low-frequency hearing capabilities of several toothed whale species as
well as in techniques required to acquire such data. Significant data have been
obtained on temporary threshold shift (TTS)1 in several marine mammal species
(Ridgway et al., 1997; Kastak and Schusterman, 1998; Kastak et al., 1999;
Schlundt et al., 2000~. Additional work needed includes (1) anatomical studies of
species with known audiograms to validate the use of anatomical features to
predict auditory capabilities and (2) studies using both behavioral and auditory
evoked potentials techniques to determine auditory capabilities of marine mam-
mals (especially of baleen whales). Additional studies are needed to determine
the abilities of marine mammals to detect natural sound in the presence of human-
generated background noise (Erbe and Farmer, 1998~.
Several papers have appeared since 1994 reporting on use of auditory evoked
potentials to study dolphins (Szymanski et al., 1995, 1998; Popov and Klishin,
1998; Popov and Supin, 1998; Popov et al., 1998~. Dolphin (1997) reviewed
auditory processing in several toothed whales (Grampus, Orcinus, Tursiops,
Delphinapterus) that were studied using evoked potential methods. He pointed
out that auditory evoked potentials can be used to study a wide range of questions
about hearing and the auditory system. These include determination of auditory
filter shapes that may provide clues about the potential masking effects of some
human-generated sounds, determination of ITS resulting from sound exposure,
and studies of the masking effects of specific sounds. Dolphin (1995, 1996) and
Dolphin et al. (1995) examined temporal processing in several whale species in
response to amplitude-modulated stimuli using evoked potential techniques.
Ridgway and Au (1999) reviewed earlier work on processing by the auditory
central nervous system and approaches to sound conduction to the dolphin ear.
Popov and Supin (1998) studied dolphin auditory evoked responses to rhythmic
sound pulses. Popov and Klishin (1998) reported on a study of common dolphin
hearing using the electroencephalogram. Popov et al. (1998) reported on fre-
quency tuning of the dolphin auditory system using evoked potential methods.
1Temporary threshold shift is a temporary increase in the threshold audible sound level presumed
to be caused by temporary inactivation of the outer hair cells at a given frequency.
2Auditory evoked potentials (AEPs) are electrophysiological recordings of minute voltages gener-
ated by neural activity in the brain in response to acoustic stimuli. AEPs can be noninvasively
recorded from the scalp skin surface and have been broadly applied with great success in humans.
The response following the presentation of a brief acoustic stimulus is a series of peaks or waves that
arise as a consequence of more or less simultaneous firings in sets of neurons located in successively
higher auditory nuclei. Evoked potentials are quite weak, meaning that multiple presentations of the
acoustic stimulus and averaging techniques must be used to measure them. Among the advantages
of AEP measurements are that (1) they require no or only minimal cooperation from the subject,
(2) responses are rapidly obtained and are highly robust, (3) response detection can be fully auto-
mated and based on totally objective acceptance criteria, and (4) tests are noninvasive and therefore
amenable to examination of protected species. AEPs have now been obtained from a wide range of
species. The availability of such data greatly facilitates comparative studies of hearing and auditory
function.
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ASSESSMENT OF CONTINUING RESEARCH NEEDS
43
Employing the auditory brainstem response (ABR), a type of evoked poten-
tial technique, Szymanski et al. (1999) reported that the ABR of killer whales
provided a means of suprathreshold hearing measurement. The ABR thresholds
of individual killer whales were, at most frequencies, within 12 dB of thresholds
measurable by behavioral responses. At the most sensitive frequency (20 kHz),
mean thresholds determined by behavioral and physiological methods differed by
only 3 dB, thus showing the usefulness of this technique as a proxy for measure-
ments of behavioral thresholds. The two killer whales studied are the largest
animals (2,000 to 3,000 kg) ever successfully investigated with evoked potential
methods (see also Szymanski et al., 1995, 1998~. Although the ABRs were lower
in amplitude than those for dolphins less than one-tenth as heavy, ABRs from the
killer whale (Szymanski et al., 1999) were adequate for determining values near
the behavioral threshold (ABRs averaged 5 dB higher than behavioral thresholds
for the most sensitive range of 18 to 42 kHz). Because ABR amplitudes appear to
correlate with the relative size of auditory structures in the brain, Szymanski et al.
(1999) suggested that successful use of this method in the larger baleen whales
may be more difficult. Experience with a young gray whale at Sea World (San
Diego) demonstrated that working with larger whales requires more sensitive
techniques, quieter conditions, and more time to use ABR techniques on whales
than is needed for smaller marine mammals. However, the promise of this
technique for validating behavioral observations on large whales indicates that
the NRC (1994) recommendation about testing hearing in beached, stranded, or
entrapped larger whales should continue to be a goal. These investigations should
build on those mentioned above using teams experienced in electrophysiological
techniques.
A comprehensive set of species groups, signal types, and biological param-
eters that should be measured for marine mammals is presented in Chapter 5.
Priority for acoustic studies should also be given to species that are (1) endan-
gered or threatened (e.g., the northern right whale, Eubalaena glacialis); and
(2) known or suspected to hear and communicate using low-frequency sound
(e.g., baleen whales, sperm whales, elephant seals).
BEHAVIOR OF MARINE MAMMALS IN THE WILD
Aim: To determine the normal behaviors of marine mammals in the wild and
their behavioral responses to human-generated acoustic signals (NRC, 1994, pp.
41-47~. This aim can be conceptualized as a number of specific topics, which are
shown here in bold print.
.
Determine how marine mammals utilize natural sound for communi-
cation and for maintaining their normal behavioral repertoires.
Although much is unknown about communication among baleen whales, and
OCR for page 44
44
MARINE MAMMALS AND LOW-FREQUENCY SOUND
little research is currently directed at this topic, some work has been published
since 1994 on species that may be affected by the Acoustic Thermometry of
Ocean Climate (ATOC) signal or low-frequency active (LFA) sonar. Gray
whale (Eschrichtius robustus) vocalizations were documented at two locations
along their southward migration by Crane and Lashkari (1996), who found that
vocal behavior varied with location. More vocalizations occurred in shallow
water than in deep water, and the type of vocalization (call structure) produced
most frequently during the southward migration differed from vocalizations in
the lagoons of Baja California where the whales calve and spend the winter.
Crane and Lashkari concluded that vocal activity is an important component of
migratory behavior in gray whales, probably for communication rather than navi-
gation. These data are important as a reminder that the potential for acoustic
interference by human-generated noise can be site-dependent and seasonally vari-
able. Clearly, there is a greater potential for disruption of normal behavior in
areas where vocalization rates are high.
Recordings of blue whale vocalizations in the Pacific Ocean have indicated
that the occurrence of a two-part call is characteristic of populations in both the
Gulf of California and the waters off California (Thompson et al., 1996; Rivers,
1997; Stafford et al., 1999~. Whether these two populations are part of a single
Pacific Ocean population or whether there is limited exchange is yet to be deter-
mined. The presence of a subspecies (pygmy blue whale, Balaenoptera musculus
brevicauda) off Australia (Ljungblad et al., 1997) adds further incentive for
documenting the occurrence and types of vocalizations of blue whales in the
Pacific Ocean (Stafford et al., 1998~.
The recent availability of the Navy's Integrated Undersea Surveillance Sys-
tem (IUSS) and other hydrophore arrays to scientific researchers has permitted
the use of passive acoustic tracking as a method for documenting migration
routes and critical habitats where baleen whales are seasonal visitors or residents
(e.g., Stafford and Fox, 1996; Stafford et al., 1998, 1999; Watkins et al., 2000~.
Although direct visual observations of vocalizing whales provide more details of
behavior compared to the use of remote monitoring (where animals are not
observed as they produce sounds), the detection of well-described species-specific
calls to track baleen whale migrations and activity using the IUSS and other
hydrophore arrays has great potential.
Several studies (listed above) have provided descriptions of vocal behavior,
and such data are critical for interpreting the biological significance of any
changes in vocal behavior induced by human-generated noise. A shift in the
focus of regulations from harassment to the biological significance of behavioral
disruption (recommended later in this report) will require a much better under-
standing of the functions of vocal behavior. Therefore, a high priority should be
given to basic research on the behavioral ecology of how marine mammals use
the sounds they produce; the results of such research would have immediate
regulatory implications.
OCR for page 45
ASSESSMENT OF CONTINUING RESEARCH NEEDS
45
· Determine the responses of free-ranging marine mammals to human-
generated acoustic stimuli, including repeated exposure of the same
individuals. How is the use of natural sounds altered by the presence
of human-generated sounds?
ATOC transmissions from a fixed source off Hawaii theoretically could
provide some data on repeated exposure to this signal; however, few individuals
were seen close enough to the Hawaii source site either in experimental (ATOC
signal on) or control (ATOC signal off) conditions to provide evidence of any
response to source levels greater than 130 dB. Individual identification of hump-
back whales was part of the MMRP, but the activities of specific individuals
exposed to the ATOC stimulus repeatedly have not been presented to date. In
addition, humpback whale song was recorded prior to and during exposure to the
ATOC signal, but these data also have not been examined in sufficient detail to
determine if the presence of the ATOC sound has a concurrent and/or long-
lasting effect on the songs of individual whales. The Quick-Look reports did
indicate that there was no change in average acoustic energy in the 200- to 800-
Hz band, a dominant frequency band for humpback song. Detailed examination
of the actual songs for changes in frequency content or amplitude of individual
components that may overlap with the broadband components of the ATOC
signal was not completed at the time of the Committee's April 1999 meeting.
Clearly, such data are unique and warrant a detailed examination. If subsequent
analyses of data support the finding of no major change in the vocalizations of
humpback whales, that would support the contention that ATOC is unlikely to
have long-term effects on this species at this important breeding site.
Two studies on white whales (Delphinaptera leucas) have provided perti-
nent data since the publication of NRC (1994~. Erbe and Farmer (1998) trained
captive animals to detect vocalizations from other white whales in the presence of
background noise to assess the maskings effects of icebreaker activities. Erbe
and Farmer found that the aeration system used to clear ice debris had the greatest
masking effect, followed by propeller noise. This innovative protocol may be
useful for determining masking effects in other toothed whales using a variety of
human-generated noises, including the ATOC signal and LEA sonar. A logisti-
cally challenging field study was conducted in the St. Lawrence estuary with
free-ranging white whales (Lesage et al., 1999~. The vocal activity of the animals
was monitored prior to, during, and after exposure to two vessels with different
Masking is the reduction in the audibility of one sound due to the presence of a second sound. of
greatest interest here is simultaneous masking, in which both sounds overlap in time, but masking
can also occur when the signal is a brief sound presented immediately before or immediately after the
masker (backward and forward temporal masking, respectively). In humans the amount of simulta-
neous spectral masking observed is related to the width of the person s auditory filter located at the
signal frequency.
OCR for page 46
46
MARINE MAMMALS AND LOW-FREQUENCY SOUND
noise signatures: a motorboat and a ferry. More than 70 recording sessions were
conducted, but only six met the criteria for a successful experimental sequence
with acceptable signal-noise ratio for analysis. Despite the small sample size
(N= three for each vessel), differences were noted in the vocalizations. Calling
rates declined in five of the six sessions as noise levels increased, the occurrence
of certain call types increased, repetition rates of certain calls increased when a
vessel was within 1 km of the white whale, and the mean frequencies produced
were higher, presumably to move the frequency of the call outside the frequency
band of the masking noise produced by the vessels.
These two studies provide evidence that specific human-generated noises
can affect the vocal activity of white whales in the short term. Although generali-
zations to other species are not without risk, baleen whales exposed to low-
frequency noise (from ATOC, LEA, ships) may respond similarly. In addition,
the field study by Lesage et al. (1999) indicates that such data are difficult but not
impossible to obtain given a sustained field effort.
Tyack (1998) and Tyack and Clark (1998) concluded that 10 of 17 singing
humpback whales exposed to low-frequency sounds from the SURTASS-LFA4
sonar system stopped singing during playback with a source level that ranged
from 155 to 205 dB, resulting in maximum received levels of 120 to 150 dB.
Four of these 10 stopped when they were joined by other whales, a behavior that
has been observed in previous studies with no human-generated sound (Tyack,
1983), and their responses cannot be definitively associated with the playback.
There was no difference in the received levels for the six whales that stopped
singing, apparently in response to playback, compared to the seven that did not
stop, suggesting individual differences in perception and/or response. In order to
evaluate the significance of disruption induced by cessation of singing, it will be
necessary to make a decision about what proportion of the population of singers
can be disrupted before the disruption will have a population-level effect.
An important task is to determine how different sound types and levels affect
migration and other movement patterns of marine mammals. The 1994 NRC
report specifically recommended shore-based studies similar to the Malme et al.
investigation of migrating gray whales. Tyack and Clark (1998) replicated the
Malme et al. (1983) study using a sound projector from the SURTASS-LFA.
Two important characteristics of the Tyack and Clark study were that (1) the
intensity of the source was adjustable and (2) its location could be changed. The
results differed considerably depending on both of those variables. When the
source level was 170 dB and was located in the migratory path of the whales,
animals deflected around the source by a maximum of only several hundred
4SURTASS-LFA is the acronym for a low-frequency active sonar system developed by the U.S.
Navy to detect submarines. This sonar uses a vertical array of sound projectors deployed from a
surface ship to broadcast sounds in the 100- to 500-Hz range. The Navy made this system available
for three marine mammal studies conducted over a period of 1 year.
OCR for page 47
ASSESSMENT OF CONTINUING RESEARCH NEEDS
47
meters. When the source level was increased to 185 dB and was located in the
migratory path of the whales, the animals changed course so as to avoid passing
within a kilometer of the source on both the onshore and the offshore sides. The
evidence suggested that most whales avoided exposure to received levels of 140
dB or more. These findings agreed with earlier findings of Malme et al. (1983,
1984) using noises associated with oil industry activities. This study confirmed
that whales change their response as source level is changed, which demonstrates
that they are responding to received level, not just distance or sound gradient.
When the source was located on the offshore side of the migratory path, there
was little evidence of any diversion in the individual migratory paths for source
levels of both 185 and 200 dB (Tyack and Clark, 1998~. This finding is espe-
cially interesting because calculated received levels were higher in these cases
than in the condition producing the strong avoidance effect when the source was
located in the migratory path at a source level of 185 dB. For example, at the
200-dB source level for offshore playbacks, the received levels measured at
ranges of 2 to 2.5 km were >140 dB. Since the offshore source was placed about
2 km offshore from the inshore source location, this means that during the off-
shore playbacks most of the whales were exposed to received levels that would
almost certainly have elicited an avoidance reaction had the source been placed in
the inshore location. Thus, for these whales there clearly was something disturb-
ing about a strange sound source located in the migratory path that was not
disturbing when that same source was on the offshore side of the migratory path.
This study illustrates that behavioral responses to noise sources may not be solely
dependent on the acoustic nature of the noise, but on the location of the noise as
well. Apparently, the 120-dB avoidance model, which seemed correct for non-
impulse noises in the migratory path, is not valid for offshore sources.
High-frequency pingers and submarine sonar pings are known to affect sperm
whale vocalization rates and behavior (Watkins and Schevill, 1975; Watkins et
al., 1985, 1993~. Low-frequency sound also may affect sperm whales because
their wide-band clicks contain energy between 100 and 2,000 Hz (Watkins et al.,
1985; Moore et al., 1993), which is suggestive of low-frequency hearing.
Gordon et al. (1996) were funded by the MMRP for two three-week cruises
off the Azores to study how sperm whales responded to experimental playbacks
of M-code sequences similar to the ATOC stimulus as well as similar stimuli of
higher frequency. Most standard playbacks were conducted at 75 Hz (the ATOC
frequency), and one each was conducted at 2, 3, and 4 kHz. There were 16
control trials. Gordon et al. observed no significant difference between playback
and control groups in blow rates of whales before diving or the relative bearing of
sperm whales with respect to the source vessel. Sperm whales produce a regular
series of clicks starting about three minutes after the onset of a dive. Gordon et al.
(1996) compared six different measures from these clicks and found no differ-
ence for the 75-Hz playbacks, although the initial click rate was higher for the
three higher frequency playbacks than during controls.
OCR for page 48
48
MARINE MAMMALS AND LOW-FREQUENCY SOUND
Many earlier reports suggest that sperm whales may silence or move out of
an area in response to manmade noise (Watkins et al., 1985; Bowles et al., 1994;
Mate et al., 1994~. The contrasting lack of response in Gordon et al. (1996) may
reflect different responsiveness to different stimuli or perhaps that different groups
of sperm whales have differing responsiveness depending on their prior exposure
history. As with the Frankel et al. (1995) study, this six-week pilot study
confirmed the utility of observations for which the study site and experimental
protocol are optimized for marine mammal studies. One of the most important
limitations of the Gordon et al. study was that the limited source level of the
sound source meant that few whales were exposed to received levels above 120
dB. Gordon et al. (1996) advocate further studies using more powerful sources
and more sensitive methods for measuring or estimating received levels and for
monitoring responses of the whales.
In another recent study of sperm whales in the Atlantic Ocean, Andre et al.
(1997) presented individuals off the Canary Islands with various noise sources to
determine if acoustic deterrence could reduce whale collisions with ferry boats.
Although there is no documentation of the received levels for the sound stimuli
used, Andre et al. observed approaches to the source, which were interpreted as
curiosity, when an artificial click "coda" was presented. The authors concluded
that sperm whales exposed to high levels of shipping noise have a high tolerance
for noise. Alternatively, those animals may have permanent threshold shifts.
There is no way to distinguish between those possibilities with the data available
at present, although postmortem examinations on the cochleae of a few animals
could resolve this uncertainty. A careful study of the response of sperm whales to
low-frequency sound seems warranted.
Several papers have suggested that beaked whales tend to strand when there
are naval operations offshore. Simmonds and Lopez-Jurado (1991) reported on
four mass strandings between 1985-1989 of Cuvier's beaked whale (Ziphius
cavirostris) on the coast of Fuerteventura in the Canary Islands that may have
been related to naval maneuvers. Frantzis (1998) reported on another mass
stranding of 12 or more beaked whales sighted over 38 km of coastline during
two days (May 12 and 13, 1996) in the Kyparissiakos Gulf in Greece. There was
no external sign of injury or disease in any of these animals. Frantzis (1998)
concluded that the mass stranding was associated with a concurrent NATO sonar
exercise. The Frantzis paper stimulated the NATO research center that con-
ducted the sonar tests to convene panels to review the data (D'Amico,1998~. The
NATO sonar transmitted two simultaneous signals, one at 450-700 Hz and one at
2.8-3.3 kHz at source levels of just under 230 dB. This combined signal lasted
four seconds and was repeated once every minute. The NATO analysis suggested
close timing between the onset of sonar transmissions and the first strandings.
Unfortunately, it was not possible to determine the received levels experienced
by the stranded whales. D'Amico (1998) states that received levels as high as
150-160 dB were estimated to occur at ranges of 50 km. Sperm whales were
OCR for page 49
ASSESSMENT OF CONTINUING RESEARCH NEEDS
49
heard within 10-25 km of the sound source, but demonstrated no obvious changes
in their clicking patterns before, during, and after sonar transmissions. Although
these papers raise concern about the effects of noise on beaked whales, they
provide little guidance regarding what exposures may be dangerous and which
are safe. There is a clear need for experimental studies of the responses of beaked
whales to carefully controlled exposures of noise.
NATO sonars have been tested in the Mediterranean Sea on many occasions
without strandings. Both Simmonds and Lopez-Jurado (1991) and Frantzis (1998)
started with rare strandings and then looked for some other rare event that might
correlate, but neither paper makes a strong case for having performed a thorough
systematic survey of when naval or sonar exercises might have occurred in these
areas in the absence of strandings. There is a clear need for studies designed to
test this association more systematically. In areas where beaked whales are
common or have historically stranded, it would be good to set up a prospective
study monitoring noise exposure, systematically logging mass strandings and
sources of loud noise such as naval exercises. Careful necropsy of stranded
animals would help test for any noise-induced injuries.
.
Determine the response of deep-diving marine mammals to low-
frequency sounds whose characteristics (source level, frequency
bandwidth, duty cycle) duplicate or approximate those produced by
acoustic oceanographers.
The only research on this issue known to the Committee was the previously
mentioned study on elephant seal deep diving conducted as part of the California
ATOC observations. This should be an area of priority for future studies since it
is directly related to the issue of the effects of the ATOC source on the animals
that may approach closest to the source.
Several other studies have reported on the effects of ATOC-like sounds on
various marine mammals (e.g., Mattlin, 1995; Aburto et al., 1997; Harvey and
Eguchi,1997~. However, none of these studies has appeared in the peer-reviewed
literature, and weaknesses in data acquisition, analysis, or interpretation limit
their usefulness. Although these studies were examined by the Committee, their
results will not be considered further. Relevant results should be published in the
peer-reviewed scientific literature, and funding agencies should provide support
for this critical component of research.
STRUCTURE AND FUNCTION OF MARINE MAMMAL
AUDITORY SYSTEMS
Aim: To determine the structure and capabilities of the auditory system in marine
mammals (NRC, 1994, pp. 47-53~.
OCR for page 50
so
MARINE MAMMALS AND LOW-FREQUENCY SOUND
This aim can also be conceptualized as a number of specific topics, which are
shown below in bold print.
.
Determine basic hearing capabilities of various species of marine
mammals.
Low-frequency audiograms have now been obtained from multiple individu-
als of several species of toothed whales and seals by using a variety of behavioral
and electrophysiological techniques. Audiograms extending below 100 Hz have
been obtained for white whales, the bottlenose dolphin (Tursiops truncates), the
false killer whale (Pseudorca crassidens), and Risso's dolphin (Grampus
griseus), as well as the harbor seal (Phoca vitulina) (Au et al., 1997; Kastak et
al., 1999~. Additional species of toothed whales must be tested because auditory
differences among species can be substantial (e.g., porpoises versus killer whales).
There is still no audiogram for any baleen whale species, something that is
understandable considering the difficulties of working with these giant species.
Since the publication of NRC (1994), several studies have been published that
reveal new information on auditory capabilities of seals and toothed whales.
Underwater thresholds for three seal species (one eared seal and two true seals)
were obtained with lower frequencies than had been tested previously (Kastak
and Schusterman, 1998~. All three species had relatively high thresholds to
sounds with a frequency of 100 Hz (89.9 to 116.3 dB, depending on the species).
In addition, Kastak and Schusterman calculated ranges at which the sound from
the ATOC source would be just detectable, not necessarily annoying, for all three
species: 9 to 34 km for the California sea lion, 160 km for the harbor seal, and 279
km for the elephant seal.5 Using a different experimental design, Kastak and
Schusterman compared low-frequency hearing sensitivity in air and in water for
the same three pinniped species. They presented the argument that in water
sound pressure is a more biologically appropriate measure than sound intensity.6
Using sound pressure sensitivity as the critical parameter, they found consistent
correlations between hearing sensitivities and the environment in which the ani-
mal spends the greater portion of its time. Thus, the elephant seal, which spends
a greater proportion of time in water, has better underwater hearing sensitivity
than the sea lion, which hears better in air than in water, and the harbor seal,
which has almost equal sensitivity in air and in water.
5Kastak and Schusterman performed these calculations assuming a simplified propagation model
of spherical spreading to a distance of 1 km (20 log R), followed by propagation described by
51Og R at greater distances. The authors did not give a reason for using this combination.
6Sound intensity is the power of the sound or pressure squared. This raises questions about the
biological relevance of comparing in-air and underwater sounds by ``correcting for,, intensity rather
than by simply comparing pressure levels.
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ASSESSMENT OF CONTINUING RESEARCH NEEDS
51
Ridgway et al. (1997) tested the hypothesis that the sensitivity of white
whale hearing might diminish with depth. To test the effect of depth, two trained
white whales made dives to a platform at 5, 100, 200, and 300 m in the Pacific
Ocean. During dives to the platform for up to 12 minutes, the whales whistled in
response to 500-ms tones projected at random intervals to assess hearing threshold
at each of the four depths. Analysis of response whistle spectra, whistle latency
to tones, and hearing thresholds showed that the increased hydrostatic pressure at
depth changed each whale's whistle response at depth but did not attenuate
hearing. Hearing is attenuated in the aerial ear of humans and other land mam-
mals when tested in pressure chambers due to changes in middle-ear impedance
that result from increased air density (Fluur and Adolfson, 1966; Pantev and
Pantev, 1979; Levendag et al., 1981~. The finding that whale hearing is not
attenuated at depth suggests that sound is conducted through whale head tissues
to the ear without requiring the usual eardrum/ossicular chain amplification of the
aerial middle ear. These first-ever hearing tests in the open ocean demonstrate
that zones of audibility for human-generated sounds are as great at 300 m, and
potentially much deeper, as in shallow water. (These tests could not have been
performed without trained whales.)
Au et al. (1997) tested two members of the dolphin family (false killer whale
and Risso's dolphin) for their responses to the ATOC signal using a behavioral
paradigm. They concluded that neither species could be negatively affected by
the ATOC experiments due to their high auditory thresholds (139 and 141 dB,
respectively) for the ATOC signal.7 Based on calculated transmission signal
loss, Au et al. concluded that at a horizontal range of 0.5 km these whales would
not hear the ATOC signal. They also concluded that the ATOC signal is unlikely
to harm baleen whales or to mask their vocalizations, but these conclusions are
based on calculations and interpretations of source levels of whale vocalizations
without using actual behavioral data. Source levels for baleen whale vocaliza-
tions have been calculated from measurements for single animals at considerable
distances from the hydrophore. Such vocalization levels may be more appropri-
ately compared to humans shouting, rather than to conversational speech levels.
Members of the same species probably are rarely, if ever, exposed to vocaliza-
tions at levels as high as the maxima reported by some researchers (cited in
Richardson et al., 1995; Au et al., 1997~. The reported maxima also may be
outliers or erroneous because available data are relatively scarce; thus, assump-
tions about noise levels that may or may not disturb whales should not be based
on such measurements.
7According to the authors, small toothed whales swimming directly above the ATOC source will
not hear the signal unless they dive to 400 m. The authors used a propagation loss model calculated
for the source off Kauai. They then stated the horizontal distance without a depth, so it is unclear
whether the animals would have to be at the surface for these measurements to be valid.
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MARINE MAMMALS AND LOW-FREQUENCY SOUND
animal should be viewed as only a temporary substitute for average hearing
capabilities across members of wild populations (such as the bottlenose dolphins
that have been studied for several years in Sarasota Bay, Florida). In the absence
of large datasets on the hearing capabilities of multiple animals, Terhune and
Turnbull suggested a "broad-brush approach" in using available data that assumes
average auditory capabilities, for example, rather than using the lowest thresholds
measured as standard values. This approach also may be useful when comparing
auditory data for various types of whales. Knowledge of the individual differ-
ences in hearing sensitivity within species may help explain the large differences
observed in the behavior of individual animals (e.g., gray whales) when con-
fronted with a noise source in their path of travel (Tyack and Clark, 1998~.
Audiograms from many individuals, preferably from the wild, are critically
important as a baseline to understanding the hearing capabilities of populations.
.
Determine sound pressure levels that produce temporary and per-
manent hearing loss in marine mammals.
In humans, exposure to intense sounds has the potential to produce a number
of temporary or permanent aftereffects, depending on the level and duration of
the exposure. These aftereffects include reductions in perceived loudness, ring-
ing in the ears (tinnitus), and changes in perceived pitch (Kryter, 1985; Ward,
1997~. The most studied aftereffect is loss of hearing sensitivity, commonly
known as temporary and permanent threshold shift (TTS and PTS, respectively).
Repeated exposures that produce TTS eventually produce a PTS. The fact that
some hearing losses last only minutes, hours, or days suggests that some cochlear
structures have the ability to recover from whatever damage is inflicted by the
exposure stimulus. In terrestrial mammals the receptor cells known as outer hair
cells are known to be far more susceptible to acoustic damage than are the less
numerous inner hair cells (Saunders et al., 1991), and temporary inactivation of
the outer hair cells is presumed to be the primary factor in exposure-induced
hearing losses of about 30 dB (in air) or less that last only a few hours. Tremen-
dous variability exists in the susceptibility of individual ears to both TTS and
PTS; in some human survey studies the subjects are even partitioned into differ-
ent groups on the basis of the apparent "toughness" of their ears. For a typical
human ear exposed daily to essentially continuous noise in the workplace, 90 dB
(in air) has been widely adopted as the point at which precautions need to be
taken (see Appendix D). Under U.S. regulations, for every additional 5 dB of
exposure, the allowable duration of exposure is halved. However, the 90-dB
value (in air) in not ubiquitous across animals and cannot be compared directly
with in-water values.
Across mammalian species there are known to be quite large differences in
the sound levels required to damage the auditory system. For example, 12 min-
utes of exposure to a 1,000-Hz tone of 120 dB (in air) sound pressure level can
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55
produce more hearing loss in a chinchilla than 12 hours of the same exposure in
a squirrel monkey (Hunter-Duvar and Bredberg, 1974~. Decory et al. (1992)
made direct comparisons of hearing loss and cochlear damage in cat, guinea pig,
and chinchilla using several exposure frequencies and a fixed duration of 20
minutes. Across exposure frequencies there was between 10 and 25 dB more
hearing loss in the chinchilla than in the cat, with the guinea pig being intermedi-
ate between the two. At the highest exposure frequency (8 kHz), there were
approximately 10 times the number of damaged hair cells in the chinchilla than in
the cat. Because of certain controls implemented by the experimenters, it is
possible to rule out any contribution of differences in the outer-ear system to
these differences in susceptibility, but it is not yet possible to say with certainty
what the relative contributions were from differences in the middle-ear system
and differences in cochlear mechanisms.
Decory et al. (1992) presented an interesting argument about the possible
contribution to damage made by the angular displacement to which a stereocilium
is exposed in each of these species, a suggestion that warrants further study and
attention by comparative anatomists studying marine mammals. The work of
Luz and Lipscomb (1973) suggests that large discrepancies exist across species
for susceptibility to impulse noise as well as continuous noise. There is some
evidence to indicate that similar differences in susceptibility to exposure-induced
hearing loss exist in marine mammals as well. Kastak and Schusterman (1996)
suggested that exposure (in air) to a band level only about 10 to 25 dB above
absolute sensitivity was adequate to produce 8 dB of TTS in a harbor seal. By
comparison, the work of Schlundt et al. (2000) suggests that for the bottlenose
dolphin the sound levels necessary to produce a small masked TTS (in water)
must be between about 115 and 150 dB above absolute sensitivity.
The existence of such large differences among species in terms of suscepti-
bility to exposure-induced hearing loss creates two problems for regulators of
human-generated sound in the ocean. First, critical exposure levels cannot be
extrapolated from a few species, although it may be possible to identify a set of
representative species for the initial studies (see Box 5.1~. Second, it almost
certainly will never be possible to specify one single value of sound level at
which damage to the auditory system will begin for all, or even most, marine
mammals.
An interesting characteristic of TTS and PTS is that for terrestrial mammals
the maximum hearing loss typically occurs in a frequency region above the
exposure frequency (McFadden, 1986~. Investigators studying TTS in marine
mammals should design their experiments to obtain information about any upward
shifts in maximum hearing loss that exist in marine mammal ears. In terrestrial
mammals, cochlear mechanics are known to be somewhat different in the apical
(low-frequency) regions of the cochlea than in the middle and basal regions.
Further, some species of bats have cochleae that are highly specialized to process
stimuli from certain narrow frequency bands that are important to their survival.
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MARINE MAMMALS AND LOW-FREQUENCY SOUND
The extent to which these differences affect the growth and spread of TTS and
PTS is not completely understood, but because there may be corresponding
regional differences in the cochlear mechanics of those marine mammals that
depend on low-frequency sounds, it will be important to study TTS following
exposures to low-frequency sounds as well as to mid- and high-frequency sounds.
A serendipitous occurrence of TTS was documented in a captive harbor seal
exposed to construction noise (Kastak and Schusterman, 1996~. Although the
exposure was in air, the TTS (also measured in air) nonetheless reveals the levels
above threshold at which the seal ear could be affected and the duration of the
effect after prolonged repeated exposure. The noise was present over 6 days with
6 to 7 hours of intermittent daily exposure. The longest continuous exposure was
believed to be about 1.5 hours. The third octave sound pressure level, centered at
100 Hz, varied from 75 to 90 dB (in air), which was only 10 to 25 dB above
threshold for this seal. On day 6 of daily exposures, an 8-dB temporary threshold
shift was measured at 100 Hz (the only frequency tested). Also, the seal's false
alarm error rate in responding to sounds increased by 23 percent, which the
authors suggested may have been the result of exposure-induced tinnitus. Hear-
ing sensitivity recovered completely in about 1 week. Kastak et al. (1999) went
on to test underwater TTS in three species of seals. They used test frequencies
ranging from 100 to 2,000 Hz and "octave-band noise exposure levels that were
approximately 60-75 dB SL (sensation level at center frequency)." They found
TTS averaging 4.8 dB for one harbor seal, 4.9 dB for two California sea lions,
and 4.6 dB for one northern elephant seal. The animals' hearing returned to
baseline levels when tested within 24 hours of noise exposure.
Schlundt et al. (2000) reported measures of TTS in four bottlenose dolphins
(three females and one male) following exposures of differing levels at differing
frequencies. The exposures were always 1 second in duration because the experi-
ment was designed to determine the effects of a sonar sound commonly used by
the U.S. Navy. The behavioral task used was imaginative and requires some
description. A platform contained two stations, each having a prescribed place
for the animal to position itself in front of a speaker. Under instructions from its
trainer, a dolphin would position itself at the first station where it would first be
exposed to a sound having a duration of 1 second. Immediately after receiving
the first sound, the animal swam to the second station where it was presented with
a series of 250-ms tones of fixed frequency and varying level. The animal was
trained to whistle whenever it heard a tone from this second speaker, and the level
of these test tones was adjusted up and down in accordance with the animal's
responses to the series of test tones. After extensive training with this procedure,
the level of the 1-second first sound was increased, with the objective of producing
6 dB of hearing loss as detected using the series of second tones. Different
animals were tested with the exposure and test tones set to 3, 20, and 75 kHz. In
part because of the background level at the test location, the series of second
tones was actually masked by a broadband noise whose level could be varied.
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Thus, the measure extracted from these data was in fact the difference in the level
of the second tone necessary for a fixed level of detection performance in the
presence of the background masker. Accordingly, it is probably better character-
ized as masked temporary threshold shift (MTTS) in order to distinguish it from
standard (unmasked) measures of TTS.
This paradigm, while unusual, may provide a more realistic estimate of
threshold shifts for animals tested in a noisy environment. The results were that
small amounts of MTTS were observed when the exposure sounds were in the
range of 192 to 201 dB, and this was true for all three exposure frequencies. (As
already noted, this corresponds to exposures approximately 1 15 to 150 dB above
absolute sensitivity, values that are higher than the nominal 90 dB commonly
cited for humans.) Further, the animals exhibited varying degrees of behavioral
disturbance at exposure levels about 12 to 15 dB below the values necessary to
produce MTTS, a fact that Schlundt et al. interpreted to mean that dolphins, and
perhaps other whales, may naturally avoid sounds that pose a threat to their
hearing. To test this possibility, whales could be monitored closely during the
ramp-up phase of experimental presentations of the ATOC signal near the test
subjects.
There is a particular priority for obtaining TTS data for endangered baleen
whales. Many environmental impact statements (EISs) suggest levels of 150 to
160 dB as safe exposure levels for marine mammals. This is particularly difficult
to justify with animals for which no audiometric data exist. Even if there is not
enough time to conduct a complete TTS study on a stranded whale, it would still
be particularly useful to test for TTS after several tens of minutes of exposure to
160-dB noise.
.
Determine condition of important cochlear structures in wild marine
mammals using postmortem examinations (this topic did not appear in
earlier reports).
For many locations around the world the ocean is an extremely noisy place
due to shipping, petroleum exploration or drilling, underwater explosions (Ketten
et al., 1993), and other human activities. Accordingly, large numbers of marine
mammals are already being exposed to anthropogenic sounds on a regular basis,
and many of these sounds are of high intensity. Unfortunately, little is known
about the auditory consequences of these exposures even though that knowledge
could be extremely informative about the possible consequences of exposure to
other intense sources such as those used for ATOC and LEA sonar. One obvious
source of information would be behavioral or physiological measures of hearing
from animals living at these especially noisy sites. In the absence of such difficult-
to-obtain information, there is a less direct, but still potentially informative,
approach that deserves attention. Specifically, postmortem examinations of the
cochleae of marine mammals have the potential to correlate noise levels and
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MARINE MAMMALS AND LOW-FREQUENCY SOUND
hearing damage. Many marine mammals of various species die each year, in
strandings, fish nets, and accidents. If the cochleae of some of these animals
could be collected, preserved correctly, and sent to scientists capable of histologi-
cal examination of the cells known to be damaged by exposure to intense sounds,
some idea about the current (ambient) level of cochlear damage and, by impli-
cation, hearing loss gradually would emerge. Eventually, this information could
i'
provide a set of baselines for use in establishing new regulations and against
which new exposures could be measured.
Much is currently known about the progression of cellular damage that occurs
in terrestrial mammals following noise exposures of various types and durations
(see Saunders et al., 1991), meaning that the cochlear physiologists have good
expectations about what to look for, and where, in the cochleae of marine mam-
mals. Damage of particular types and extents will be definitive evidence of
permanent extensive hearing loss, and less severe damage will be evidence of less
extensive hearing loss. Of special interest should be the cochleae of young
marine mammals because they are the least likely to have cochlear deterioration
attributable to factors other than noise exposure (such as the presbyacusis attrib-
utable to aging).
In terrestrial mammals, at least, hearing loss induced by a number of agents
s known to progress from the basal (high-frequency) part of the cochlea toward
the apical (low-frequency) part. Assuming that this pattern of progression exists
in marine mammals as well, discovering damage to apical regions of the cochlea
that is not accompanied by correspondingly greater damage to basal regions will
be strong evidence that the damage was induced by an intense low-frequency
sound source. Once frequency maps are established for the cochleae of various
representative mammals (Ketten, 1994, 1997), localized damage at a particular
region of the cochlea can be traced back to sound sources with maximum energy
in a particular frequency band. It is important to undertake a parallel effort to
monitor noise exposure in areas where strandings are well monitored. Regular
monitoring of noise, strandings, and cochlear damage is required to move research
from an anecdotal correlation toward causal links between noise and strandings.
When implementing this procedure, the so-called half-octave shift in PTS needs
to be taken into account; see McFadden (1986~.
Because this idea about gathering information concerning the current state of
hearing loss in marine mammals arose late in its deliberations, the Committee
was not able to determine how best to implement a program of collecting cochleae
from dead marine mammals. Clearly, the SWAT (Standard Whale Auditory
Test) teams described elsewhere in this report should include personnel capable
of extracting and preserving the peripheral auditory systems of marine mammals.
In addition, it might be possible to train other personnel that are likely to come
into contact with potential specimens. All interested governmental agencies
should be advised about the importance of this collecting work and should be
encouraged to implement policies that minimize any existing impediments to its
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59
efficient progression. Because of the impossibility of predicting the location of
specimens that might become available for this study, and because specimens can
deteriorate relatively rapidly, transportation of auditory specimens across inter-
national borders for purposes of histological examination should be made as
simple as possible. This is especially important for vulnerable endangered
species, where the Convention on International Trade in Endangered Species of
Wild Fauna and Flora (CITES) permit processes inhibit timely delivery of samples
to appropriate experts.
.
Determine morphology and sound conduction paths of the auditory
system in various marine mammals.
No studies have been published since 1994 that reveal new information
about the structural details of the auditory system in marine mammals, although
ONR has funded a project in the laboratory of Darlene Ketten (Woods Hole
Oceanographic Institution) to examine potential auditory pathways in the lower
jaw and skull of dolphins as part of a project to model sound conduction in these
animals. The existence of a second pathway to the ear may be confirmed by these
studies (Ketten, 1994, 1997~.
· Determine temporal-resolving power for various marine mammals
(this topic did not appear in earlier reports).
In humans and other terrestrial mammals, hearing sensitivity varies with
signal duration. Specifically, the longer the duration, the less strong the signal
needs to be for equal detectability. This relationship holds out to a limiting
duration of about 300 to 500 ms, beyond which the signal level for equal detect-
ability remains constant. What is most interesting about this trade-off between
duration and level is that level declines (or increases) about 3 dB for every
doubling (or halving) of duration below the limiting duration. That is, power is
traded almost perfectly for time (e.g., Plomp and Bouman, 1959), suggesting that
energy is the feature of the signal that determines detectability. This temporal
integration function rises about 15 dB as the signal duration decreases from 300
ms to 10 ms in any animal using signal energy as the basis for its performance.
The implications of temporal-resolving power differ for marine mammals
with low-frequency hearing versus those with high-frequency hearing. Malme et
al. (1983, 1984) reported that the levels required to elicit a response in gray
whales need to be as much as 50 dB greater for short impulses than for continuous
sounds. These data suggest that this species may be disproportionately insensitive
to very short sounds; that is, their temporal integration function appears to be
much steeper or displaced from that of typical terrestrial mammals. Data on the
hearing capabilities of other baleen whales may suggest that long temporal inte-
gration functions, with a corresponding relative insensitivity to transient sounds,
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MARINE MAMMALS AND LOW-FREQUENCY SOUND
is to be expected in other baleen whales. One implication is that these species
should have generally poor temporal resolution; for example, they should have
difficulty detecting a brief silent gap in an ongoing waveform.
In contrast to baleen whales, characterized by their sensitivity to low-
frequency sounds, the toothed whales, characterized by high-frequency hearing,
do appear to exhibit short temporal integration functions. Measures of temporal
resolution capability and temporal integration times were obtained in several
species of toothed whales by Dolphin (1995, 1996) using evoked potential tech-
niques. The species examined demonstrated temporal resolution capabilities
exceeding those observed in humans and other terrestrial mammal species. Thus,
one would expect these species to exhibit high sensitivity to brief, transient
sounds. Moreover, the possession by dolphins of the capability for both very
high temporal resolution and extremely sharp frequency resolution is enigmatic
and points out our lack of understanding of the auditory processing of these
animals. Accordingly, it would be extremely valuable to obtain information
about temporal processing in a wider range of species from each of the groups
identified in Box 5.1.
Although the evidence suggesting differences in temporal processing capa-
bilities between certain marine mammals and terrestrial mammals is unquestion-
ably scarce, a logical argument can be made for such a difference. Species that
are highly dependent on low frequencies may have acquired long time constants
for temporal integration, making them highly insensitive to very short sounds
(the rise time of their auditory systems may be so long that the response to
impulsive sounds is weak), whereas those species that depend on high frequen-
cies and/or echolocation have evolved auditory systems with short time constants
and high temporal resolution capabilities. If this is true, it would have major
impact on what kinds of sounds are regarded as dangerous for different species;
namely, it may be that brief sounds, even quite intense ones, are not unduly
dangerous to hearing in some species of baleen whale. These facts emphasize
that temporal resolution can vary substantially across species having hearing that
is specialized for operation in different frequency regions and they lend plausibil-
ity to the speculation above about temporal resolution possibly being poor in
those marine mammals specialized to communicate with low-frequency sound.
EFFECTS OF LOW-FREQUENCY SOUNDS ON THE FOOD CHAIN
Aim: To determine whether low-frequency sounds affect the behavior and physi-
ology of organisms that serve as part of the food chain for marine mammals
(NRC, 1994, pp. 53-54~.
The most serious effects of noise on potential prey species are those that
involve growth and reproduction. Increases in noise (above ambient levels) have
been implicated in reduced growth and reproduction in a variety of marine organ-
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61
Ems in tanks, although these data are still very limited and in need of replication.
A single study of the effects of intense sound on the auditory systems of fresh-
water fish has been published (Hastings et al., 1996~. In this study, freshwater
fish were exposed to varying levels of a sinusoidal sound, and some evidence of
auditory damage was reported for sounds above 180 dB when fish were exposed
to continuous signals for 4 hours and then allowed to survive for several days
before damage was assessed. It should be noted that these sounds were very
different in intensity, duty cycle, and duration from those used in the MMRP
studies and the fish used in this experiment could not escape the sound source.
Additional studies are needed, particularly of fish species that are endangered,
important commercially, or are a component of the food chain of marine mammal
species.
Growth rates in two fish species, sheepshead minnows (Cyprinodon
variegates) and killifish (Fundulus similis), were significantly lower in aquariums
exposed to noise 20 to 30 dB above ambient levels in the natural habitat (Banner
and Hyatt, 1973~. Tanks with noise levels 20 dB higher than ambient levels
reduced the viability of eggs in sheepshead minnows. Evidence reviewed by
Corwin and Oberholtzer (1997) suggests that fish and perhaps some shark and
amphibian species have the capacity to regenerate damaged hair cells in their
auditory and balance organs (see also Lombarte et al., 1993~. To the extent that
this ability is widespread in those species, they may be at lesser long-term risk
from exposure to intense sounds than are marine mammals, which are presum-
ably like terrestrial mammals in being incapable of regenerating new receptor
cells to replace damaged ones (e.g., Hastings et al., 1996; Corwin and Oberholtzer,
1997~.
No assessments of pre-ATOC shark abundances were made, nor has the
potential attraction of sharks by low-frequency ATOC sound been investigated,
despite extensive data in the literature showing that low-frequency sounds, such
as those used by ATOC, attract sharks (e.g., Myrberg, 1972, 1978; Myrberg et al.,
1976~. The potential for redistribution of sharks cannot be ignored, and some
effort should be made in the future to monitor any ATOC source with appropriate
methods (methods that would not alter the behavior of sharks and other organ-
isms in significant ways) to determine if sharks are attracted to the site.
Effects of intense sound have been observed in a shrimp species (Crangon
crangon) (Lagardere, 1982~. Shrimp exposed to noise levels 20 to 30 dB higher
than normal ambient levels exhibited reduced growth and reproduction and
increased aggression and mortality relative to a control group.
Although population surveys have indicated the presence of endangered turtle
species at both ATOC sites (the Kauai EIS mentions green sea turtles [Chelonia
mydas], leatherback turtles [Dermochelys coriacea], olive Ridley turtles
[Lepidochelys olivacea], and hawksbill turtles [Eretmochelys imbricataj) no
specific results for studies of hearing or behavioral observations on any shark or
turtle species were presented by the MMRP scientists.
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MARINE MAMMALS AND LOW-FREQUENCY SOUND
POTENTIAL NONAUDITORY ACOUSTIC EFFECTS ON
MARINE ANIMAL HEALTH
Extremely intense sound can result in injury to many bodily organs and
physiological processes. Aside from the direct effects of nearby blasting from
explosives, intense sound can cause injury to lungs and other air-containing
spaces. In addition, there may be direct effects on the nervous system. Extremely
intense sound can also affect the vestibular system and can cause disorientation.
In humans these vestibular impacts result in readily observable nystagmus, an
uncontrolled movement of the eyes (Stephens and Ballam, 1974~. Studies of
these effects in humans and other terrestrial species are being funded by ONR.
Research by Crum and Mao (1996) suggests the possibility of potential
effects that only occur in certain physiological states that would be very difficult
to study in nonhumans. For example, human divers are susceptible to decom-
pression sickness ("bends"), a disabling and sometimes fatal condition in which
bubbles of nitrogen gas form in the blood, joints, and other tissues. Low-frequency
sound might induce bends episodes in human divers whose tissues are saturated
with gas due to breathing pressurized gas at depth, that would not otherwise
occur. Crum and Mao (1996) showed experimental data suggesting that intense
(160 to 220 dB) low-frequency sound may induce bubble growth in tissues (see
also Lettvin et al., 1982), and therefore divers ensonified with low-frequency
pulsed sound when they are near decompression limits could be severely injured
(Crum and Mao, 1996~. This is unlikely to be a problem with the ATOC sources
because they are so far offshore from dive sites and in deep water, but it may be
a problem with more powerful shallow-water sources, such as SURTASS-LFA.
Although marine mammals do not carry a tank of pressurized breathing gas
as human divers do, they make frequent dives to depths greater than 100 m,
which may produce over 200 percent supersaturation of nitrogen in muscle tissue
after repetitive dives (Ridgway and Howard, 1979, 1982~. In Ridgway and
Howard's 1979 study, dolphins made 23 to 25 dives to 100 m (lo atmospheres) in
1 hour. Dolphins did not suffer from decompression sickness even with muscle
nitrogen at supersaturated levels that would produce bends in humans (Ridgway
and Howard, 1979~. However, Lettvin et al. (1982) and Crum and Mao (1996)
suggested that sound exposure could induce bubble growth in blood. Crum and
Mao (1996) suggested that this might be an issue for both humans and marine
mammals. Therefore, it should be considered whether intense low-frequency
sound might cause bubbles in the circulatory systems of whales returning to the
surface after a long series of deep, but rapid, dives similar to those studied by
Ridgway and Howard (1979~. At the present time, bubble formation in diving
marine mammals must be considered as conjecture based on findings in unrelated
studies. However, research on marine mammals should probably consider the
issue after ONR studies on human subjects have been completed. If results from
research on humans suggest that marine mammals could experience such effects,
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63
sophisticated electronic tags that record depth of dive and other relevant data
might reveal the extent to which such a nonauditory threat may be realistic in
wild populations.
As part of the SURTASS-LFA program, ONR funded research on the effects
of low-frequency sound on human divers. These studies show that the likelihood
of scuba divers aborting a dive is lowest at 250 Hz but rises for source frequen-
cies both above and below 250 Hz. Sound levels were relatively nonaversive
until reaching a received sound pressure level of 148 dB, at which point 15
percent of dives were aborted. Aversion at lower frequencies resulted from a
sensation of vibration in the air-filled cavities in the head, chest, and abdomen.
No vestibular effects were observed for sound up to 157 dB.
Research on marine mammals should also be devoted to evaluating other
physiological measures of general health, both short-term and long-term, that can
be monitored non-invasively over time with tags. For now, these measurements
are limited to heart rate and respiration. Data on stress and stress indicators in
marine mammals is sorely lacking, and there is not even a baseline from which to
determine normal values. At the same time, the significance of such reactions to
stress in terms of reproduction and survival should be assessed.
LONG-TERM ACOUSTIC MONITORING OF CRITICAL HABITATS
The issue of cumulative impacts from human-generated noise is best dealt
with as a habitat degradation issue. Undersea noise should be monitored in
important marine mammal habitats (after these have been identified). This moni-
toring effort should be designed in parallel with surveys of marine mammal
distribution, abundance, and strandings using methods that allow temporal and
spatial analysis of how noise may lead to changes in these population character-
istics. These data are particularly important for populations that are either not
recovering or are declining due to habitat degradation and other causes. Monitor-
ing should include the ambient noise field, marine mammal vocalizations, and
transient noises, particularly in locations and times of the year in which marine
mammals are known to be common. This monitoring optimally should also
include or be coordinated with other assessments of habitat quality such as prey
fields and chemical pollutants. Coordination of noise monitoring with stranding
networks would enable more systematic and controlled evaluation of whether
noise influences strandings and whether cochlear damage in stranded animals is
associated with acute noise exposure.
NMFS, the Navy, and other agencies with responsibilities for marine mam-
mals or that conduct or permit activities that introduce significant levels of sound
to the ocean should evaluate the costs and benefits of an array of acoustic receivers
designed to monitor both human-generated sound in the ocean and the vocaliza-
tions of whales in acoustic hotspots (NRDC, 1999~. One possibility is to use
existing arrays such as the IUSS (JOI, 1994; Clark, 1995; Gisiner, 1998) devel-
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MARINE MAMMALS AND LOW-FREQUENCY SOUND
oped by the U.S. Navy to detect submarines. Evaluation of the appropriateness of
the IUSS should determine whether the bandwidth and geographic coverage of
the IUSS is adequate for the task of monitoring ambient noise and marine mam-
mals or whether it is necessary to design and build an array of sensors specifically
to monitor marine mammals. Such a system could be automated to activate when
significant sounds are detected. Whales could be located and tracked in real time
and in three-dimensional space, thus identifying natural paths and avoidance
paths. This capability was demonstrated in the Whales '93 program in which the
IUSS was used to routinely detect, locate, and track blue, finback, and humpback
whales in the North Atlantic Ocean (JOI, 1994~. Hundreds of thousands of whale
vocalizations were documented, allowing the description of seasonal movements
of the whales. Autonomous underwater recorders, sonobuoys, or towed arrays of
hydrophores can be used in areas where (or at times when) more intensive moni-
toring is desired (Richardson et al., 1986; Thomas et al., 1986; Moore et al.,
1999~.
Representative terms from entire chapter:
baleen whales
