16 Marine Technology Society Journal
tion in auditory sensitivity), temporary thresh-
old shift (TTS. i.e. reduction in auditory sen-
sitivity with eventual recovery), and chronic
stress effects that may lead to reduced viabil-
ity. The most likely perceptual effects would
be masking of biologically significant sounds
(e.g. communication signals, echolocation,
and sounds associated with orientation, find-
ing prey or avoiding natural or manmade
threats) while behavioral effects could include
disruption of foraging, avoidance of particu-
lar areas, altered dive and respiratory patterns,
and disruption of mating systems. Indirect
effects might include reduced prey availabil-
ity resulting in reduced feeding rates.
There are a number of existing reviews of
this topic. The comprehensive book on ma-
Sea Mammal Research Unit, Gatty Marine
Laboratory, University of St Andrews
Song of the Whale Research Team,
International Fund for Animal Welfare
Acoustic Research Laboratory, National
University of Singapore
Pelagos Cetacean Research Institute
Mark P. Simmonds
Natural Resources Institute, University
Sea Mammal Research Unit, Gatty Marine
Laboratory, University of St Andrews
A Review of the Effects of
Seismic Surveys on Marine Mammals
This review highlights significant gaps in our knowledge of the effects of seismic air
gun noise on marine mammals. Although the characteristics of the seismic signal at differ-
ent ranges and depths and at higher frequencies are poorly understood, and there are often
insufficient data to identify the appropriate acoustic propagation models to apply in particu-
lar conditions, these uncertainties are modest compared with those associated with bio-
logical factors. Potential biological effects of air gun noise include physical/physiological
effects, behavioral disruption, and indirect effects associated with altered prey availability.
Physical/physiological effects could include hearing threshold shifts and auditory damage
as well as non-auditory disruption, and can be directly caused by sound exposure or the
result of behavioral changes in response to sounds, e.g. recent observations suggesting
that exposure to loud noise may result in decompression sickness. Direct information on
the extent to which seismic pulses could damage hearing are difficult to obtain and as a
consequence the impacts on hearing remain poorly known. Behavioral data have been col-
lected for a few species in a limited range of conditions. Responses, including startle and
fright, avoidance, and changes in behavior and vocalization patterns, have been observed in
baleen whales, odontocetes, and pinnipeds and in some case these have occurred at ranges
of tens or hundreds of kilometers. However, behavioral observations are typically variable,
some findings are contradictory, and the biological significance of these effects has not
been measured. Where feeding, orientation, hazard avoidance, migration or social behavior
are altered, it is possible that populations could be adversely affected. There may also be
serious long-term consequences due to chronic exposure, and sound could affect marine
mammals indirectly by changing the accessibility of their prey species.
A precautionary approach to management and regulation must be recommended. While
such large degrees of uncertainty remain, this may result in restrictions to operational prac-
tices but these could be relaxed if key uncertainties are clarified by appropriate research.
rine mammals and noise by Richardson and
colleagues (Richardson et al., 1995) summa-
rized most of the relevant work up to that date.
Since then a number of reviews have focused
on air gun noise and marine mammals, in-
cluding Evans and Nice (1996), Richardson
and Würsig (1997), and Gausland (2000).
Harwood and Wilson (2001) consider the is-
sue in a risk assessment framework, while an
expert panel have provided recommendations
on levels of exposure and mitigation proce-
dures likely to minimize risk to marine mam-
mals (HESS, 1997). A series of reports by the
National Academy of Sciences have reviewed
and summarized information relating under-
water noise and marine mammals and made
recommendations for future research (e.g.,
arine seismic surveys produce some of the
most intense manmade noises in the oceans
and these surveys often operate over extensive
areas for extended periods of time. The juxta-
position of intense sound sources and acous-
tically sensitive marine mammals must give
rise to concerns about possible adverse im-
pacts. Powerful sounds can potentially have a
number of effects on marine mammals. In
this review, we divide possible effects into four
categories: physical (including physiological)
effects, perceptual effects, behavioral effects,
and indirect effects. Possible physical and
physiological effects include damage to body
tissues, gross damage to ears, permanent
threshold shift (PTS. i.e. permanent reduc-
Winter 2003/04 Volume 37, Number 4
NRC, 2000; NRC, 2003). Relevant chapters
from the latest of these are summarized else-
where in this volume (Wartzok et al., 2004).
Here our intention is to summarize the exist-
ing information including recent findings,
make it available to an audience that includes
non-biologists, and comment on some of the
implications for regulation and management.
Methods for Investigating the Effects
of Manmade Noise on Marine Mammals
Marine mammals, which spend most or
all of their lives at sea and much of that time
submerged, must rank amongst the most dif-
ficult of research subjects. We believe that,
especially for a non-biological readership, a
brief overview of research approaches that
might be used to study these animals will help
to set the context for a review of the poten-
tial effects themselves. If nothing else, this
might explain why so many uncertainties still
exist in this field.
Studies on trained captive animals allow
detailed observations and measurements of
psychometric parameters. In this case, mea-
surements of auditory sensitivity, auditory
function, and the effects of noise exposure
are extremely relevant. There are significant
shortcomings, however. Only a limited range
of species is routinely kept in captivity, and it
is unlikely that some groups, offshore spe-
cies and the great whales for example, ever
will be. Sample sizes are usually small; typi-
cally data come from only one or two indi-
viduals, which may not be representative of
animals in the wild. Captive studies can pro-
vide examples of responses (often dramatic)
to high levels of exposure but they do little to
elucidate disruption of natural patterns of
behavior and extrapolating from the behav-
ior of trained animals in small enclosures to
the real world is problematical.
Extrapolation and Modeling
Approaches involving physical and physi-
ological modeling and extrapolation from
other species have some scope for predicting
the occurrence of physical phenomena such
as trauma and threshold shift but they are of
limited value in predicting disturbance reac-
tions, which are likely to vary greatly and will
depend on species and context.
Observing Behavior in the Field
Measuring behavioral responses requires
data to be collected from unrestrained ani-
mals at sea. Different approaches have their
own strengths and weaknesses.
■ Visual observation: Observation from ships
has often been used to study the behavior of
cetaceans. Unfortunately, even large whales
are difficult to observe at sea. They are visible
for only brief periods at the surface where they
perform a small subset of their behaviors that
may be poor indicators of disturbance. Fur-
ther, the presence of a vessel close enough to
allow visual observation can influence the be-
havior of the subjects. This approach is of little
value for studying pinnipeds, which are par-
ticularly difficult to observe at sea.
Aircraft have the advantages of provid-
ing higher vantage points and can be less likely
to affect behavior than ships (Richardson et
al., 1995). However, air surveys are expen-
sive and observation time will be limited by
restricted range and endurance.
Observations from coastal vantage points
do not rely on expensive platforms and do
not affect the behavior of the target animals.
Theodolites can be used to accurately locate
and track animals seen at the surface. Inevi-
tably though, observations are restricted to
inshore waters with adjacent high vantage
points, and even then it can be difficult to
follow the behavior of individual animals.
■ Passive acoustic monitoring: For the more
vocal species, acoustic monitoring can pro-
vide researchers with a variety of behavioral
cues. Acoustic monitoring can be carried out
using remote hydrophones or receivers (Clark
& Fristrup, 1997; Clark & Charif, 1998; Culik
et al., 2001) or from relatively inexpensive
small vessels that are quiet and are unlikely
to affect the behavior of the study animals.
Acoustic monitoring has several advantages
compared to visual methods: 1) the range for
acoustic detection is often greater than visual
range; 2) many species are audible for a greater
proportion of time than they are visible at
the surface; 3) monitoring can continue
through the night and in poor weather con-
ditions (although background noise does in-
crease in rough weather, which reduces acous-
tic range, this is typically less pronounced
than the effect of the same weather on visual
detectabilty); and 4) data collection and
analysis can be readily automated. However,
not all marine mammal species are vocal, and
the significance of changes in vocal behavior
can be hard to interpret. Acoustic monitor-
ing can be readily combined with visual ob-
servations and the two approaches should be
seen as complementary.
■ Telemetry: Both direct real-time tracking
using acoustic and VHF telemetry and remote
tracking using satellite transmitters have been
widely applied to study pinnipeds and, more
recently, cetaceans. Telemetry techniques, par-
ticularly satellite-linked tracking systems, can
provide large quantities of reliable data, includ-
ing information on underwater behavior (Mar-
tin et al., 1998; Fedak et al., 2001), on physi-
ological responses such as heart rate (e.g.,
Thompson et al., 1991; Thompson and Fedak,
1993; Thompson et al., 1998), and on the
physical environment (Lydersen et al., 2002).
Tags incorporating hydrophones and sound
recording devices have recently been developed
(Fletcher et al., 1996). Because they provide
both detailed data on animal movements and
record received sound and the subject’s vocal-
izations, they are well suited for noise effects
studies. Indeed such tags are already being used
to measure responses of sperm whales to seis-
mic air guns (Johnson & Tyack, 2003).
Pinnipeds can be captured on or close to
land and good results have been obtained by
gluing transmitters to their fur (Fedak et al.,
1983). However, the lack of an effective
method for attaching tags remains a major
constraint for cetacean telemetry. Long-term
cetacean attachments have been achieved with
implantable tags (e.g. Watkins et al., 1999;
Mate et al., 1999), however, these methods
are invasive and space constraints within the
dart limit the sophistication of the telemetry
packages. A better approach for cetacean ef-
fects studies may be short-term attachments
of archival tags using suction cups (e.g., Baird,
1998; Johnson and Tyack, 2003).
Tag costs and problems of attachment
mean that marine mammal telemetry stud-
ies usually have small sample sizes and the
use of tags also requires ethical evaluation.
18 Marine Technology Society Journal
■ Experimental control: Ideally researchers
should have full control over when and where
a seismic source is active during field trials. If
correctly designed, the resulting Controlled
Exposure Experiments (CEEs) provide a
powerful method for demonstrating cause
and effect (see Tyack et al., 2004). Financial
constraints may dictate that this approach is
only possible with small-scale sources and in
a limited set of conditions. Investigations of
responses to full-scale seismic arrays will usu-
ally have to be conducted around ongoing
surveys. Long-term effects will be almost
impossible to investigate using CEE.
Effects of Air Gun Noise
Underwater explosions cause tissue dam-
age and can be lethal. The effects of shock
waves from explosions have been explored
using submerged terrestrial animals (Goertner,
1982; Richmond et al., 1973; Yelverton et al.,
1973), and dolphin carcasses (Myrick et al.,
1990). However, pressure pulses from air guns
have longer rise times and are therefore less
likely to cause damage than pressure waves
from high explosives. To date there is no evi-
dence that seismic pulses cause acute physical
damage to marine mammals.
Human divers exposed to pulses of very
intense low frequency sound have experi-
enced non-auditory physiological effects, in-
cluding resonance of the lungs and other cavi-
ties and symptoms of dizziness, nausea and
visual disruption. Possible sources of such
pulses include a new generation of Low Fre-
quency Active Sonar Systems (Cudahy &
Ellison, 2002). The potential for relatively
short seismic pulses to cause similar effects
has not yet been investigated.
Another potential mechanism by which
powerful sounds could cause tissue damage is
by sound-induced growth (rectified diffusion)
of gas bubbles in super-saturated tissues of div-
ing mammals. Although marine mammals
are breath-hold divers and are thus less likely
to suffer the bends than human divers breath-
ing compressed air, it is believed that during
long sequences of dives, their body tissues be-
come super-saturated (Ridgway & Howard,
1982). Crum & Mayo (1996) calculated that
exposure to 500 Hz sounds at SPLs of 210 dB
re:1µPa could cause bubble growth that could
induce the ‘bends’ in marine mammals. [All
decibel levels (dBs) are referenced to 1 micro
Pascual (1µPa) unless otherwise stated in the
text.] They considered that this effect was un-
likely at SPLs below 190 dB re:1µPa. How-
ever, as bubbles get larger the so-called “Laplace
pressure” resulting from surface tension in the
bubble wall, which serves to discourage the
expansion of small bubbles, will reduce and
passive diffusion from saturated tissue into
bubbles may then be sufficient to maintain
bubble growth. The potential for noise from
seismic air guns to cause such effects has not
A series of incidents in Greece, the Baha-
mas, Madeira, and the Canary Islands
(Frantzis, 1998; Balcomb & Claridge, 2001;
Jepson et al., 2003) have served to establish
that military sonar can cause cetaceans, in most
cases beaked whales, to strand. Autopsies of
some of these animals reveal signs of physical
damage including hemorrhages in the acous-
tic fats, sub-arachnoid bleeding and gas and
fat emboli in certain tissues (Evans and En-
gland, 2001; Jepson et al., 2003). The extent
to which these are a direct result of sound en-
ergy or are a secondary behaviorally-mediated
effect is not clear. For example, panic could
lead to high levels of stress resulting in inter-
nal bleeding and/or could cause rapid surfac-
ing or changes in patterns of diving behavior
that might trigger decompression sickness.
Two incidents hint at the possibility of
similar links between air guns and beaked
whale strandings. In 2002 two beaked whales
were found stranded in the Gulf of California
close to an area in which a scientific survey,
using a powerful air gun array, was being con-
ducted by the RV Maurice Ewing (Malakoff,
2002). The same vessel had been potentially
linked to a beaked whale stranding event in
the Galapagos in 2000 (Gentry, 2002) . It
should be stressed that a causal link was not
established in either case, but concern was suf-
ficient for U.S. courts to agree to a restraining
order until a more complete investigation
could be completed. This is perhaps the stron-
gest indication that air guns could lead directly
to stranding and cetacean mortality.
If, as these incidents suggest, changes in
animals’ behavior can lead to physical dam-
age, the zones of influence models typically
used for regulation, in which it is assumed
that physical damage will be restricted to lim-
ited areas very close to powerful sound
sources, may have to be revised. As we will
see, behavioral responses can occur at ex-
tended ranges and are often highly variable.
It is hardly surprising that ears, which have
been adapted to be exquisitely sensitive to
sound, are also vulnerable to being damaged
by it. Underwater explosions can result in
gross tissue damage in ears. For example, half
of a sample of 10 Weddell seals (Leptonychotes
weddellii) collected in McMurdo Sound af-
ter a series of dynamite explosions, had tissue
damage in their ears (Bohne et al., 1985;
1986). Similarly, Ketten et al. (1993) found
damage consistent with blast injury in the
ears of humpback whales (Megaptera
novaeangliae) trapped in fishing gear off New-
foundland after blasting operations in the
area. To date there is no direct evidence of
damage to the ears of marine mammals re-
sulting from seismic sound sources.
■ Noise-induced hearing loss; temporary and
permanent threshold shifts: Exposure to noise
of sufficiently high intensity causes a reduc-
tion in hearing sensitivity (revealed as an up-
ward shift in the hearing threshold). This can
be a temporary threshold shift (TTS), with
recovery after minutes or hours, or a perma-
nent threshold shift (PTS) with no recovery.
PTS may result from chronic exposure, and
sounds that can cause TTS usually cause PTS
if the subjects are exposed to them repeatedly
and for a sufficient length of time. Very in-
tense sounds, however, can cause irreversible
cellular damage and instantaneous PTS.
TTS appears to be associated with meta-
bolic exhaustion of sensory cells and certain
anatomical changes and damage at a cellular
level. Excessive metabolic and electrome-
chanical response activity also leads to swell-
ing in the hair cells, in neural connections,
and in the vascular system of the cochlea. PTS
Winter 2003/04 Volume 37, Number 4
may be accompanied by more dramatic ana-
tomical changes in the cochlea including the
disappearance of outer hair cell bodies and,
in very severe cases, a loss of differentiation
within the cochlea and degeneration of the
FIGURE 1 Schematic representation of routes of extrapolation and data needs (bold boxes on the right) to estimate safe
levels of exposure to seismic pulses for marine mammals.
(A) + (B) ➞ (C)
(A) Existing information on safe levels of exposure to
particular sounds in terrestrial mammals and humans
combined with (B) our existing knowledge/data on the
characteristics of seismic sources received at marine
mammal ears is used to derive (C) acceptable levels
of exposure (DRC) to marine seismic sources for hu-
mans and terrestrial mammals.
(C) + (D) ➞ (E)
(C) Estimated acceptable levels of exposure to marine
seismic surveys for humans and terrestrial models
combined with (D) our current knowledge of auditory
sensitivity in a range of marine mammal species would
be used to estimate (E) safe levels of exposure (DRC)
to marine seismic surveys for different marine mam-
(E) + (F) ➞ (G)
(E) Estimates of safe levels of exposure combined with
(F) estimates of exposure (based on our knowledge
of source levels, propagation conditions, seismic ves-
sel working procedures, and animal behavior) would
be used to draw up (G) guidelines and practical codes
of conduct to minimize the risk of hearing damage .
Human Terrestrial DRC
Human Terrestrial DRC for
DRC’s for Marine Mammals
• Blue whale
• Harbour Porpoise
Predicted zones of hearing
damage, discomfort etc
• Operating conditions
• Exposure patterns
Marine mammal auditory
As no direct investigations of threshold
shifts induced by air guns have been made in
marine mammals, any consideration of safe
levels of exposure must depend to a greater
or lesser extent, on inference and extrapola-
tion. A schematic representation of this pro-
cess is shown in Figure 1. This chain of infer-
ence requires the input of usually uncertain
data and the making of assumptions at each
step. The conclusions arising from such a
process must be treated with caution as the
errors introduced at each stage may become
Human or Terrestrial Mammal
safe exposure levels
(Damage Risk Criteria–DRC)
Human or Terrestrial DRC for
20 Marine Technology Society Journal
compounded. In the following sections we
examine some of the steps in this process in
■ Hearing loss in terrestrial mammals (A):
Threshold shifts in man, including those
caused by chronic exposure to industrial
noise, have been an area of intense research.
Extensive reviews are provided by Kryter
(1985, 1994). Typically, and especially when
human experimental subjects are involved,
measurements of TTS are made in controlled
conditions and these are used to infer the risk
of PTS from higher levels of exposure. Often
the goal is to calculate Damage Risk Criteria
(DRC): levels of exposure that should not be
exceeded without risking hearing damage.
Ward (1968) (cited in Richardson et al.,
1995) investigated human DRC for impulse
noise in air, based on empirical observations
of TTS, and derived a predictive formula
for PTS using peak pressure levels, pulse du-
ration, and number of pulses as parameters.
Risk was found to increase with both the
number of pulses and with their duration.
The threshold for damage diminishes by 2
dB for each doubling of pulse length, up to
a pulse length of 200 ms, beyond which
there was no further decrease. Thus, for ‘safe’
exposure to 100 pulses the peak pressure
level is 164 dB re: 20µPa when pulses were
25 ms long, and 138dB re: 20µPa for pro-
longed (>200 ms) pulses. As the number of
pulses is reduced, the DRC is adjusted up-
ward, by 5 dB per 10-fold reduction in pulse
number. Thus for a single 200 ms pulse,
the DRC is 148 dB re: 20µPa. Exposure to
a single pulse at this level might be expected
to cause damage.
There are indications that DRC for im-
pulsive noise currently applied to humans
may underestimate the risk of hearing dam-
age from impulsive noises. Procedures for
predicting TTS and PTS are based on the
equal energy hypothesis, i.e. that threshold
shift should be proportional to the product
of intensity and time. Thus, impulsive noises
are assumed to have a much-reduced poten-
tial to cause threshold shifts because of their
short duration. However, the relationship of
TTS to the characteristics of pulsed sound is
complex (Melnick, 1991). In humans, the
levels of TTS resulting from prolonged ex-
posure reached an asymptote (indicative of
PTS) 16 times faster for impulsive (impact)
noise than for continuous noises (Laroche et
al., 1989). This may be because pulses cause
greater displacement of the basilar membrane
(increasing the potential for sensory cell dam-
age) than exposure to continuous noise.
■ Nature of the seismic signals received at
marine mammal ears (B): Seismic gun ar-
rays are designed to deliver a very well defined
and uniform sound pulse in the desired direc-
tion—downwards. However, the characteris-
tics of sounds projected in other directions are
very different. Marine mammals will be dis-
tributed in a variety of positions relative to a
seismic array and the signal they receive may
have a complicated and variable nature. In deep
water for example, it may include both com-
ponents with a sharp onset and short dura-
tion received directly and longer pulses with
slower rise times received by reflection from
the seabed. These components may be sepa-
rated by different time intervals, depending
on water depth and the position of the ma-
rine mammal receiver within the water col-
umn. Measurements by Goold and Fish
(1998) have shown that air gun arrays can
produce significant sound energy up to, and
probably beyond, 22kHz. Well above the
lower (~200Hz) frequencies at which air guns
are designed to provide most energy. In addi-
tion, many other sound sources are associated
with a seismic survey, including networks of
high frequency transponders used to track the
positions of arrays of hydrophone streamers.
All small cetaceans and pinnipeds for which
audiograms have been measured are much
more sensitive to sound at these higher fre-
quencies than at 200Hz.
It is unclear which measurements of a seis-
mic pulse provide the most reliable indications
of its potential to impact the hearing sensitiv-
ity of different species of marine mammal, but
at short ranges, where hearing damage may
occur, the peak broad band pressure and pulse
rise time and duration seem to be the most
relevant measures (see section A, above). It
should be emphasized, however, that the ef-
fects of noise with the characteristics of air gun
pulses on hearing sensitivity have not been
measured, even in easily studied terrestrial
mammals and man, and extrapolation from
existing data on the effects caused by generic
‘impulsive noise’ may be misleading.
■ Marine Mammal Auditory Sensitivity
(D): A basic measure of auditory sensitivity
is the audiogram: a plot of auditory thresh-
old against frequency. Audiograms have been
measured for many small cetaceans and pin-
nipeds that can be kept and trained in cap-
tivity, although for most species only a few
individuals have been measured (Moore,
1997). The use of electrophysiological tech-
niques to measure evoked auditory poten-
tials allows thresholds to be measured much
more quickly, at least for the smaller species
and at higher frequencies (Dolphin, 1997).
No equivalent measurements have been made
of the hearing sensitivities of the great whales.
Instead auditory parameters are inferred from
the frequency range of their vocalizations,
from levels of background noise, from field
observations of responses to biologically sig-
nificant sounds (Frankel et al., 1995), and
from models of the physical characteristics
of the inner ear (Helweg, 1999). If auditory
systems of different species are assumed to
have similar dynamic ranges then audiogram
data, combined with threshold shift data from
other species, might indicate the levels of
sound at particular frequencies that could
cause threshold shifts in marine mammals.
It is almost certain that the dynamic range of
marine mammal auditory systems varies con-
siderably. However, given the current lack of
direct observations, extrapolations like this,
with appropriate caveats, may often be the
best that can be attempted.
■ Direct measurements of noise-induced TS
in marine mammals: Only recently have
experiments to measure threshold shifts di-
rectly been conducted with marine mammals.
Initial work with odontocetes has been driven
by concerns about effects of military sonar
and explosions. Schlundt et al. (2000) mea-
sured masked hearing thresholds of bottle-
nose dolphins and belugas before and after
exposure to 1 sec tones at 0.4, 3, 10, 20, and
75 kHz. Levels between 192 and 201 dB
caused a 6 dB reduction in sensitivity except
at 400 Hz, where no animals showed evi-
dence of threshold shifts. There was evidence
of some inter-individual variation in sensi-
tivity: one dolphin showed a threshold shift
Winter 2003/04 Volume 37, Number 4
at 75 kHz at 182 dB re:1µPa while another
showed no shift at a maximum exposure of
193 dB re:1µPa. Threshold shifts described
in this work are termed “masked threshold
shifts” because background noise was broad-
cast during the experiment to provide a con-
sistent noise floor in the test facility. Masked
TTS are generally smaller than the non-
masked TTS that would be induced by the
same level of fatiguing noise. Finneran et al.
(2000a) measured masked underwater hear-
ing thresholds of dolphins after they had been
exposed to sounds resembling distant explo-
sions. No threshold shifts were evident after
exposure to single pulses. This group is con-
tinuing this investigation of responses to pow-
erful single pulses using a seismic water gun
as a sound source (Finneran et al., 2000b).
(Water guns are not routinely used as sound
sources by the seismic industry nowadays.)
Au et al. (1999) explored the effects on bottle-
nose dolphins of longer exposures to broader
band noise. They subjected individuals to a
5-10kHz (octave band) fatiguing source for
at least 30 minutes over a one hour period.
No TTS was evident at a received level of
171dB. However, a fairly substantial thresh-
old shift of 12-18dB occurred at 179dB re:
There have been no direct observations
of noise-induced PTS in cetaceans. However,
André et al. (1997) reported patterns of cell
damage, consistent with PTS effects, within
the cochlea of a mother and calf sperm whale
(Physeter catodon) that died after being struck
by a high-speed ferry. They proposed that this
might have been caused by long-term expo-
sure to noise from the relatively high level of
shipping in the Canaries.
Similar noise exposure experiments have
been conducted with pinnipeds (Kastak et
al., 1999). Detection threshold increases of
4.8 dB (harbor seal), 4.9 dB (sea lion), and
4.6 dB (elephant seal) were recorded in har-
bor seals, elephant seals, and California sea
lions, respectively, exposed to 20-25 min. of
octave band noise with mid frequencies rang-
ing from 100 Hz to 2 kHz, at octave band
sound levels of 60-75 dB above the thresh-
old level at the central frequency. All animals
showed full recovery after 24 hrs (Kastak et
al., 1999). Responses of California sea lions
to single impulses from an arc gap transducer
(a device designed to frighten pinnipeds away
from fishing gear) were investigated by
Finneran et al. (2003). Exposure levels were
equivalent to 178-183 dB re:1µPa. While the
two test animals showed avoidance responses,
no TTS was observed.
It is clear that threshold changes can be
induced in both odontocetes and pinnipeds
by exposure to intense short tones and sounds
of moderate intensity for extended periods.
Exposures to single short pulses have not in-
duced threshold shifts. However, it is diffi-
cult to extrapolate from these findings to the
situation typical of a seismic survey, where
animals will receive many pulses over the
course of an exposure.
■ Relative sensitivity of marine mammals
to hearing damage: It has been suggested
that cetacean ears may be less vulnerable to
acoustic damage than those of terrestrial
mammals. However, there is no direct evi-
dence to support this contention. The middle
and inner ears of cetaceans are located out-
side the cranium and are enclosed in two
fused, dense, bony capsules. The middle ear
is enclosed by the tympanic bulla while the
inner ear is within the periotic bulla. These
bones are massive by comparison to homolo-
gous structures in terrestrial mammals, and
this may be an adaptation to withstand pres-
sure changes during diving. However, noise-
induced threshold shifts result from damage
to sensory cells of the organ of Corti, which
sits on top of the basilar membrane within
the cochlea. These structures are as delicate
and vulnerable in cetaceans as in terrestrial
mammals. In odontocetes, the organ of Corti
is very well developed and exhibits conspicu-
ous hypercelluarity. The stria vascularis—a
well vascularised region that runs through
the cochlea and maintains the high potas-
sium concentration within the scala media,
which is essential for the triggering of the
ear’s sensory hair cells—is also very well de-
veloped. It might be argued that this would
make the ear less vulnerable to metabolic ex-
haustion but it could not protect the stereo-
cilia of the hair cells from the physical dam-
age that often underlies PTS, particularly that
caused by relatively short exposure to intense
In terrestrial mammals, the ear can be
protected from intense noise by the opera-
tion of the ‘stapedius reflex’. This involves
the contraction of small muscles that run
between the walls of the middle ear chamber
and the ossicles. In terrestrial mammals, the
ossicles act as a series of levers matching the
low impedance of the external medium (air)
to the high impedance of fluid filled cochlea.
Contraction of these muscles stiffens the os-
sicular chain and reduces the transmission of
sound. In marine mammals, the internal and
external media are both liquid and thus there
is not the same requirement for impedance
matching. The ossicles are relatively massive
and in odontocetes the ossicular chain is stiff-
ened. Thus, although the stapedius muscles
are present, it can be argued that their con-
traction would be less effective in disrupting
the transmission of acoustic signals to the
cochlea. In addition, there is an unavoidable
delay between sensing an intense sound and
implementing the auditory reflex—in hu-
mans this is of the order of 50-100 ms. Con-
sequently, even in terrestrial mammals, the
stapedius reflex has only a limited potential
for protecting the inner ear from short rapid
onset sounds such as seismic pulses.
There is currently no firm evidence to
suggest that cetacean ears are less vulnerable
to the effects of intense noise than terrestrial
mammals and man, or that applying dam-
age risk criteria developed for humans will
necessarily lead to particularly conservative
conclusions for cetaceans.
■ Extrapolations from human criteria to as-
sess risks of threshold shifts in marine mam-
mals (E): It is not clear which measures best
describe a transient sound’s ability to cause
threshold shifts. Some evidence suggests that
peak pressure may be a more appropriate
measure than total energy for predicting in-
stantaneous damage. For any sound type,
there is an intensity threshold above which
damage occurs and below which threshold
shifts are the result of metabolic processes.
Most human research and noise regulation
criteria address long-term exposure to mod-
erate noise sources. However, animals close
to an operating air gun array would receive
very high levels for a relatively short period.
Thus, when considering the potential for seis-
22 Marine Technology Society Journal
mic guns to cause TTS or PTS in marine
mammals, it may be that the risks associated
with short exposures to very high levels are
of greatest concern. As already noted, long-
term exposure to moderate levels may not
adequately predict the effects of shorter ex-
posure to more intense sounds.
Richardson et al. (1995) considered the
application of the human damage risk crite-
ria (derived by Ward, 1968) to marine mam-
mals. They allowed for differences in hearing
thresholds between man and various marine
mammals by expressing critical sound levels
in DRCs relative to the likely best hearing
thresholds for the species under consideration.
(An implicit assumption here is that the dy-
namic range of all marine mammals is the
same, and is similar to that of humans. The
added uncertainty due to this simplification
must be born in mind when considering the
results.) They considered that, for the pur-
poses of this exercise, marine mammals could
be considered in two groups: sensitive spe-
cies, with lowest hearing thresholds of around
40 dB re:1µPa, and less sensitive species, with
best hearing thresholds at 70 dB re:1µPa.
Comparisons between different species are
then made as dBs over threshold level. Thresh-
old levels for humans at frequencies of best
sensitivity are 0 dB re: 20 µPa, so it is neces-
sary to add to that 40 dB or 70 dB (depend-
ing on the assumed sensitivity of the species)
to any human criteria when applying them
to marine mammals. Based on these assump-
tions, Richardson et al. (1995) derived a table
of DRCs for exposure to different numbers
of ‘long’ (200 ms or more) or ‘short’ (25 ms)
pulses, and these data are presented here as
Table 1. This suggests that sensitive marine
mammals might exceed DRC from a single
200 ms pulse with a peak received level of
188 or 218 dB re:1µPa, for more or less sen-
sitive marine mammals respectively (=148 dB
single pulse DRC for humans + 40 or 70 dB).
Further extrapolations can be made to
explore DRC for exposures to different pulse
lengths and numbers of pulses. For example,
consider a typical 10msec air gun pulse.
(152 dB re: 20
Pa is given as a threshold
level for 100-1.5 ms pulses in humans For 10
msec pulses subtract 6 dB = 146 dB (Ward
1968). For a single pulse add 10 dB = 156 dB,
then add 40 dB or 70 dB = 196 dB and 226
dB dB re:1
Pa as DRCs for more sensitive or
less sensitive marine mammals respectively)
For a 260 dB re: 1µPa peak-to-peak
source, and assuming spherical spreading,
these levels would be exceeded out to ranges
of 1,585 m and 50 m respectively. It should
also be appreciated, though, that an animal
might receive several loud pulses from an
For an animal to the side of the array,
pulse length is greater than 200 ms and DRCs
from Table 1 would apply directly. DRCs are
188 dB or 218 dB for more or less sensitive
marine mammals. Assuming spherical
spreading, these levels would be exceeded out
to ranges of 1,260 m and 40 m.
If exposure to 100 pulses is considered,
then DRCs from Table 1 would be 178 dB
or 208 dB and, for spherical spreading, these
levels would be exceeded out to ranges of
c.4,000 m and 130 m.
A seismic survey vessel making 5.5 knots
and emitting one shot every 10 secs will travel
2.8 km in the 1,000 secs required for 100
shots. Exposure of stationary sensitive ma-
rine mammals with a 40 db threshold would
then exceed the levels prohibited by these
DRCs out to a range of ~3,700 m. (An as-
sumption of spherical spreading at such
ranges may overestimate attenuation and if
so then effects at even greater ranges would
Davis et al. (1998) considered the impli-
cations of Ridgway et al.’s (1997) finding of
a 192 dB threshold for TTS from a 1 sec
pulse. They suggested that, because of the
shorter duration, seismic pulses would have
to have to be 10 dB louder, 202 dB re:1µPa,
to achieve the same sound exposure levels.
(This 10 dB increment assumes the seismic
pulse is only 0.1 sec in duration. However,
in deep water, the primary pulse and echo
can last for 0.4 secs, in which case only around
4 dB should be added to Ridgway et al.’s
(1997) threshold for TTS.) More impor-
tantly, these calculations make the assump-
tion that the damaging effect of a pulse is
directly proportional to its energy. This may
not be the case with transients for which peak
pressure seems to be the most important fac-
tor. For example, as mentioned above, Ward
(1968) found that in humans there was no
additional damage from impulse noises once
pulse length exceeded 200 ms. This then
would seem to be the most appropriate inte-
gration time to apply, suggesting a possible
addition of 3 dB to Ridgway et al.’s (1997)
value to allow for a 0.1 sec pulse length and
giving a threshold for TTS of 195 dB re:1µPa.
This is at least within the range of values sug-
gested by Richardson et al’s (1995) extrapo-
lation from human DRCs
Clearly, extrapolations such as these, be-
tween different species, different media and
noise types, are highly speculative. They could
be either significantly over- or under-estimat-
ing the real risks. Given the current state of
knowledge, it is not possible to reach firm
conclusions on the potential for seismic pulses
to cause threshold shifts or hearing damage
in marine mammals. However, extrapolations
made here and elsewhere do serve to indicate
that the risk of seismic sources causing hear-
ing damage to marine mammals cannot be
dismissed as negligible.
TABLE 1 Inferred Auditory Damage Risk Criteria for humans and marine mammals
exposed to noise pulses underwater. After Richardson et al. (1995).
Number of Pulses DRC for human in air Speculative DRC (in dB re. µPa) for marine
(dB re. 20µPa) mammal listening in water with hearing
threshold of 40 and 70 dB re. 1µPa
40dB re. 1
Pa 70 dB re. 1
100 long (>200 ms) 138 178 208
10 long (>200 ms) 143 183 213
1 long (>200 ms) 148 188 218
1 short (25 ms) 174 214 244
Winter 2003/04 Volume 37, Number 4
Perceptual Effects: Auditory Masking
Background noise can reduce an animal’s
ability to detect certain other sounds by
masking. Generally, noise will only mask a
signal if it is sufficiently close to it in fre-
quency, i.e. within that signal’s ‘critical band’.
At low frequencies, critical bands are broad
and have a constant bandwidth. At higher
frequencies, bandwidths are narrower and
their width scales with frequency. Johnson
et al. (1989) found that in beluga whales
bandwidths were fairly constant below 2
kHz, while data for pinnipeds (summarized
by Richardson et al. 1995) suggest broad
critical bands below ~200 Hz. Thus, marine
mammals might be expected to be most sus-
ceptible to masking of low frequency sounds
by low frequency noise, such as seismic. A
masking bandwidth of 1/3 octave at higher
frequencies has often been assumed. How-
ever, in their review of this topic, Richardson
et al. (1995) found masking bandwidths are
typically narrower than this and often >1/6
octave, at higher frequencies. Studies of
masking have usually considered the mask-
ing of a pure tone by other tones or by noise
in a frequency band around it. The situa-
tion is more complex when, as would be the
case for masking of most biologically signifi-
cant sounds by seismic sources, both the
noise and the signal are broadband and the
noise is intermittent rather than continuous.
Signals that are structured, stereotyped, and
repeated will be less susceptible to masking
because they have in-built redundancy. The
effects of masking can also be reduced when
the noise and the signal come from different
directions and the receiver is able to deter-
mine the direction of one or both. In effect,
the signal-to-noise ratio is then reduced in
the direction from which the signal is arriv-
ing. Directional hearing, however, has not
been investigated in marine mammals at the
low frequencies where most seismic source
energy is centered.
There is no direct information on the
extent to which seismic pulses mask biologi-
cally significant sounds for marine mammals.
At greater ranges from the source the main
potential for masking will be at the lower fre-
quencies where masking bands are wider and
susceptibility to masking may thus be greater.
Baleen whales that are believed to be low fre-
quency specialists might thus be most vul-
nerable. Most of their vocalizations are be-
low 1 kHz and some, such as blue
(Balaenoptera musculus) and fin (B. physalus)
whales make predominantly low frequency
calls (Clark, 1990). It has been suggested that
baleen whales could use low frequency sound
to communicate over great distances (Payne
& Webb, 1971), and monitoring of whale
calls using the military SOSUS hydrophone
arrays (Clark & Fristrup, 1997), which lis-
ten within the deep water (~1000m) SOFAR
channel, lends some support to this sugges-
tion. Recent attempts to monitor baleen
whales off the west coast of the British Isles
using SOSUS array hydrophones (Clark &
Charif, 1998) were hampered for long peri-
ods by interference from oil-related seismic
surveying. This suggests that the ability of
baleen whales to monitor their acoustic en-
vironment in oceanic waters could be simi-
larly compromised by seismic surveys. The
intermittent nature of seismic pulses might
be expected to reduce their potential for
masking. However, the length of a seismic
pulse increases with range from the source so
that at range it may approach the 1-sec dura-
tion of the 20 Hz pulses produced by fin
whales, possibly increasing the potential for
masking. Phocids, especially elephant seals,
are another group with good low frequency
hearing that would be expected to be more
susceptible to low frequency masking.
It is not possible, given the current state
of knowledge, to properly assess the poten-
tial for biologically significant masking by
noise from seismic sources. On the one hand,
mammals show a number of adaptations to
enable them to minimize the effects of mask-
ing. On the other, it is likely that being able
to detect a variety of sounds at very low lev-
els is important for their well-being and sur-
vival. Indeed, the fact that such sensitive hear-
ing and sophisticated mechanisms for mini-
mizing the masking effects of noise have
evolved is one indication of the importance
of this for their biological success. If this is
the case, then any reduction in a marine
mammal’s ability to detect biologically sig-
nificant signals could reduce its viability and
the noise from seismic surveys could be hav-
ing deleterious effects on marine mammals
over very substantial ranges.
Many studies have measured changes in
behavior in response to exposure to seismic
noise. Table 2 summarizes the findings from
some of those that provide data on received
noise levels and/or ranges from sources for
■ Bowhead whales (Balaena mysticetus): Oil
exploration in the Bering, Chuckchi, and
Beaufort Seas has prompted considerable re-
search to investigate possible disruption of
the behavior of endangered bowhead whales.
Research has included both opportunistic
observations made during seismic surveys
(e.g. Reeves et al., 1984; Richardson et al.
1986; Richardson et al., 1999), and experi-
mental exposure to air guns (Richardson et
al., 1986; Ljungblad et al., 1988; Richardson
& Malme, 1993). Richardson et al. (1991,
1995) provide comprehensive reviews of
much of the earlier work.
Initial studies in the 1980s indicated that
bowheads typically exhibited overt avoidance
behavior at ranges as great as 6-8 km, corre-
sponding to received noise levels of 150-180
dB re:1µPa, though some observations hinted
at avoidance at greater ranges, e.g. >20 km
by Koski & Johnson (1987). However, ob-
servations of migrating animals made be-
tween 1996-9, which were able to look for
reactions at greater ranges, revealed a higher
sensitivity (Richardson et al., 1999). Bow-
heads avoided the area within 20km of seis-
mic sources ranging in size from 560-1500
in3; at this range received levels were typi-
cally 120-130 dB re:1µPa rms. In 1998 num-
bers were also lower at ranges between 20
and 30km. Beyond the avoidance zone, den-
sities were higher, providing strong support
for the existence of long-range avoidance.
Changes in behavior characteristic of distur-
bance, including reduced surface interval and
dive duration and lower numbers of blows
per surfacing, have been recorded at ranges
of up to 73 km from seismic vessels (Malme
et al., 1988) where received levels were be-
tween 125 and 133 dB re:1µPa.
24 Marine Technology Society Journal
■ Gray whales (Eschrichtius robustus): East-
ern gray whales migrate annually along the
western seaboard of the United States. The
regular and predictable movement of thou-
sands of animals close to land-based obser-
vation points provides ideal opportunities for
conducting controlled and replicated experi-
ments. Reactions of gray whales to air guns
were monitored as they moved along the
Californian coast by Malme et al. (1984).
Observation teams on the shore tracked in-
dividual whales as they swam past a moored
source-vessel during periods when an air gun
was firing and periods when it was silent.
Animals that responded to the guns slowed
and turned away from them and some moved
into areas where the topography shielded
them from the noise of the guns. Received
levels for avoidance reactions by 10%, 50%
and 90% of the animals were 164, 170 and
190 dB re:1µPa. A smaller-scale experiment
with gray whales summering in the Bering
Sea yielded similar results with 10% and 50%
avoidance at 163 and 173 dB re:1µPa, re-
spectively. Würsig et al. (1999) made obser-
vations of the behavior of western gray whales
while seismic surveys were being conducted
in their summer foraging grounds off
Sakhalin Island. They found indications of
avoidance at ranges up to 24km and altered
behavior (faster and straighter swimming and
shorter blow intervals during seismic noise)
at ranges of over 30 km. During a seismic
survey in the same region in 2001, 3-5 gray
whales were apparently displaced from the
seismic survey area to the main known feed-
ing area for the Sakhalin Island population
(Johnson, 2002). Mitigation measures should
have ensured that animals were not exposed
to levels above 163dB (the level causing 10%
avoidance during experiments described
above). Thus, this observation may indicate
greater levels of disturbance from more ex-
Summary of observations of behavioural change in marine mammals in response to air guns and seismic surveys
North West Cape,
1 hr exposure
1 hr exposure
2,120 cu. In.
20 kHz pulse
(263 dB re.
1 µPa –m)
1.64l, 226 dB
560-1500 cu. in
1.64L (226 dB)
44l (258 dB re.
1 µPa2-m p-p)
0.33L, (227 dB re.
1 µPa2-m p-p)
1,600 cu. in.
(215 dB re.
1 µPa 1-m p-p).
Single gun or
(215-224 dB re.
Single gun or small
array (215-224 dB re.
1 µPa-1 m)
(236 dB re. 1µ Pa-
1 m p-p horizontal)
· 178 (75 kHz)
dB-186 (3 kHz dB
· 112 dB
· 180 dB
· 170 dB
· 164 dB
· 173 dB
· 163 dB
· 125-133 dB
· 170 dB P-P
· 162 dB P-P
· 157 dB P-P
· 168 dB P-P
· 159 dB P-P
· 143 dB P-P
200 dB rms
190 dB rms
180 dB rms
160 dB rms
3- 4 km
Reduced vocalisation rate within vocal
range and/or exclusion within 1 km.
Behavioural avoidance responses at
Cessation of vocalisation in response
to some instances of air gun activity
10 % avoidance by migrating whales
10% avoidance by summering whales
Whales abandoned foraging site close to sur-
vey area and moved to main foraging area
Behavioural changes. Changes in blow
rates and dive patterns.
Active avoidance. Swimming away from
the guns and behaviour disrupted for 1-2 hrs.
No avoidance behaviour but significantly
shorter dives and surfacing periods.
Short-term startle response. No clear
avoidance at levels up to 172 dB re.
1m Pa effective pulse pressure level.
Stand-off (General avoidance)
Course alterations begin
Closest approach 10 km?
Cessation of vocalisations for c.1 hr.
Resumption of vocalisations and
movement away from source.
Avoidance. Change from feeding to
transiting behaviour. Haulout.
Apparent recovery c 20 mins after trial.
Initial fright reaction. Bradycardia.
Strong avoidance behaviour
Cessation of feeding
Partial avoidance at <150mMore seals
seen swimming away while guns firing
Ridgeway et al.
Bowles et al.
Malme et al.
Malme et al.
et al. (1995)
Malme et al.
McCauley et al.
McCauley et al.
Thompson et al.
Thompson et al.
Harris et al.
Winter 2003/04 Volume 37, Number 4
■ Humpback whales: McCauley et al. (1998)
reported observations of humpback whales,
migrating off Western Australia, made dur-
ing both full-scale seismic surveys and experi-
mental exposures to a single air gun. Obser-
vations were made from three different plat-
forms: aircraft, the seismic survey vessel, and
an independent tracking vessel.
Comparison of the onshore-offshore dis-
tribution of sightings made during pre-seis-
mic aerial surveys and the distribution of
sightings from the seismic survey vessel did
not indicate any gross disruption of the
whale’s migration route. However, all pods
followed by the independent tracking vessel
were observed to respond to the seismic ves-
sel. One whale showed a dramatic alteration
of behavior, swimming at high speed (10-15
knots) very close to the surface before pass-
ing 1,500 m ahead of the seismic vessel. It
eventually slowed when some 3 km beyond
the seismic vessel and resumed its previous
course when 6 km south of it. Two other pods
showed less dramatic course changes at ranges
of 5-8 km, passing 3-4 km behind the sur-
vey vessel. The fourth pod followed an er-
ratic zigzag course and eventually passed 3
km behind the survey vessel. On two occa-
sions, animals spent an unusually high pro-
portion of time at the surface. The authors
speculated that this could be due to reduced
sound level in surface waters.
Observers on the seismic vessel made pro-
portionally more sightings within 3 km and
relatively fewer at ranges of greater than 3
km during periods when guns were off. Sight-
ing rates at ranges >3 km were some 3 times
higher during “guns on” than “guns off ” pe-
riods. This is consistent with whales avoid-
ing the survey vessel out to ranges of >3 km.
Total sightings rates were highest during ‘tran-
sition periods’: the periods when guns were
turned on and when they were turned off. It
was suggested that this could be a startle ef-
fect or curiosity, causing whales to come to
Controlled exposure experiments using
a small air gun array were conducted in an
adjoining bay. The source vessel approached
groups of humpback whales while a dedicated
observation vessel tracked their movements.
Whales generally showed speed and course
changes to avoid coming closer than 1-2 km
to the air gun vessel. However, on several
occasions whales were observed to approach
and circle the seismic vessel at ranges within
100-400 m (expected exposures 192-177 dB
In summary, humpback whales showed
avoidance behavior at a range of 5-8 km from
a full-scale array and maintained a stand-off
range of 3-4 km. Typical received levels at 5
km were measured as 162 dB re:1µPa2 peak-
to-peak. During the trials with a smaller air
gun, avoidance was usually evident at 2 km
at which received levels were 159 dB re:1µPa2
McCauley et al. (1998) suggested that
different classes of humpback whales might
exhibit different levels of sensitivity. For ex-
ample, adult males seeking mates might be
least likely to alter their behavior in response
to seismic surveys. The authors also suggested
that males might even confuse seismic pulses
with the noise made by the flipper slaps and
lob-tails of competitors.
This comprehensive study demonstrates
how valuable it can be to integrate CEE and
observational approaches and how detailed
behavioral observations of a few individuals
can assist with the interpretation of more
■ Blue whales: McDonald et al. (1995)
tracked the locations, and from these, in-
ferred the movements of blue whales by ana-
lyzing data from an array of seismometers
mounted on the seafloor. One blue whale
was tracked while an active seismic survey
vessel was moving through the area. The
source was a relatively low-powered four-
air gun array with a total capacity of 1,600
in3 and a source level of 215 dB re:1 µPa
peak-to-peak @ 1 m over a 10-60 Hz band.
Initially, the whale was tracked moving at a
speed of about 10 km/hr on a course con-
verging with that of the vessel. At a range of
10 km from the seismic vessel (estimated
received level of 143 dB re:1 µPa peak-to-
peak in the 10-60 Hz band ) the whale
stopped vocalizing and remained silent for
an hour before resuming calling at a range
of 10 km. Its track then diverged from that
of the seismic vessel by about 80° and from
its original course by c.120o. This apparent
long-range avoidance of the seismic vessel
may indicate that blue whales are rather
more sensitive to air gun noise than hump-
backs, bowheads, or gray whales.
■ Other rorquals: Stone (2003) summa-
rized reports from observers on seismic ves-
sels operating in UK waters between 1998
and 2003, collated by the UK Joint Nature
Conservation Committee. When sightings
of minke whales (Balaenoptera
acutorostrata), sei whales (B. borealis), and
fin whales were combined, ranges to ani-
mals were higher for sightings made during
surveys than those made at other times—
Toothed Whales and Dolphins
The effects of seismic surveys on
odontocetes have generally been less thor-
oughly investigated than effects on baleen
■ Dolphins and porpoises: Goold (1996)
monitored acoustic activity of common dol-
phin (Delphinus delphis) before, during, and
after seismic surveys off the coast of Wales.
Acoustic contact with dolphins was lower
during the seismic survey than before it, and
was lowest during periods when the guns were
actually firing. In addition, fewer dolphins
were observed bow riding during seismic sur-
veys. These results were taken to indicate that
within 1 km dolphins found the signals from
a seismic source aversive.
Gordon et al. (1998) reported on both
experimental exposures to harbor porpoises
in inshore waters around Orkney, UK, using
a small source (3 x 40 cu. in. air guns; source
level approx 228 dB re: 1 µ Pa zero-to-peak
@ 1 m), and on harbor porpoise detection
rates made during commercial seismic sur-
veys. In both cases, porpoise groups were de-
tected acoustically using semi-automated de-
tection equipment (Chappell et al., 1996).
During experiments in inshore waters the
sound source was slowly brought into a bay
while a small boat conducted continuous
acoustic survey lines within the bay. No
changes in the rate of acoustic contact were
observed during two hour periods before,
during, and after the controlled exposure.
Harbor porpoises were not excluded from an
area of preferred habitat by short-term expo-
26 Marine Technology Society Journal
sure to this modest source. The authors cau-
tion, however, that these results can only be
applied to the very precautionary experimen-
tal approach that they employed which in-
volved using a small source and short expo-
Detections of porpoises were also made
during full-scale seismic surveys to the north
of Shetland. The same acoustic detection
equipment was deployed from a guard vessel
that kept station about one mile ahead of the
seismic vessel. At this range, there were no
significant differences in acoustic detection
rates for porpoises during periods when the
guns were firing and when they were off (dur-
ing turns between lines for example). This
might be taken as lack of evidence for avoid-
ance by harbor porpoises at ranges of a mile
and more, though it is of course possible that
avoidance could have occurred at shorter
ranges than this.
Probable avoidance of active seismic
sources by odontocetes is suggested by analy-
sis of the reports of observers on seismic ves-
sels off the UK collated by the UK Joint Na-
ture Conservation Committee (Stone 2003).
Encounter rates for both white-beaked
(Lagenorhynchus albirostris) and Atlantic white-
sided dolphins (L. acutus) were significantly
lower during seismic surveys than at other
times and distance to sightings was significantly
higher for these two species and for killer
whales, bottlenose dolphins, other dolphins,
and porpoises when guns were firing.
■ Sperm whales: Sperm whales are the larg-
est odontocetes and are thought to have bet-
ter low frequency hearing than smaller
odontocetes. They may therefore be more
vulnerable to disturbance from seismic sur-
veys. Observations of responses to seismic
surveys are contradictory however. Some
suggest a strong effect. For example, Mate et
al. (1994) reported that sperm whale density
in a preferred area in the Northern Gulf of
Mexico decreased to approximately 1/3 of
pre-survey levels for the two days after the
seismic survey started and to zero for five days
after that. The authors do point out that these
observations were serendipitous and it would
be wrong to assume that any causal relation-
ship was demonstrated. Sperm whales were
also reported to temporarily vacate the wa-
ters off Kaikoura, New Zealand, after a seis-
mic survey (Liz Slooten, pers. comm. Cited
in IFAW, 1996).
Indications that sperm whales in the
Southern Ocean respond to seismic surveys
at extreme ranges are provided by Bowles et
al., 1994. They observed that sperm whales
ceased vocalizing during some, but not all,
periods when a seismic survey vessel was
heard firing at a range of 370 km. These were
apparently not startle or curiosity responses
to a novel stimulus, as the seismic source had
been audible intermittently over the two
weeks in which they had monitored acousti-
cally in the area, and indeed had been sur-
veying for some time before the start of their
study. The sound source was later found to
be an array of 8 x 16l Bolt air guns with an
estimated source level of 263 dB re:1µPa @
1m. At these extreme ranges the seismic
pulses had a duration of c. 3 secs, ranged in
frequency from 30-500 Hz and received lev-
els of 120 dB re:1µPa were measured at a
range of 1,070 km.
In contrast to these reports of extreme
sensitivity, other observations suggest that
sperm whales show little response and are not
excluded from habitat by seismic surveys (e.g.
Rankin & Evans, 1998; Swift, 1998). Swift
(1998) used acoustic monitoring techniques
to determine the relative abundance and dis-
tribution of sperm whales before, during, and
after a three week seismic survey on the
Rockall bank west of Scotland. Observations
extended over seven weeks to include one
week pre-survey, three weeks during the sur-
vey, and a week of post-survey monitoring.
Acoustic detection rates were actually higher
during the seismic survey period than the
weeks before or after the survey. It is possible
that whale density did actually increase in
response to the seismic survey but perhaps
more probable that changes in detection rates
were the result of a seasonal change in sperm
Swift (1998) also found no significant
difference in detection rates between ‘guns
on’ and ‘guns off ’ periods during the seismic
survey itself, suggesting a lack of short-term
responses as well. However, it should be re-
membered that, using hydrophones, these
researchers were able to detect sperm whales
at ranges of c. 5 miles and this may have made
changes in behavior and distributions at lesser
ranges more difficult to detect.
Madsen et al. (2002) describe how a seis-
mic survey was shot in the area over a 13 day
period while they were involved in a detailed
study of sperm whale acoustic behavior in
the area and they were thus able to look for
responses to these opportunistic exposures.
Ranges of the whales being observed from
the seismic vessel, which was using a 62L ar-
ray, ranged from 20-140km and maximum
measured exposure levels were 130 dB re:
1µPa rms. Whales did not fall silent when
air guns started and the authors did not de-
tect any changes to their normal vocal be-
havior in response to air guns. Further, whale
watchers operating in the area encountered
whales in much the same locations in the
canyon both before and during the survey.
■ Pinnipeds: There have been surprisingly
few studies of the effects of air gun noise on
pinnipeds, even though members of this
group have good underwater hearing and
their feeding grounds often overlap with seis-
mic survey areas. When Richardson et al.
(1995) reviewed the subject, they could only
find two anecdotal reports, and both sug-
gested that seals did not react strongly to
seismic noise. Recently, detailed observa-
tions of the behavioral and physiological re-
sponses of harbor and grey seals (Halichoerus
grypus), have been reported by Thompson
et al. (1998). These researchers conducted
1 hr controlled exposure experiments with
small air guns (source levels of the air guns
used were 215-224 dB re: 1µPa peak-to-
peak) to individual seals that had been fit-
ted with telemetry devices. The telemetry
packages allowed the movement, dive be-
havior, and swim speeds of the seals to be
monitored and thus provided detailed data
on their responses to seismic pulses. Two
harbor seals equipped with heart rate tags
showed evidence of a fright responses when
playbacks started: their heart rates dropped
dramatically from 35-45 beats/min to 5-10
beats/min. However, these responses were
short-lived and following a typical surfac-
ing tachycardia; there were no further dra-
matic drops in heart rate. In six out of eight
trials with harbor seals, the animals exhib-
Winter 2003/04 Volume 37, Number 4
ited strong avoidance behavior, swimming
rapidly away from the source. Stomach tem-
perature tags revealed that they ceased feed-
ing during this time. Only one seal showed
no detectable response to the guns and ap-
proached to within 300 m of them. The be-
havior of harbor seals seemed to return to
normal soon after the end of each trial.
Similar avoidance responses were docu-
mented during all trials with grey seals: they
changed from making foraging dives to v-
shaped transiting dives and moved away from
the source. Some seals hauled out (possibly
to avoid the noise); those that remained in
the water seemed to have returned to pre-
trial behavior within two hours of the guns
falling silent. The authors comment that re-
sponses to more powerful commercial arrays
might be expected to be more extreme, longer
lasting, and to occur at greater ranges. These
represent some of the most detailed and dra-
matic short-term responses to air guns ob-
served from any marine mammal.
By contrast, sightings rates of ringed seals
(Phoca hispida) from a seismic vessel in shal-
low arctic waters showed no difference be-
tween periods with the full array, partial ar-
ray, or no guns firing (Harris et al., 2001).
Mean radial distance to sightings did increase
during full array operations, however, sug-
gesting some local avoidance.
Habituation, Sensitization, and Indi-
vidual Variation in Responsiveness
It is clear from the above review that,
even within species, the behavioral responses
of marine mammals to seismic survey noises
are quite variable. A range of factors may
affect an animal’s response to a particular
stimulus including: 1) its previous experi-
ence of it; 2) any associations it may have
made with that signal; 3) the individual’s
auditory sensitivity; 4) its biological and
social status, including its age and sex; and
5) its behavioral state and activity at the time.
Thus, by their very nature, behavioral re-
sponses are likely to be unpredictable. (See
Wartzok et al.’s paper in this volume for a
Habituation occurs when an animal’s re-
sponse to a stimulus wanes with repeated
exposure in the absence of unpleasant asso-
ciated events. Animals are most likely to ha-
bituate to sounds that are predictable and
unvarying. The opposite process is sensitiza-
tion, when experience of a signal leads to an
increased response. This may occur when an
animal learns to associate a sound with a
harmful or unpleasant event. Animals might
be expected to respond to such signals when
they are just audible. In the case of seismic
sounds, an animal that had been exposed to
levels of sound at a level high enough to cause
discomfort might show avoidance responses
at a lower level on subsequent exposures,
while other animals, which had only been
exposed to lower levels, might become ha-
bituated. Thus, quite different response be-
haviors could become established in differ-
Within a species, different classes of indi-
viduals might be expected to be differentially
vulnerable and/or responsive. For example, a
mother nursing a young calf might be expected
to be more likely to show avoidance behavior
than a male guarding a breeding territory. Fi-
nally, the animal’s behavioral state might make
it more or less likely to exhibit disturbance
behavior: animals that are resting or engaged
in some non-essential activity may show greater
behavioral change than animals highly moti-
vated to perform an important activity, such
as feeding or mating.
Chronic Effects and Stress
Extended exposure to even quite low lev-
els of sound can cause stress and lead to health
problems in humans. Elevated levels of noise
can impair both mental and psychomotor
functions in man (Kryter, 1994). In mammals,
stress is often associated with release of the
hormones ATCH (adrenocorticotrophic hor-
mone) and cortisol. This has been shown, for
example, during transportation in pigs (Dalin
et al., 1993; McGlone et al., 1993) and goats
(Greenwood & Schutt, 1992). Increases in
hormone levels are typically also associated
with changes in behavior, e.g. increased ag-
gression, changes in respiration patterns or
social behavior and may lead to a reduction in
the effectiveness of the immune system.
Thomas et al. (1990) attempted to mea-
sure stress induced in four captive belugas
by playback of recordings of drilling plat-
form noise (source level 153 dB
re:1µPa@1m). Levels of catecholamines in
blood samples did not increase during the
experiment. However, the playbacks were
relatively short and these captive animals
may have already adapted to living in a noisy
and stressful environment.
In spite of the potential difficulty of dem-
onstrating cause and effect in marine mam-
mals in the field, the potential for noise-in-
duced stress to have effects on so many as-
pects of the health of individuals and popu-
lations makes it a matter of real concern.
Noise may indirectly impact cetaceans
through its effects on prey abundance, be-
havior, and distribution. Bony fish may be
particularly vulnerable to intense sound be-
cause most of them possess an air-filled swim
bladder. Although marine fish typically have
less acute hearing than marine mammals,
many are more sensitive than odontocetes in
the range 100-500 Hz where most seismic
sound is produced. Effects of air gun pulses
on fish range from serious injury at short
ranges, to avoidance behavior, possibly at the
range of many kilometers (Turnpenny &
Nedwell, 1994). Reduced catch rates have
been reported for several species of fish in
areas of seismic surveying activity (see review
in McCauley, 1994). For example, catch per
unit effort for rockfish (Sebastes spp.) declined
by 50% during a series of controlled expo-
sure experiments using air guns (Skalski et
al., 1992). Acoustically estimated fish popu-
lations were reduced by 36% for demersal
species, 54% for pelagic species, and 13%
for small pelagic species during a 3-D survey
in the North Sea compared with pre-shoot-
ing abundance (Bohne et al., 1985). Re-
corded cod and haddock catches were re-
duced by 50% within a 20 nm radius of an
operating seismic vessel, and reduced by 70%
within the 3 x 10 nm seismic shooting area
(Engas et al., 1993). Long-line catches of both
species were also reduced by 44% inside the
seismic shooting area but were not affected
at a range of 18 nm. Scanning electron mi-
croscopic examination of the ears of pink
snapper (Pagrus auratus) after exposure to air
gun pulses has revealed ablated hair cells and
28 Marine Technology Society Journal
extensive damage to the sensory epithelia
(McCauley et al., 2003).
These studies indicate a variety of effects
from seismic pulses on potential marine mam-
mal prey species. If seismic surveys cause fish
(or other animals) that are the prey of marine
mammals to become less accessible, either
because they move out of an area or become
more difficult to catch, then marine mammal
distributions and feeding rates are likely to be
affected. Conversely, it might be possible that
damaged or disoriented prey could attract
marine mammals to a seismic survey area, pro-
viding short term feeding opportunities but
increasing levels of exposure. There have as yet
been no attempts to investigate such indirect
effects on marine mammals.
Effects of Seismic Disturbance
on Human Utilization of Marine
In addition to strong ethical reasons for
wishing to avoid disrupting marine mam-
mal populations with seismic noise, there
may also be immediate economic and po-
litical concerns when human utilization of
a marine mammal is potentially affected.
Marine mammals are exploited both con-
sumptively (hunting of seals and whaling),
and non-consumptively (nature tours and
Marine mammals continue to be hunted
by both commercial and aboriginal opera-
tions in many parts of the world. Changes in
distribution, abundance, or behavior of ma-
rine mammals that make them less accessible
to local hunters will reduce commercially
valuable catches that in some cases are also
considered to be of significant subsistence or
cultural value. Avoiding the potential disrup-
tion of Inuit whaling and sealing operations
is an important concern during oil explora-
tion activities off Alaska, for example.
Whale watching is a significant and rap-
idly increasing sector of the tourist industry.
Hoyt (1995) estimated worldwide revenues
to be US$ 504 million, and the activity has
continued to expand since then. The impor-
tance of seals as subjects for wildlife tourism
has been less well appreciated, but Young
(1998) estimated the gross annual value of
seal watching in the UK and Ireland to be
£38 million. Clearly, changes in a local popu-
lations, abundance, and distribution, or in
the approachability of marine mammals
could all affect the viability of specific ma-
rine mammal tourism operations.
Discussion and Comment
Marine Mammals in a Three-
The Particular Vulnerability
of Deep Divers
As seismic exploration increasingly moves
into deeper offshore waters, the magnitude
of the third dimension, depth, becomes more
significant. All marine mammals dive, in fact
many will spend the majority of their lives
underwater, and some can spend significant
times at very substantial depths. Sperm
whales, for example, regularly make dives in
excess of 1,000 m (Watkins et al., 1993) and
have been recorded down to 2,500 m (Norris
& Harvey, 1972). Beaked whales are also
known to be impressive deep divers (Hooker
& Baird, 1999), possibly exceeding the abili-
ties of sperm whales. Seals may be even more
accomplished divers than cetaceans. For ex-
ample, elephant seals regularly dive to depths
greater than 1,700 m (Delong and Stewart,
1991; McConnel et al., 1992), and most
phocid seals spend >80% of their time sub-
merged and most of that at depth.
Deep divers are worthy of special con-
sideration for a number of reasons. Diving
takes them into regions in which received
sound levels are higher than those measured
or predicted close to the surface, including
the zones beneath air gun arrays in which
most sound is focused. A diving animal is
also committed to a strict energy budget, to
ensure that the oxygen stores within its body
are managed to allow it to dive to a certain
depth for a particular length of time and
return to the surface.
A diving mammal leaves the surface with
stores of oxygen in its blood and muscles
that must sustain it through its entire dive.
During dives, energetic activities are mini-
mized and movement will tend to take place
at close to the most energy-efficient swim-
ming speed. It is possible for muscles to re-
spire for short periods without oxygen
(anaerobically) but this incurs an ‘oxygen
debt’ which is expensive to ‘repay’ both in
terms of energetic and time budgets (Fedak
& Thompson, 1993; Thompson & Fedak,
2001). From the perspective of an animal
wishing to avoid loud noise sources, this may
mean that strategies involving energetically
costly activities, such as rapid swimming,
may be precluded, particularly towards the
end of dives when oxygen stores will be
minimal. Their options for avoiding loud
noise sources are tightly constrained and the
consequences of their taking avoiding ac-
tion may be more serious than they would
at first seem. An air-breathing diver must
ultimately return to the surface to have ac-
cess to air. This may force the animals to
swim towards the noise source.
Generally, of course, submerged divers are
not visible at the surface, and some divers, such
as elephant seals, hooded seals, sperm whales
and beaked whales may routinely perform
dives of between 30 mins and an hour. In such
cases, the fact that observers have not seen them
at the surface before starting a seismic line is
no guarantee that they are not within the ‘dan-
ger zone’. For some species, notably the sperm
whale, which is highly vocal, acoustic moni-
toring can provide helpful information on the
presence of submerged animals (Leaper et al.,
1992; Lewis et al., 1999). Unfortunately, this
is unlikely to be a reliable method for detect-
ing the presence of beaked whales or seals.
Zones of influence
A concept widely used in regulation and
management is that of zones of influence,
within which different types of effects would
be expected to occur. If a uniform field of
propagation and attenuation is assumed (and
ignoring the third dimension of depth), these
can be represented as a series of concentric
circles around a noise source, whose radii are
the ranges at which the level of the sound
might be expected to have fallen to a certain
threshold level. Four zones suggested by
Richardson et al. (1995) are:
1) the zone of audibility (the area within
which the sound is both above the
animal’s hearing threshold and detectable
above background noise)
Winter 2003/04 Volume 37, Number 4
2) the zone of responsiveness (the region
within which behavioral reactions in re
sponse to the sound occur)
3) the zone of masking (the zone within
which the sound may mask biologically
4) the zone of hearing loss, discomfort, or
injury (the area within which the sound
level is sufficient to cause threshold shifts
or hearing damage)
The radius of the circle defining each zone
will depend on the characteristics of the sound
itself, the susceptibility of the animals being
considered, and the acoustic propagation char-
acteristics in the survey area. In devising man-
agement guidelines and regulations that are
appropriate for a particular survey, managers
will often use threshold sound levels for cer-
tain effects (based perhaps on research in a
different area) and calculate the ranges at which
the sound level from the particular source be-
ing used will fall to this threshold in the sur-
vey area being considered. In these situations,
the nature of propagation conditions in the
survey area becomes critical. Propagation con-
ditions can vary widely from location to loca-
tion and depend on a variety of factors (Urick,
1983). Differing propagation conditions will
have a magnified effect (squared if consider-
ing area, cubed if volume) on zones of influ-
ence since these represent areas which rapidly
increase or decrease with even small changes
in radius. For example, the radius of a zone of
behavioral influence for an air gun array with
a threshold of 140dB could vary by 4000 times
between different likely propagation condi-
tions, in which case the area and number of
individuals affected would vary by a factor of
16 million. This highlights the importance of
making empirical measurements of propaga-
tion loss and applying appropriate models in-
formed by up-to-date oceanographic data
when management involving a zones of influ-
ence model is used.
Biological Significance of Possible
Effects of Seismic Pulses for
Individuals and Populations
There are both ethical and legal reasons
for being concerned about the welfare of in-
dividual animals. In addition to these, are
concerns for the health and viability of popu-
lations and species. Much legislation is
couched in conservation terms, while public
opinion often responds strongly to animal
welfare issues. As we have seen, research is
beginning to provide evidence for and against
the existence of short-term effects on indi-
viduals. When it comes to assessing the bio-
logical significance of these, we have to rely
on biological interpretation, modeling, and
extrapolation. One might expect serious ef-
fects to cause changes in the size of popula-
tions, but, given the poor precision with
which the size of any marine mammal popu-
lation can be measured, and the fact that in
many parts of the world there are no reliable
estimates for many populations, only very
large changes in population size would be
identified. As many marine mammal popu-
lations have very small rates of increase, bio-
logically important changes in these rates
would be especially difficult to identify, par-
ticularly in a timely fashion. Further, it is not
good practice to wait for such major impacts
before taking management action.
■ Hearing damage: Hearing is the most im-
portant sensory modality for marine mam-
mals underwater and the ability to hear well
is vital for many important aspects of their
lives such as finding food, navigating, locat-
ing mates and avoiding predators. It would
seem indisputable, then, that any reduction
in hearing ability would very seriously com-
promise the viability of individual animals.
If a significant proportion of the population
was affected in this way, there could be del-
eterious conservation consequences as well.
Exposure to high levels of noise could also
have animal welfare implications if, for ex-
ample, it induced panic or caused pain.
■ Perceptual and behavioral effects: Mask-
ing of biologically significant sounds by back-
ground noise is equivalent to a temporary loss
in hearing acuity, but little is known about
the importance to marine mammals of hear-
ing low-level sounds in background noise.
The very fact that they have developed such
sensitive hearing, and seem to be adept at
detecting signals in background noise, sug-
gests that this is an important ability for them.
The significance of disruption of behav-
ior in part depends on the importance of the
behavior affected. Small-scale course changes
to avoid surveys during migrations, such as
those measured for gray whales by Malme et
al. (1986), might, in themselves, have few
long-term consequences for individuals or
populations. The consequences might be
more serious in areas where many surveys are
occurring simultaneously. In some cases, al-
terations in migration paths could move ani-
mals into dangerous areas. For example,
Simmonds & Mayer (1997) suggested that
seismic surveys being conducted to the west
of the British Isles might have contributed to
recent live multiple sperm whale strandings
in the North Sea if they caused southward-
moving animals to divert to the east of their
normal course and into the shallow North
Sea. As discussed above, information from
recent mass strandings of beaked whales (e.g.,
Jepson et al., 2003) hints at the possibility
that noise-induced changes in behavior could
lead deep diving cetaceans to develop decom-
pression sickness. Disturbance could also lead
to disruption of feeding and deep diving ani-
mals could be particularly vulnerable. If
sperm whales use their vocalizations to
echolocate, as most believe they do, then the
cessation of vocalizations observed by Bowles
et al. (1994) in the Southern Ocean in re-
sponse to seismic noise at ranges of hundreds
of kilometers, would have stopped those ani-
mals feeding. In this case, the effects were
evident at such extended ranges that hun-
dreds or thousands of animals might have
been affected. Seismic surveys could influ-
ence the availability of prey, especially fish.
Reduced feeding by marine mammals may
eventually reduce their reproductive rates and
increase mortality. Most marine mammals are
adaptable and opportunistic feeders, and the
large whales, in particular, have evolved to
survive for extended periods without feed-
ing. However, certain classes e.g. newly
weaned phocid seal pups may be particularly
susceptible to reduced feeding rates.
Disruption of social organization could
have severe consequences for those animals
for which long-term social groupings seem
to be important for survival, such as the
toothed whales. Long-range communication
is important in keeping cetacean groups to-
gether (Payne, 1995). Acoustic disturbance
may disrupt social groups, while increased
30 Marine Technology Society Journal
background noise could hamper the ability
of members of dispersed groups to find each
other and keep in contact using vocalizations.
Mothers and their dependent calves may
be particularly vulnerable to disturbance. In
some species of odontocetes calves remain
with their mothers for several years. Disrup-
tion of this bond could prevent calves from
suckling and may lead to increased threat of
These potentially damaging or disturb-
ing effects of seismic surveys cannot be con-
sidered in isolation. Marine mammals are
subject to a range of natural and, to an in-
creasing extent, anthropogenic threats. It is
the combination of all of these that may lead
to biologically significant effects. Some fac-
tors will interact and may act synergistically.
For example, chronic effects due to distur-
bance, stress, or chemical contamination may
weaken the immune systems of individuals
making them more vulnerable to disease.
With marine mammals becoming subject to
an increasing number of new threats whose
effects are likely to be cumulative, it is im-
portant to minimize the impacts of all and
any of them wherever possible.
There is at present, little or no direct evi-
dence for biologically significant effects of
seismic surveys on marine mammals but it
must be appreciated that none of the research
projects that have been conducted so far have
been capable of adequately testing for effects
at this level. The fact that plausible cases can
be made that observed or possible responses
could result in biologically significant effects,
is an indication that this is a potential prob-
lem that deserves to be taken seriously.
Spatial and Temporal Scales
The spatial and temporal scales at which
the potential effects of seismic surveys
should be investigated are daunting. Sound
from air guns may be audible to marine
mammals at ranges of several hundreds of
kilometers. In some cases (e.g. bowhead
whales, Richardson et al., 1999; Malme et
al., 1988; and sperm whales, Bowles et al.
1994), behavioral responses have been mea-
sured at ranges of many tens, or even hun-
dreds, of kilometers from the source. How-
ever, few studies have attempted to measure
effects at these ranges. Studies tend to focus
on smaller numbers of animals close to seis-
mic surveys and it is possible that very sub-
stantial numbers of marine mammals are
subject to unmeasured effects, perhaps in-
ducing stress, over huge areas. In the tem-
poral dimension, in some commercially
promising regions, such as the Gulf of
Mexico and the North Sea, many seismic
surveys may be being conducted simulta-
neously throughout most of the summer
months of each year for many years. The
modern trend towards using 4-D seismic
surveys to monitor patterns of oil field ex-
ploitation will mean that seismic surveys will
become a regular occurrence in many oil
fields. Long-term studies to assess impacts
at these temporal and spatial scales have not
been conducted, and in nearly all cases
baseline data, from the time before surveys
start, are completely lacking.
Making Better Use of Data Collected
on Seismic Surveys
In many regions regulators require seis-
mic operators to carry trained marine mam-
mal observers to keep watch for marine
mammals for mitigation purposes. This re-
source can provide high quality visual (and
in some cases acoustic) data collected sys-
tematically by trained observers and this can
potentially be used to provide information
on the offshore distribution of marine mam-
mals and on behavioral responses to air gun
pulses. The scheme run in the UK by JNCC
provides a good example of how such data
can be collated (e.g. Stone, 2003). However,
it is likely that even more useful data could
be collected if specific observation proto-
cols were developed by experts with appro-
Implications for Management
Concerns about the conservation of ma-
rine mammals have usually focused on cases
where animals suffer dramatic effects, such as
mortality from hunting or fisheries bycatch.
Management regimes have been established,
with varying levels of success, to address such
issues. It is possible that, at short ranges, seis-
mic survey noise could cause similar acute
problems. Of potentially greater concern is the
possibility that alone, or in combination with
other factors, air gun noise will have less dra-
matic chronic effects such as: excluding ma-
rine mammals from important areas at sig-
nificant times, interfering with their migrations
and movements, contribute to overall habitat
degradation, disruption of biologically signifi-
cant behaviors, and increased levels of stress.
Although such effects appear less severe than
direct mortality or injury, they affect many
more individuals and extend over significant
periods of time. Cumulative effects could re-
sult in the reduction of reproductive rates,
which are generally very low in marine mam-
mals, and increases in mortality. Chronic prob-
lems of this kind are a legitimate conservation
concern but they are difficult to manage within
This review has emphasized the paucity
of knowledge and the high level of uncer-
tainty surrounding potential effects of sound
on marine mammals. This problem is com-
mon to many conservation issues and has led
to the development and adoption of a pre-
cautionary approach in many national and
international agreements (Hey, 1991a; Hey,
1991b provide reviews). Mayer & Simmonds
(1996) considered the role of precaution in
cetacean conservation and used, as one case
study, an example of acoustic disturbance, the
‘Heard Island Experiment’—an experiment
designed to transmit loud, low frequency
sound underwater from Heard Island (near
Antarctica) to 18 detection sites around the
world. The experiment raised concerns about
the impacts of anthropogenic sounds on ce-
taceans. It highlighted the lack of informa-
tion on the biology and population distribu-
tion of the marine mammals in the study area
and the widely differing scientific opinions
on the effects of noise on them.
While so many uncertainties surround
the effects of air gun noise on marine mam-
mals, it is important that their use should be
managed in a precautionary way to safeguard
both individuals and populations. The most
immediate and effective method of reducing
impacts would be to minimize the number
of surveys and the power of the sources em-
ployed. Encouraging companies to share the
results of past and future surveys would be
one mechanism for achieving this. When
Winter 2003/04 Volume 37, Number 4
surveys must be undertaken, they should be
governed by appropriate regulations and
codes of practice that are based on either good
empirical observations or on precautionary
assumptions about sound propagation and
the auditory sensitivity, behavior, and vulner-
abilities of marine mammals. Such an ap-
proach should stimulate all stakeholders to
strive to expand and refine our knowledge
about the effects of anthropogenic noise and
can help to clarify how research resources can
best be used to reduce overall uncertainty.
When precautionary management is applied,
new research to decrease uncertainty in our
understanding of key parameters will usu-
ally lead to regulations that are less onerous
and disruptive for industry while providing
effective protection to marine mammals.
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