Then we humans dunk our heads underwater, the ocean seems relatively silent. This misconception occurs because our ears are optimized to hear in air and have poor sensitivity in the much denser medium of water. In reality, the oceans are full of sounds. Natural sources of underwater sound include breaking waves and surf, rain striking the sea surface, ice cracking and groaning in the higher latitudes, and the distant rumble of storms and earthquakes. Besides these physical sources, there is also a rich biological repertoire. There are sounds of snapping shrimp, grunting fishes, squeaking and popping sirenians, and the amazingly varied vocalizations of pinnipeds and cetaceans. Walruses (Odobenus rosmarus) display by knocks and mews: bearded seals (Erignathw bariatiis) emit elaborate trills during their breeding season; toothed whales whistle, send bursts of staccato-like click trains, and echolocate; and large whales moan, groan, and sing for group cohesion, sexual displays, and communication (Tyack, 2000). Some researchers suspect that strong low-frequency sounds of certain baleen whales may also function as active sonar, helping them to navigate across wide open ocean spaces or around ice, or to locate silent conspecifics.
Unfortunately, the industrialized world has created other sources of noise underwater (Fig. 1). There is motorized shipping, underwater blasting, and offshore drilling, dredging, and construction. These activities produce underwater sounds incidentally, not purposefully. Several other types of underwater sounds are created purposefully: fathometers and sonars of many types operating at frequencies ranging from very high to low; air gun pulses for oil and gas exploration; pingers used to locate underwater equipment and to alert marine mammals to the presence of fishing nets; acoustic harassment devices (such as seal bombs) to keep marine mammals away from mariculture facilities: and sounds used for ocean science measurements (such as ATOC). Fish and marine mammals have evolved with the rich physical and biological cacophony of nature and are presumably well adapted to those sounds. However, most anthropogenic (human-generated) sounds first appeared in the past 100 years or so and are increasing in intensity and geographical extent decade by decade (Gisiner et al, 1999; Jasny, 1999).
I. Importance of Sound to Marine Mammals
Marine mammals rely on underwater sound for communicating, finding prey, avoiding predators, and probably navigating. Other senses are available to them, but sound is the most important one at distances or in environments where the senses of touch, taste, and sight are not available. It is unclear how much sea otters (Enhijdra lutris) and polar bears (Ursus mar-itimus) rely on sounds underwater. However, it is well known that pinnipeds (sea lions, fur seals, seals, and walruses), sirenians (manatees and the dugong, Dtigong dugon), and cetaceans (dolphins, porpoises, and whales) use sound both passively, when listening to the environment, and actively, when communicating. The toothed whales also echolocate to find prey, detect predators, and maneuver in the environment.
The acoustic frequencies that are most important vary with the type of marine mammal. Baleen whales tend to use lower frequencies of sound: usually below 1 kHz and reaching down into the infrasonic range (<20 Hz) in fin and blue whales (Balaenoptera phijsalus and B. musculus). Toothed whale communication is mainly above 1 kHz, and the echolocation sounds of most species are at very high frequencies, above 20 to 30 kHz.
Figure 1 General types of natural and human-made sounds in the world’s oceans.
The latter sounds are above the frequency range audible to humans (“ultrasonic”). Pinnipeds and sirenians tend to be intermediate between baleen and toothed whales with respect to the frequencies of their calls (mainly at 0.5-10 kHz) and optimum hearing range. The optimum frequencies for pinnipeds and sirenians are similar to those for humans.
While sounds of sirenians, some pinnipeds, and some small odontocetes are weak and audible only within a few tens of meters, most cetacean sounds are much stronger (Richardson et al, 1995). Received sound levels and maximum detection distances depend not only on the strength of the sound at the source (“source level”), but also on the frequency and on physical properties of the environment through which the sound propagates. Nevertheless, smaller dolphins and poipoises can be heard to several hundred meters. Killer whale (Orcinus orca) “screams,” social sounds of pilot whales (Globicephala spp.) and the staccato click sounds of sperm whales (Physetcr macrocephalus) reach to several kilometers. Communication sounds of many odontocetes and mysticetes have source levels of 160 to 180 dB re 1 jj-Pa at 1 m distance. The clicks of bottlenose dolphins and sperm whales can be much more intense, >220 dB in die same units (Au et al, 2000; M0hl et al, 2000). Echolocation clicks of some smaller odontocetes can have similarly high peak levels, but are very brief and do not contain much energy. In a few species of seals, e.g., bearded and Weddell seals (Leptonychotes weddellii), the males appear to use complicated tonal sounds as advertising displays to warn off other males (and possibly to attract females); these sounds can also be quite intense. These seals, which defend underwater territories (or “maritories”), are exceptional among the pinnipeds, however.
A. Potential Types of Noise Effects
Anthropogenic sounds can result in a variety of effects whose consequences to marine mammals can range from nil to severe, depending on the type and received level of the sound:
Tolerance (No Overt Response): Mammals exposed to audible levels of man-made sounds often exhibit no obvious responses, continuing their normal activities without moving away. A marine mammal hearing a sound does not always react overtly to it. However, responses can be subtle so a detailed behavioral and physiological study is needed before it is legitimate to conclude that there is no response. Animals might tolerate noise in order to remain in a preferred location, such as a feeding area, even if the noise caused stress or other inconspicuous effects.
Changes in Behavior or Activity: Alterations in behavior are common when marine mammals are exposed to man-made sounds. Sometimes the effects are subtle, discernible only by detailed observation and statistical analysis, e.g., changes in surfacing/respiration/dive cycles. More conspicuous effects include changes in activity, e.g., from resting or feeding to alert, facing toward the noise source, and so on.
Avoidance Reactions: Upon exposure to strong man-made sounds, marine mammals engaged in feeding, social interactions, or other “normal” activities often interrupt those activities and swim away. When migrating whales approach a noise source, they typically deflect a few degrees off their “normal” course and swim to one side or the other of the noise source.
Masking: Masking is the process whereby sounds of interest to a listener are obscured by interfering sounds. If those sounds are important to the listener (e.g., predator calls), masking could have a serious effect (Richardson et al, 1995).
Hearing Itnpairment: Animals (including humans) exposed to strong sounds can incur a reduction in their hearing sensitivity. This impairment is often temporary, provided the sound levels are not too high or too prolonged. However, repeated exposure to strong sounds, and even a single brief exposure to an extremely strong sound (e.g., nearby explosion), can cause permanent hearing impairment.
Nonauditory Phtjsiological Effects and Stress: In humans, exposure to very strong underwater sounds can cause resonances in lung cavities and other types of nonauditory physiological effects. Chronic exposure to strong noise may also at times cause stress reactions. These phenomena are almost entirely unstudied in marine mammals.
B. Noise Definition, Categories, and Levels
The term “noise” indicates an unintentional or unwanted sound, possibly disagreeable or noxious. We tend to think of noise as sound we do not like, but this is a subjective impression; a sound that is noise to one person (or animal) may be an important signal (or music) to another. For example, the songs of humpback whales (Megaptera novaeangliae) are undoubtedly important to them. We do not know whether these strong and— in some areas—seasonally persistent sounds are regarded as “music” or as noise by pilot whales and bottlenose (Tursiops spp.), spinner (Stenella longirostris), and spotted dolphins (S. attenuata and S. frontalis) that are attempting to communicate in the presence of this background of whale song. However, industrial sounds are likely to be perceived by marine mammals as noise. Nevertheless, some anthropogenic noises may themselves be important cues to mammals: a boat is approaching, take evasive action; a seismic vessel is active, do not get too close, etc. Thus, noise can have an important signaling function. In human terms, it is probably good that we hear an approaching truck or train while we are on foot or on a bicycle nearby.
Sound strength can be measured near die source or at some greater distance. By convention, source levels usually are reported for a standard distance of 1 m. The received sound level tends to diminish witli increasing distance. In interpreting quoted sound levels, it is essential to know whether the value is a source or received level and, for die latter, the distance from the source. The received levels 10, 100, and 1000 m from a source are often about 20, 40, and 50-70 dB less than the corresponding source level. A further complication is that sound levels are measured in many different and sometimes difficult-to-convert units. Decibel (dB) values are meaningless unless the reference level is also specified. This is typically 1 jj-Pa for underwater sound and 20 jj-Pa for airborne sound, but other references are sometimes used, especially in the older literature. Even when the distance from source and the reference level are stated, other complications can make it difficult or impossible to compare acoustical measurements from different studies. For example, simple comparisons of levels in water vs air are usually of doubtful validity.
The levels quoted in this article concern underwater sound measured in dB re 1 jj-Pa and, except for values shown in Fig. 1, are overall broadband levels (i.e., including a wide range of frequencies). In the case of source levels, the levels are standardized to a distance of 1 m.
One way of categorizing anthropogenic noises is whether they are transient or continuous. Transient noises are generated by helicopters or planes passing by, sonar pings, explosions, seismic surveys, and ocean acoustic studies. The source (and received) levels of some of these transient sources can be high (Richardson et al, 1995). However, because these sounds are brief and intermittent, the average sound level over an extended period is lower than the peak level. Transient sounds may have less effect on animal behavior and hearing as compared with continuous sounds having a similar received level.
The strongest transient sounds in the sea come from explosions, which produce shock waves as well as very strong (but brief) underwater sounds. Nominal source levels can be on the order of 270-280 dB—higher than for any other anthropogenic sound. The shock waves from explosions can injure, stun, and even kill nearby fishes, sea turtles, humans, and marine mammals. The distances within which death or injury will occur depend on the size and depth of the explosion, the type of animal, and other factors. Aside from explosions, some of the strongest sources of underwater sound are seismic surveys (typically using air gun arrays to produce low-frequency noise pulses) and military search sonars operating at medium and low frequencies. Air gun arrays and sonars can produce transient sounds with effective source levels of 230 dB or more. Sounds received from these sources diminish rapidly with increasing distance, but often are detectable as much as 100 km away. There are many other types of transient anthropogenic sounds with a wide variety of levels and characteristics.
Continuous sounds are caused by drill ships, dredging, and vessels of all sizes underway. Of these sound producers, large tankers, container ships, and icebreakers working in ice are among the strongest sources. Their source levels are on the order of 200 dB or more. Although not as high as for some transient sources, this level is sustained without gaps. Much of this sound energy is at infrasonic frequencies, which are apparently important to at least some of the baleen whales, but may be irrelevant to dolphins that have little or no hearing sensitivity at those frequencies. Much of the shipping is concentrated along defined shipping lanes, where sound levels will wax and wane as individual ships pass. However, at least in the Northern Hemisphere, the overall din of vessels pervades not only the lanes themselves, but major parts of the ocean basins involved (Fig. 2).
The strongest components of sound from many of the major anthropogenic sources are below 1 kHz, in the same frequency band as is used by most of the baleen whales for their communication calls. For seismic surveys and large ships, most sound energy is below 200 Hz. A recently acknowledged source of strong low-frequency sound is the low-frequency active (LFA) sonar used to detect quiet submarines. The U.S. Navy’s LFA projectors produce a beam of intense sound (source level up to about 240 dB) at frequencies in the 100- to 500-Hz band. However, many of the strongest military sonars operate in the midfrequency (a few kilohertz) range. Likewise, the sounds from outboard engines operating at high speed, snowmobiles traveling on ice, and acoustic harassment devices at maricul-ture facilities are in the low kilohertz range. Sounds at medium and especially at low frequencies tend to propagate for longer distances than sounds at high frequencies (e.g., above 10 kHz). At high frequency, sound is absorbed into seawater, and the absorption rate increases rapidly with increasing frequency. Hence, high-frequency pinger and sonar sounds (including dolphin clicks) generally diminish to low or undetectable levels within 1 km or less and thus do not present potential problems except at close range.
C. Animals of Concern
Because most, if not all, marine mammals rely on underwater sound for various purposes, any strong anthropogenic sounds at relevant frequencies might have an effect. Animals attuned to different sound frequencies will be most affected by different types of sounds. Mid- and high-frequency sounds as produced by small vessels and mid- and high-frequency sonars are likely to affect dolphins, porpoises, pinnipeds, and sirenians. In contrast, large whales are more susceptible to lower frequency sounds such as those from large vessels, drilling operations, and sounds specifically designed to propagate long distances through the water and/or bottom, such as LFA sonar, marine seismic exploration, and ocean tomography (e.g., ATOC).
Although it was once thought that nearshore animals are likely to be most vulnerable because of the concentration of industrial activities in nearshore waters, this is no longer as clear. Marine mammals on the high seas may be affected as well, given the occurrence in deep water of shipping, military operations, acoustic oceanography projects, and (increasingly) oil industry operations. In general, it is probable that mammals of the southern oceans are still affected less often and less intensively by anthropogenic sounds than in the northern oceans, simply because of the preponderance of human occupation, industry, and shipping in the north. However, underwater noise is now everywhere and has the potential to affect all animals that can hear it and that need to communicate through it.
II. Effects on Hearing
Most, if not all, marine mammals have evolved special adaptations to hear well underwater (Au et al, 2000). However, the adaptations that make their ears sensitive to pressure fluctuations in the water may also increase the risk of damage from exposure to strong waterborne sound and shock waves.
The most drastic effects come from shock waves, which can damage not only the ear but also other organs, in extreme cases causing death. Explosions produce mechanical and pressure trauma effects that can cause profound physical injury or direct death of marine mammals. It has also been suggested that some deaths of beaked whales (Ziphiidae) in the Mediterranean Sea, the Bahamas, and elsewhere were attributable to strong noise from sonar trials. The evidence is circumstantial and inconclusive for the Mediterranean. Preliminary accounts of the Bahamas incident indicate that sonar sounds may have caused injury to the auditory system and associated structures, although not enough to kill the animals directly.
Sounds that are not strong enough to cause profound physical injury or outright death may nonetheless impair hearing, resulting in either temporary or permanent threshold shifts (TTS and PTS, respectively). TTS occurs in humans and animals subjected to intense sounds for short periods of time or to less intense sounds for longer times. The temporary loss of hearing acuity by humans listening to a rock concert is the classic example. Mild TTS has been demonstrated in bottlenose dolphins (Tursiops truncatus) and beluga whales (Delphi-napteras leucas) exposed to a single 1-sec pulse of strong sound (192-201 dB) and in dolphins and various pinnipeds exposed to lower sound levels for 20-50 min (Kastak et al., 1999; Schlundt et al., 2000). Hearing thresholds returned to preexposure levels within about 12 hr, and these brief exposures caused no long-term hearing impairment.
Figure 2 Major international shipping lanes in North American waters.
Temporary hearing impairment could be deleterious to animals that rely strongly on their abilities to detect sound. In addition, data on the sound levels and durations at which TTS begins are useful in identifying situations when there is concern about permanent hearing damage (PTS). However, it is not known how much additional exposure (higher levels and/or longer durations) would be necessary before PTS would occur. In general, permanent damage typically results from a loss of sensory hair cells within the inner ear. The loss is most pronounced in the part of the inner ear responsible for detecting sound at the frequency of the injurious stimulus. In terrestrial mammals, lost hair cells do not grow back, and this is likely the case for marine mammals as well. (Interestingly, at least partial regeneration occurs in birds and fishes.) Older bottlenose dolphins in captivity are known to have reduced hearing sensitivity (especially at the higher frequencies), presumably at least in part because of the cumulative effects of sound (Ridgway and Carder, 1997). This loss is similar to the progressive hearing loss in humans, or presbycusis.
The initial TTS results from small odontocetes confirm that sound levels necessaiy to cause TTS are correlated with the duration of exposure. More data are needed to quantify this relationship and to determine the TTS thresholds for repeated sounds such as seismic and sonar pulses. Sound levels necessary to cause TTS and PTS in baleen whales and sirenians are entirely unknown.
III. Nonauditory Physiological Effects
Even less is known about potential effects of strong noises on nonhealing physiology than on hearing. Human and animal studies show that strong sounds can affect the vestibular system (and thus sense of balance), air sinuses and adjacent tissues. neural transmission (skin tingling in divers), and reproductive functions. Studies of some terrestrial animals exposed to strong noise have shown reduced sperm production, menstrual irregularities, abortions, and stillbirths. Most of these drastic effects come from intensive low-frequency sounds and attendant physical vibrations of body tissues. It is not known whether the low-frequency underwater sounds of ATOC, LFA, or seismic exploration could elicit similar effects in marine mammals. If so, these effects would probably be limited to animals close to the sound source.
It is possible that strong sound could cause bubbles to form in blood or tissues, and that animals might succumb to a lodging of these bubbles in the brain and elsewhere (Crum and Mao, 1996). This situation is analogous to bubble formation in human divers when they return too rapidly from depth—the condition known as the bends.
Stress is one possible outcome of exposure to sounds that are disturbing to animals. Stress could, in theory, inhibit normal social interactions, feeding, reproduction, longevity, and a host of other important functions (Curry, 1999). However, noise-induced stress is not well understood even in humans and terrestrial mammals, and there are essentially no data on noise-induced stress in marine mammals. Research is needed.
IV. Effects on Behavior
A. General
Short-term behavioral and avoidance reactions to noise have been studied more than damage to hearing and other physiological effects, to the point that there are now at least some data on noise-induced disturbance in every group of marine mammals. For most marine mammals, it is relatively easy to observe some aspects of their distribution and behavior, as they need to come to the surface to breathe. Nevertheless, much of their time is spent below the waves, and our brief glimpses of them at the surface can present a biased view. Also, short-term reactions (or lack thereof) are not necessarily a good indicator of long-term effects, and the latter have rarely been studied directly. This section does not attempt to review the numerous studies on behavioral and distributional responses to underwater noise; for more details, see Richardson et al. (1995).
Mammals often appear to become alert to novel sounds when they are of low intensity, but this alertness quickly wanes if there is no danger connected with the sound. Thus, the droning of ships or drilling platforms in the distance often appears to be ignored due to habituation or sensory adaptation. Some mammals may avoid the immediate area around the noise source, but remain within the area where the sounds are at least faintly audible. When sounds are tolerated, it is possible (though unproven) that they may elicit stress or other unseen physiological reactions.
Marine mammals often react more dramatically when received sound levels are high (indicative of a strong or nearby source) or when they are increasing (indicative of an approaching source). Bowhead whales (Balaena mysticetus) migrating toward a drill ship or a marine seismic operation typically deflect their course so that their closest point of approach as they pass the noise source is at least 20 km away. At that distance, the received level of the strongest air gun pulses was near 130 dB, averaged over pulse duration. Migrating gray whales (Eschrichtius robustus) show similar deflections of their migration route as they approach a simulated seismic operations or LFA sonar, but they seem to tolerate higher sound levels than bowheads. These deflections by bowhead and gray whales are not a sudden fright reaction, but an edging away— analogous to a person walking along a sidewalk, seeing a disturbance ahead, and crossing to the sidewalk on the other side of the road. Also, at least in bowheads, there is a decrease in durations of surfacings and dives, and number of breaths per surfacing. In other words, the whales cycle through their surfacing/dive repertoire more rapidly. There must be at least subtle commensurate changes in activities such as feeding, socializing, or rest.
Cetacean responses to noise are quite variable, depending on the circumstances of exposure and the activities of the animals at the time. There are anecdotal reports from Heard Island in the Southern Ocean, and from the Gulf of Mexico, that sperm whales are sometimes sensitive to air gun pulses at even greater distances than bowheads. In contrast, observers on seismic vessels near the United Kingdom found that sperm whales did not show strong avoidance when the air guns were operating. The beluga whale is another example of a species whose responses to anthropogenic noise are highly variable. Belugas sometimes tolerate exposure to large fleets of fishing vessels, but in other circumstances flee when exposed to faint sounds from approaching ships 35-50 km away (Finley et al., 1990).
A noise that is associated with danger, such as a catcher boat of a whaling fleet or a purse-seine vessel that encircles dolphins to catch the tuna underneath, can trigger strong avoidance reactions at distances of 10 km or more. Reactions may become stronger upon repeated exposure to aversive stimuli; in this case, “sensitization” is said to occur. At least some dolphins seem to distinguish vessels based on their sounds and react differently to boats that habitually harass the animals (such as aggressive tour operators or researchers who tag or collect biopsy samples) vs boats that approach slowly and carefully. In the latter case, the marine mammals may have learned that the vessel is not harmful—a case of behavioral adaptation.
In contrast to the extreme examples just cited, some marine mammals tolerate exposure to high levels of sound. Seals and sea lions attracted to locations where prey fish are concentrated often tolerate exposure to high levels of noise from acoustic harassment devices designed to disperse pinnipeds. Also, during the Heard Island study, hourglass dolphins (Lagenorhijnchus cruciger) approached to within several hundred meters of a powerful source of low-frequency sound (57 Hz; source level 209-220 dB), whereas pilot (G. melas), southern bottlenose (Hyperoodon planifrons), and minke whales (Balaenoptera sp.) were seen less often during sound transmissions than during silent periods (Bowles et al., 1994). The hearing systems of the small hourglass dolphins probably were not very sensitive to low-frequency sounds; they may have been curious about but not discomfited by the noise. However, it is also noteworthy that, when visible near the surface, they would not have been exposed to levels much above 160 dB given the nature of the sound field around this particular sound source. It is possible that they exhibited a vertical avoidance response, staying near the surface simply because the noise was less strong there.
Marine mammals sometimes approach and tolerate a sound source for their own benefit. Bottlenose dolphins that feed on prey stirred up by shrimp fishers know the sounds of all aspects of the trawling and net-lifting operations; they move from vessel to vessel according to stage of trawling, even from several kilometers away Killer whales and several pinniped species of the northeast Pacific similarly react to net lifting like the gong of a dinner bell. Many species of dolphins and some species of sea lions and fur seals approach vessels to bow ride or wake ride near the vessel, apparently “for fun.”
While this discussion centers mainly on underwater sounds, in-air noises can affect pinnipeds on land, occasionally with serious consequences. A low-flying aircraft, the sudden honk of an automobile horn, or the noisy approach of humans on foot can cause animals on land to stampede into the water. If this occurs on a birthing/nursing beach, adults sometimes trample pups in their msh to escape the perceived danger; however, it is not always clear whether sound or sight (and. at times, substrate vibration) is more responsible for the stampede and loss of pups.
B. Communication
Because sound is such an important sensory modality for almost all marine mammals, it stands to reason that noise has the potential to disrupt the efficiency of communication (and echolocation). Indeed, there is some evidence for this. Some animals fall silent when they perceive danger (or a novel sound). During the aforementioned Heard Island study, sperm and pilot whales were heard 24% and 8% (respectively) of 1137 min when there was no sound, but none were heard to vocalize during 2202 min with sound transmissions. Although these data suggest a strong effect of noise on communication, they are not yet sufficient to indicate at what received levels the whales became quiet (Bowles et al, 1994).
Acoustic reactions to intense sounds vary among species and with the activity of the animals when they perceived the sound. Belugas are vocally active when they detect ship sounds, but narwhals fall silent. Dusky dolphins (Lagenorhijnchus ob-scurus) resting in small groups fall silent when they perceive danger. However, those same dolphins will continue their social whistling and burst-pulse “chatter” when they are engaged in high-energy social and sexual activities. They seem less easily disturbed at these higher energy times.
Masking of sounds useful to animals by stronger anthropogenic noise is another important phenomenon. We humans have the ability to converse with each other even at noisy parties and can separate useful speech from an amazing background din (the well-known “cocktail party” phenomenon). Marine mammals can probably do likewise, for they have evolved in a world where physical and biological sounds other than their own abound. Nevertheless, studies of captive dolphins and pinnipeds have confirmed that, in marine as well as terrestrial mammals, strong man-made noise can physically “drown out” other sounds at similar frequencies. The noise reduces the maximum distance at which one animal can hear calls from another animal or some other environmental sound that may be important (Richardson et al, 1995). There are indications (not fully proven) that gray whales, belugas, and bottlenose dolphins sometimes shift the primary frequencies of their sounds to reduce overlap with the frequencies of back ground noise. If so, this suggests that some cetaceans may be able to reduce the negative effects of noise masking, but this suggestion needs further study.
C. Social Structure
Marine mammals are generally very social. If noise disrupts social structure, and if the effect is sufficiently strong and long-lasting, then it could be detrimental to the well-being and survival of individuals and perhaps ultimately populations. Noise disturbance can cause some degree of social disruption, but the consequences are largely unknown. When a supply ship comes within 2-A km of a group of bowhead whales, the whales scatter in all directions. They are “socially disrupted” for as much as a few hours. However, we have no idea how important this disruption may be to the well-being and productivity of the whales. We can guess that they will be harmed if this kind of disruption happens day after day to animals that feed more efficiently when together than when apart. Social disruption also includes the accidental separation of a nursing mother from her pup or calf. When this happens, the probability of pup or calf survival may be reduced substantially.
We do not know to what extent the masking of communication sounds by noise can cause social dismption. Blue whales and fin whales can sometimes hear each other over distances of several hundred kilometers. If there are important social interactions over those long distances (not proven), these interactions may be impeded by the masking effects of background noise, especially if that noise is continuous. For example, these whales may not be able to find each other as efficiently for feeding and mating purposes given the masking effect of the inaninade noise. If whale societies are mediated in large part by acoustic contact, as we suspect (Wells et al, 1999; Tyack, 2000), then background noise that diminishes the distances over which whales can communicate also diminishes the spatial scale of their societies.
D. Habitat Use
While short-term disruption of behavior by noise is known to occur, it is less clear how this may translate to overall use of habitat and to long-term disturbance. It was mentioned earlier that migrating bowhead whales “edge away” from seismic exploration and drill ships, in most cases maintaining a minimum distance of about 20 km. It is unlikely that this results in any major harm to the animals. However, if there were a similar and prolonged displacement from a localized area of important feeding habitat, then the potential for harm would be great. At this point, we know that some whales move away from sources of strong noise, but we do not know whether this significantly reduces their yearly food intake or causes other important disruption.
Similarly, human disturbance of island spinner dolphins has affected their use of nearshore bays that appear to be important to them for daytime rest (after feeding in deep water at night). It is possible to argue that this reduced use of safe havens might impact their survivability as individuals and therefore as populations. However, to date, we have no information on such a population-wide effect.
The concept of habitat available vs habitat disturbed is an important one in conservation biology. The purist would argue that any habitat disrupted is too much, and compromises the natural world. The pragmatist argues that we humans are so overpoweringly disruptive that we must be content with keeping at least a minimum of area available for the animals in question to survive. In the case of marine mammals, that means keeping some critical habitats relatively free of strong noises that create unacceptable disruption in short-term behavior, long-term behavior, aspects of physiology, and hearing. The task is a difficult one and needs further research, monitoring, mitigation, and political enforcement to have a chance of success.
V. Research and Potential Mitigation
The first step in mitigation is to develop a better knowledge base. Detailed discussions oi research needs can be found in NRC (1994, 2000) and Richardson et al (1995). We need better information on auditory sensitivity, especially of large whales, and the levels and frequencies of noises that cause TTS, PTS, and nonhealing physiological disruptions in all marine mammals. Researchers have acquired the technical capability to study noise-induced stress and other aspects of physiology, with miniature data loggers and telemetry transmitters attached to animals under field conditions. This is a relatively new avenue of research for marine animals and is likely to grow in the near future. Studies of short-term behavioral disruptions are worthwhile, but it is long-term behavioral, social, communication, and habitat disruption that are of greatest need of study. It is our challenge to monitor the health of marine mammals as related not only to anthropogenic sound pollution, but also to other aspects of habitat degradation.
Potential mitigation avenues are many. Naval vessels are already engineered to reduce sound emissions, e.g., by physical decoupling of rotating machinery from the hull, propeller design. and emission of screens of air bubbles. This knowledge needs to be taken into the private sector, which is responsible for the vast majority of shipping noise. Bubble screening has been shown to strongly decrease the noise from a stationary pile-driving activity in Hong Kong waters, and this technique could be effective in dampening industrial sounds in many other situations (Wiirsig et al, 2000). A controversial technique that is often employed is “ramping up” of sounds in order to alert animals of impending strong noise, essentially to chase them away from the zone of most influence. This approach probably is effective for some species and situations (e.g., baleen whales vs airguns). However, in other situations it may attract curious marine mammals into a zone of danger. Further study is needed to determine the situations when ramping up is a useful mitigation technique.
In projects where sound is emitted purposefully, e.g.. marine seismic exploration, ocean science studies, and navy LFA sonar, refinements in equipment and operational procedures may be possible that will reduce the exposure of marine mammals to noise. Some efforts have already been made to reduce source levels to the minimum that will be effective and to improve beam forming so as to reduce sound radiation in unnecessary directions. Also, these (and some other) noisy activities are amenable to regulation as to where and when they are used, especially as pertains to the seasonal migration of sensitive baleen whales. It is common for noisy projects to be restricted to certain times of year and/or to certain areas in order to reduce impacts on marine mammals.
It is up to scientists to help provide badly needed information on disturbance reactions and zones of potential influence, and up to politicians and regulators to write and enforce legislation to curb the uncontrolled proliferation of ocean noise. Combined efforts by all will be needed in order to provide adequate protection for marine mammals while avoiding unnecessary restrictions on worthwhile human activities.
