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--> 3 Topics for Future Research As discussed earlier in this report, data on the effects of low-frequency sounds on marine mammals are scarce. Although we do have some knowledge about the behavior and reactions of certain marine mammals in response to sound, as well as about the hearing capabilities of a few odontocete and pinniped species, the data are extremely limited and cannot constitute the basis for informed prediction or evaluation of the effects of intense low-frequency sounds on any marine species. In this chapter the committee identifies several areas in which more research is needed in order to provide a better understanding of the effects of low-frequency sounds on marine mammals and their prey. The next two major sections—Behavior of Marine Mammals in the Wild, and Structure and Function of the Auditory System—describe several of the proposed studies that focus on the acoustic behavior, disturbance responses, and hearing of marine mammals. Marine mammals are not the only marine species that may be affected by intense low-frequency sounds. Although other marine species are outside the direct charge of this committee, many are part of the food chain for marine mammals. If low-frequency sounds change the behavior of, or damage, organisms in the food chain of marine mammals, they may significantly alter the ability of marine mammals to survive, as discussed in the third section of this chapter, Effects of Low-frequency Sound on the Food Chain. The final section, on De-
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--> velopment and Applications of Measurement Techniques, describes a number of tools that need to be developed so the basic data that are needed can be obtained. The intent here is to identify general research needs that are crucial to a full evaluation of the effects of intense low-frequency sounds on a variety of marine mammals and their prey; accordingly, specific experiments are not described in detail. The order of presentation of topics in the following sections is not meant to suggest a hierarchy of importance. Indeed, any such hierarchy depends upon the goals of the policy makers. For example, if the overriding concerns were with potential damage to the auditory systems of species by low-frequency sound, the most important studies might be those described under the heading Temporary Threshold Shift in the section on the Auditory System. However, if the overriding concern is to identify the risks posed for survival of a specific species by disruption of its ecological niche, research of the sort described in the section Effects of Low-frequency Sound on the Food Chain would have high priority. This committee believes that all of the recommendations described below need to be addressed in order to find out how low-frequency sounds affect marine mammals. Behavior of Marine Mammals in the Wild Aims: To determine the normal behaviors of marine mammals in the wild and their behavioral responses to human-made acoustic signals. Rationale: There is a substantial lack of knowledge about the normal behaviors of most marine mammals, the role of natural sounds in their lives, and their responses to human-made acoustic stimuli. The primary reason for this lack of knowledge is that marine mammals spend much of their time below the water surface and often at depths that have been, and commonly still are, generally inaccessible to humans. Still, it is crucial to acquire appropriate baseline data on various behavioral dimensions so that any future behavioral changes can be evaluated adequately. The purpose of the proposed studies is to document the normal behaviors of marine mammals and their responses to natural and human-made stimuli. In the four subsections that follow, the committee suggests several studies employing underwater sound playback and related simulation techniques as methods for testing the reactions of some marine mammals to human-made noise. Observations of reactions to actual hu-
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--> man activities are also very desirable. However, playbacks or other simulations are often more practical and have some advantages. For example, (1) It is uncommon for marine mammalogists to have full control over the location and timing of expensive operations such as large ships, seismic vessels, icebreakers, and so forth, especially over a sufficiently long period to obtain meaningful sample sizes. Hence, observations of marine mammals near such operations are usually opportunistic and/or few in number. (2) Although observations near full-scale human activities may be more realistic than those near simulated activities (e.g., playbacks), their very realism can lead to interpretation problems, such as an inability to distinguish acoustic from visual effects. Ideally, controlled or opportune observations near actual human activities would be combined with controlled simulation experiments to take advantage of the complementary advantages and disadvantages of the two approaches (e.g., Richardson et al., 1986). Natural, Ecologically Important Signals Aims: To determine how marine mammals utilize natural sounds for communication and for maintaining their normal behavioral repertoire. It is well known that many marine mammal species utilize sounds for signaling other members of the same species (intraspecific behaviors) (reviewed in Richardson et al., 1991). For example, mysticetes and some pinnipeds (e.g., Weddell seals) generate and utilize sounds in the normal course of mating and territorial behaviors. Most of these sounds are typically at low to moderate frequencies (tens to thousands of hertz). Odontocetes not only use moderate-frequency sounds for social communication, but they also use higher frequencies (ultrasonic by human standards) for echolocation to detect the presence of objects in their environment. There is also evidence that marine mammals utilize sounds for interspecific communication: for example, for detecting predators such as killer whales. Although there has been extensive research on utilization of sound by captive and coastal animals, less has been done with open-ocean species in the wild. Research on acoustic communication in wild marine mammals has been hindered by problems in following animals and in identifying when a particular animal is producing a sound. Considerable additional work on the ecological significance of natural sounds is essential to any prediction, or evaluation, of stimuli that might disrupt a wide range of behaviors, including those that are sound-dependent. For example, the functions of the low-frequency
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--> sounds made by mysticetes and the ranges over which they are used are too poorly understood to infer the biological impact if these signals were masked by noise. Further, once predictable responses to natural acoustic stimuli are identified (e.g., the attraction of animals of the same species by mating calls), this information could be used in designing experiments to determine how other sounds might mask or otherwise change a species's perception of intraspecific signaling. Investigations on natural sounds of marine mammals might capitalize on existing hydrophone arrays (e.g., the integrated undersea surveillance discussed in the section on Measurement Techniques below), or utilize towed or site-specific arrays. Hydrophone arrays have advantages over single hydrophones: arrays allow localization of the source or sources of calls by triangulation techniques, thereby allowing the matching of received calls with particular animals, their locations, and human activities. Acoustic monitoring should be supplemented, wherever possible, with visual and electronic monitoring so that the behavioral responses to natural or human-made sounds or their interruption can be learned. The ongoing development of increasingly sophisticated tags should be of considerable benefit to this work (see subsection on Tag Development under Measurement Techniques). Accordingly, tag technology deserves increased support by the relevant funding agencies such as the Office of Naval Research, the National Marine Fisheries Service, and the Minerals Management Service. Habituation to Repeated Human-made Sounds Aims: To determine the responses of free-ranging marine mammals to human-made acoustic stimuli including repeated exposure of the same individuals. How is the use of natural sounds altered by the presence of human-made sounds? In addition to the need for research aimed at correlating specific behaviors with passively monitored natural sounds that are produced or used by marine mammals, there is a need for studies in which human-made sounds of various types are presented repeatedly to marine mammals while the responses of the animals are monitored. At present, only very limited data exist on whether sounds having different levels, spectra, or temporal patterns will alter behavior in different ways, and if so, in what species. There are virtually no data on the reactions to repeated exposure to the same sound: that is, do the animals habituate to the sounds? Habituation is the phenomenon of progressive waning of behavioral responsiveness to repeated re-
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--> ception of a stimulus not accompanied by any perceived deleterious effect. Habituation has been observed in many types of animals including marine mammals. Responses of marine mammals to some human-made noises may diminish as the animals learn that these sounds are innocuous. To date, most tests of the reactions of marine mammals to human-made noises have examined only the initial response on first presentation of the noise stimulus. Thus, these results may overestimate the effects of the noises over the long-term (that is, the animal may habituate to the sounds). Clearly, initial responses of marine mammals to sounds may not be indicative of long-term effects. Such studies need to be done over extended periods of time with the same groups of animals. Where appropriate, tagging should be employed to monitor the behavior of animals before, during, and after ensonification with artificial sounds (including simulations of real-world sounds such as supertankers) and to monitor sound levels adjacent to the animals. Comparison of the responses of known individuals to successive exposures will provide the most important data in these experiments. In effect, each individual will (in part) serve as its own control. Additional control data of two types should also be obtained, however. (1) Data should be collected in the test area in the year prior to the playback test using the same type of instrumentation that will be used during the playback period (temporal control). (2) Data should also be collected on mammals in a similar control area during both the preplayback and the playback period but without use of the sound stimulation (simultaneous control). Such a design would help eliminate the potentially confounding effects of natural temporal changes and of the presence of observers or tags. Through the use of telemetry or other tags, investigators will be able to obtain more detailed data on dive history, location, and aspects of animal physiology in response to low-frequency sound signals. The same instrumented individuals should be exposed to the sound on a number of occasions. Such experiments might use a local population of marine mammals engaged in feeding or breeding activity. The committee suggests using sound sources capable of producing sounds of various sorts, including sounds of high intensity and low frequency. A test area of about 10 km square could be ensonified to an intensity level of 126 dB (re 1 μPa—water standard) or greater. Observers on one or more vessels or on shore could observe the animals intermittently. Telemetry receivers on land, in air, or at sea could receive, via radio link, data from tagged animals including ideally the received levels of both the ambient noise and
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--> test signals. This type of study might be possible at a location where a suitable acoustic source is to be installed and operated for an acoustical oceanography experiment, thereby making dual use of some logistical arrangements. The approach described above offers several advantages. (1) Unambiguous measurements could be obtained of the sound-pressure levels received by individual animals. (2) Data could be obtained from several animals over an extended period of time, thereby improving the chances of obtaining statistically reliable effects. (3) Changes in responsiveness over time could be determined (i.e., habituation could be assessed). Differential Responses of Migrating Whales to Various Human-made Sounds Aims: To determine how different sound types and levels affect migration and other movement patterns of marine mammals. Research with gray whales off the coast of central California has demonstrated statistically that received sound levels of 120 dB (re 1 μPa—water standard) can alter paths of migrating whales (Malme et al., 1983, 1984). The paths of these whales are so regular and the numbers of whales so large that it was possible to measure minor course deflections induced by playback of noise stimuli. Such studies suggest that sounds can alter, if only temporarily, the behavior of this species. However, to date, only one impulsive stimulus and five more continuous stimuli have been tested on these animals. This type of experimental method is useful because it provides the opportunity to determine the relative reactions of large samples of baleen whale species to a variety of signals. The work can be done with existing technology, with a high probability of success, and with modest logistical support. Tests of this sort performed with tonal stimuli also have the potential to provide important indirect information about the audiograms in these animals, including data on minimum auditory sensitivity at low frequencies. Further work with additional stimuli and species would reveal whether the deflections observed in migrating gray whales also occur with sounds having spectral characteristics, durations, and duty cycles other than those already tested. Comparisons should be made between the effects observed with stationary and moving sound sources, and between single and multiple, simultaneous sources.
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--> Responses of Deep-diving Marine Mammals to Low-frequency Sounds Aims: To determine the responses of deep-diving marine mammals to low-frequency sounds whose characteristics (source level, frequency, bandwidth, duty cycle) duplicate or approximate those produced by acoustic oceanographers. Sound sources used in acoustic systems designed to monitor ocean warming or for various other ocean acoustical purposes are often placed in the deep sound channel. This channel lies about 700 to 1,500 m below the surface at tropical and temperate latitudes. It is known, or believed, that sperm whales, beaked whales, and elephant seals are capable of diving to depths of 1,000 m or below, and that white whales, pilot whales, bottlenose dolphins, Weddell seals, and other species can dive to depths of at least several hundred meters. If intense sounds were present during these dives, the animals would encounter large local variations in sound levels that might alter their behavior during diving and feeding. At any given range from the sound source, those species that dive deep enough to enter the sound channel would be exposed to the highest sound levels. Since some of these species may be highly dependent on food obtainable only at great depths, they may be placed at considerable risk by intense low-frequency sounds. At polar latitudes, where the sound channel is often within 200 m of the surface, most species might be influenced by sounds concentrated in the near-surface sound channel. Any changes induced in the behavior of the food species in response to intense sound could also adversely affect marine mammals. In order to test these possibilities, it would be necessary to deploy one or more sound projectors into the sound channel (when at an accessible depth) in an area where the target species (marine mammals and prey species) occur, and to document the reactions of those animals to the sounds. The projector would probably need to be deployed from a vessel so that it could be moved to locations near tagged animals equipped with data loggers. Behavior should be documented before, during, and after exposure, and the degree of behavioral response should be examined in relation to sound exposure level. It would be desirable to project low-frequency sounds comparable in level and other characteristics to those used by acoustical oceanographers. These sounds should be projected near the depth where operational systems are deployed. This type of study might be done in conjunction with the use of an operational low-frequency acoustic system.
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--> Behavioral data should be obtained for periods while the animals are visible at the surface and while at depth, using instrumentation attached to the animals. From records of position in three dimensions, the positions and tracks of the animals relative to the sound source could be determined before, during, and after exposure to low-frequency sound. Ideally, the recording/telemetry system would also have the ability to determine the sound levels received by each animal under observation. This type of field study might be most easily implemented on deep-diving pinnipeds such as elephant seals, which are readily tagged when they are out of the water. The ongoing study of sperm whales in deep water close to Dominica (West Indies) supported by the Office of Naval Research already includes many of the elements mentioned above. It appears to have the potential to provide the desired data, but it needs the development of improved recorder/telemetry systems. Structure and Function of the Auditory System Aims: To determine the structure and capabilities of the auditory system in marine mammals. Rationale: To develop an understanding of how human-made signals can affect the behavior of marine mammals, an extensive body of data is needed regarding the basic mechanisms of hearing in these species. Such data are required before informed speculation is possible about whether low-frequency sounds can alter or damage hearing abilities and/or impair the ability of animals to communicate with one another. Basic Studies of Audiometry Aim: To determine basic hearing capabilities of various species of marine mammals. Audiometric functions show the weakest sounds that an animal can hear over the frequency range to which it is sensitive. Although audiometric data do exist for some pinnipeds and several odontocetes (see Figure 2 in Chapter 1), there are very few data on their sensitivity at low frequencies, and there has been little replication within or across species (see subsection on Replication of Data, below in this section). There are no data on hearing abilities of marine mammals as a function of depth—an especially significant question in the case of species that dive to great depths. Also, no data are available on
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--> the absolute auditory thresholds of any baleen whale. Still, there is a variety of indirect evidence suggesting that baleen whales are sensitive to low-frequency sounds. For example, large baleen whales produce very low frequency sounds that, in some cases, have been associated with behavioral observations suggesting that the whales can hear their own sounds (Cummings and Thompson, 1971; Clark and Clark, 1980; Thompson et al., 1986). Two problems with obtaining behavioral data on baleen whales are their size and the attendant difficulty (or impossibility) of working with them in controlled situations. Thus, for the foreseeable future, the only reasonable way to obtain data on these large animals is through use of the evoked potential technique, as described in the next section. For all species, sensitivity data eventually must be coupled with more detailed studies of masking and critical-band estimates to allow predictions of the minimum signal-to-noise ratios detectable by these animals with particular signals and ambient noise conditions, including low-frequency data. These data, plus knowledge of ocean sound propagation and ambient noise conditions, would allow prediction of the maximum distance at which a given signal might be detectable by an animal. Measurements on Ensnared or Beached Marine Mammals Aim: To determine hearing capabilities of larger marine mammals that are not amenable to laboratory study. As noted above, very little is known about the hearing and auditory systems of large cetaceans such as baleen and sperm whales. Special opportunities exist to obtain some of the necessary information about the hearing of these cetaceans through studies of ensnared or beached animals using a Stranded Whale Auditory Test (SWAT) team, as discussed in the section below on Measurement Techniques. Although it is possible that the beached animal may be ill or damaged, the auditory system may still be normal. Strandings, beachings, or entrapment of large cetaceans may be due to many different causes (Geraci, 1978; Klinowska, 1986; Geraci and Lounsbury, 1993) in addition to illness. Such causes as shipping vessel strikes, fishing gear entrapments, ice entrapments, isolation or orphaning of individuals, and numerous other factors that lead to a whale being marooned need not have any major effect on the animal's auditory system. Thus, it may be possible to obtain valuable data on hearing capabilities of cetaceans in these circumstances, and often this may be the only way to obtain such data.
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--> Measurements of hearing sensitivity, temporary threshold shift, or masked threshold obtained on one individual animal characterize only that animal and may not be good estimates for the species as a whole (see subsection on Replication of Data, below). Because the animal may have a temporary or permanent hearing deficit, it would be necessary to study several individuals of each species of interest. Regarding beached or ensnared animals, there is no reason to believe that stress, injury, or disease will significantly affect the auditory system in the majority of individuals (unless, of course, the stranding is related to a hearing defect). Auditory evoked potentials (AEP) can provide objective information about the peripheral auditory system (Bullock and Ridgway, 1972; Davis and Hirsh, 1976; Elberling and Don, 1987). Many features of the AEP, and especially the short-latency waves, are reasonably consistent across species (Allen and Starr, 1978) including cetaceans (Ridgway et al., 1981). Some components are unaffected by level of consciousness and do not require a behavioral response (Picton et al., 1974; Ridgway et al., 1981). AEPs have been used in humans to assess hearing in sleeping infants (Calloway, 1975; National Research Council, 1987) and to determine brain damage or brain death (Starr and Achor, 1975; Anon, 1968). AEP auditory information on just a few whales may provide information of critical importance in assessing potential effects of human-made low-frequency sound on mammals that are impractical to study under controlled conditions. Dozens of individuals of several species of large whales become ensnared in fishing gear each year. For example, as many as 65 large baleen whales are ensnared off Newfoundland each June/July season (Beamish, 1973; Lien, in press). Several hours are required for people to release these animals. During the release process, it may be possible to equip the whales with electrophysiological monitoring apparatus and to obtain auditory measurements using evoked-response procedures. Major difficulties with all such work are the background noise present (here due to the release process) and adequate control of the stimulus. Awbrey et al. (1988) and Johnson et al. (1989) have shown that adequate stimuli can be presented to the whale with speakers in air and background noise can be monitored and masked. Given the current dearth of available information, and the great difficulty in working with large whales, valuable data should be obtainable from such studies. In addition, live whales sometimes beach themselves or become isolated in tidal basins or on ice (Geraci, 1978; Lien and Stenson, 1989). On occasion, neonates or other young, apparently healthy,
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--> marine mammals of species not normally held in captivity are brought to tanks for temporary care. Replication of Data Aims: To determine audiometric data on multiple animals in order to understand intraspecific variance in hearing capabilities. Hearing capabilities of terrestrial mammals generally vary among individuals of a single species, and as a consequence, measurements are generally made on several individuals in order to determine the mean and variance for a species. There is no reason to expect marine mammals to differ in this regard. Data on several individuals are necessary before it will be possible to ascertain whether, for example, measurements made after acoustic trauma are a consequence of that stimulation or whether they simply reflect normal variance within a species. Although data are available for several different measures of underwater hearing by odontocete cetaceans and pinnipeds (Terhune and Ronald, 1972, 1975; Thomas et al., 1988, 1990; Moore and Schusterman, 1987), most studies have dealt with only one or two individual animals (but see Johnson  and Seeley et al.  for Tursiops; White et al. , Awbrey et al. , and Johnson et al.  for white whale; and Ridgway and Joyce  for the gray seal). Thus, few data exist on intraspecific variability in hearing capabilities of marine mammals. The need for data from multiple specimens is further supported by the results from two frequency discrimination studies that used different individual Tursiops (Jacobs, 1972; Thompson and Herman, 1975). The results from these two studies are quite different, possibly reflecting intraspecific variability in hearing capabilities in the species. Temporary Threshold Shift Aim: Determine sound-pressure levels that produce temporary and permanent hearing loss in marine mammals. At this time, essentially nothing is known about the auditory aftereffects of exposure to intense sound in marine mammals, fish, or invertebrates. In land mammals, short exposures to intense sounds (greater than about 90 dB re 20 μPa—air standard) can produce a temporary hearing loss that recovers to normal within minutes, hours, or days, depending on the magnitude of the exposure and on the individual.
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--> Basic Studies of the Anatomy and Physiology of the Auditory System Aims: To determine morphology and sound conduction paths of the auditory system in various marine mammals. Very little is known about hearing mechanisms in marine mammals (see Popper, 1980; Ketten, 1991). Although it is known that the overall structure of the middle and inner ears is similar to that of terrestrial mammals, there are only very limited data on the detailed structure of the cochlear and more peripheral auditory structures. One set of questions concerns how sound actually gets to the inner ear, and whether the pathways differ for different frequencies and/or in various marine mammal groups. The mechanism of stimulation of the inner ear is still controversial, and it is not clear whether the ear is stimulated by normal conduction mechanisms (via the middle ear), by bone conduction, or by some other method. Future studies need to be directed at these and other basic questions on the function of the peripheral auditory system in marine mammals, including basic analysis of the structure of the auditory apparatus in a wide variety of marine mammals. The ideal way to answer these questions would be through studies on captive specimens, but this has not been feasible with large cetaceans. Thus, studies on larger species, and especially mysticetes, might use the SWAT team approach (see subsection SWAT Team in the section on Measurement Techniques) whereby investigators, using an already prepared setup, go to the sites of strandings and record acoustic responses from entrapped animals with the specific purpose of trying to ascertain the function of specific parts of the auditory system (see subsection on Ensnared or Beached Marine Mammals, above in this section). Clearly, the paucity of data on the auditory system of any large marine mammal is so severe that data obtained using ensnared or beached animals would provide at least an initial estimate of these animals' hearing capabilities. Through repeated studies of this type, it should be possible to get a close approximation of hearing range and sensitivity of several different species. In addition, it may be possible to obtain valuable data in this way that correlate hearing loss in marine mammals with ear pathology. In particular, stranded animals that die should be used as a source of ear tissue for use in anatomical studies. Analysis of ear anatomy from these specimens would help to determine whether the hearing ability of a specimen was normal. If the tissue suggested ear pathology, it should be ultimately possible to correlate various hearing deficits with various pathologies. Of course, this would require a sufficient
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--> ''library'' of data on hearing sensitivity in a species and data on ear pathology of the same animals. Effects of Low-Frequency Sounds on the Food Chain Aim: To determine whether low-frequency sounds affect the behavior and physiology of organisms that serve as part of the food chain for marine mammals. Rationale: Almost all marine mammals are predators feeding on fishes, squids, crustaceans, and other animals, and their continued survival depends on the survival and abundance of this prey. Many prey species may themselves be affected by intense low-frequency sounds, and, depending on the extent to which their availability is altered, there could be negative consequences for marine mammals as well. Sound is important for normal behavior of many species of bony and cartilaginous fishes (reviewed in Demski et al., 1973; Myrberg, 1981). Fish use sound for a variety of reasons, including but not limited to prey detection, intraspecific communication, maintenance of schools, and predator avoidance. The sensory receptors of fish that are found in the inner ear and lateral line organs—the systems involved with detection of acoustic and hydrodynamic signals—employ the same type of sensory hair cell as found in the mammalian ear (reviewed in Popper and Fay, 1993; Popper and Platt, 1993). These cells are potentially subject to the same types of damage from exposure to intense sound as hair cells of the mammalian ear (e.g., Yan et al. 1991). Since the ear and lateral line organs are extremely important for the normal life of fish, damage to these systems would severely affect their ability to survive and reproduce, thereby affecting the food supply of marine mammals (see Popper and Platt, 1993). Although only a few data are available on the use of sound by most marine invertebrates, including prey of marine mammals (e.g., Budelmann, 1992), some groups (e.g., cephalopods) do have highly evolved statocysts and lateral line-like systems, features which have many structural and functional similarities to the vertebrate ear and lateral line (e.g., Budelmann and Bleckmann, 1988). As in fish, damage to the major sensory receptors of these species could harm a major food source for many cetaceans. Beyond damage to the receptors, intense sounds may alter the behavior of fish and invertebrates by affecting their ability to detect behaviorally relevant signals. For example, there is evidence that many species of deep-sea lantern fish, the myctophids, may have
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--> good hearing (Popper, 1977) and may use sound for communication (Marshall, 1967). These species make up a significant portion of the diet of a number of pinnipeds and cetaceans as well as many other fishes that are also marine mammal prey, as well as being commercially valuable (e.g., salmon, cod, hake, rockfishes, tunas) (Nafpaktitis et al., 1977). Indeed, Fitch and Brownell (1968) concluded that "if the day ever arrives that man finds it economically feasible to harvest fishes from the scattering layer (often composed predominantly of myctophids), uncontrolled exploitation could have a disastrous effect on our dolphin, porpoise, and whale populations." In addition, high-intensity sounds may result in damage to other organ systems of these prey animals. There is laboratory evidence that such sounds can affect egg viability and growth rates of fish and invertebrates (Banner and Hyatt, 1973; Kostyuchenko, 1973; Lagardere, 1982). Thus, intense sounds may affect the availability of organisms in the food chain of marine mammals even if these organisms do not have auditory receptors. This committee recommends that research be conducted on the ways in which fishes, squids, and crustaceans (especially krill) respond to relevant human-made sounds at levels above the ambient noise level, and on the levels that cause chronic or acute aftereffects of stimulation. Development and Application of Measurement Techniques Aims: To develop tools that can enhance observation and data gathering regarding marine mammal behavior or that can protect the animals from intense human-made sounds. Rationale: One of the major problems in understanding the behavior of marine mammals, as well as in determining the effects of human-made sounds on their behavior, is the difficulty of observing their behavior in the open ocean. A number of tools are needed to enable investigators to maintain long-term observations of animals with minimal human presence. At the same time, it may be possible to develop techniques to mitigate the effects of the intense sounds by "warning" animals away from sites where such sounds are to be used. Tag Development Aims: To develop tags that can be used for long-term observations of marine mammals, including studies on physiological
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--> condition, location (in three dimensions), sound exposure levels, and acoustic behavior. Difficulties in observing marine mammals at sea have seriously limited our ability to study their normal activities and responses to human activities. Most marine mammals live below the surface of the water much of the time, and even when at the surface, the animals may not be observable if they are far off shore or moving rapidly, or if there is bad weather, darkness, or ice. Major advances in knowledge of the behavior of pinnipeds have occurred with recent improvements in the design and application of recoverable time/depth recorders, and more recently with satellite-linked time/depth recorders, that can be placed on pinnipeds with relative ease when they come ashore. These devices would be more suitable for studying effects of low-frequency sounds if they incorporated additional sensors and, in some cases, included a provision for remote release and data recovery. Advances in knowledge of the behavior of cetaceans have been much less dramatic because of problems with the deployment, attachment, and recovery of tags. Accordingly, the committee recommends a focused effort to develop improved deployment, attachment, and telemetry methods for tags. For many pinniped as well as cetacean applications, satellite-linked tags are needed. However, the data capacity of the existing ARGOS system is seriously limited. A replacement system, expected in 1996, will increase data transmission capabilities and allow two-way communication between the satellites and transmitters. Optimized data compaction algorithms are needed aboard the tags. Ultimately and ideally, new satellite systems for getting data from the tags are needed to enable much higher data rates to be obtained. Tags incorporating additional sensors are a prerequisite for much of the high-priority research on effects of low-frequency sounds on marine mammals in the wild (see the first section in this chapter). Tags are needed that would directly or indirectly provide precise three-dimensional position data (latitude, longitude, depth). It would be extremely valuable if such a tag could also measure and record the sound levels received by the animal at different times. Ideally, the same sensor could determine sound levels before, during, and after exposure to human-made noises, and could monitor vocalization behavior of the tagged animal. Other desirable parameters include measures of animal motion (e.g., heading, pitch, speed, acceleration), foraging behavior (e.g., stomach temperature, mouth opening
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--> and closing), and physiological condition (e.g., heart rate). Tags incorporating even one or two of these new capabilities would be very useful. Integrated Undersea Surveillance System (IUSS) Aim: To develop means of using in-place acoustic monitoring devices to study marine mammal movement and behavior on an ocean basin scale and of following individuals or groups of animals for extended periods and distances. Marine mammals have traditionally been studied by using visual observation or remote tags. Passive acoustic localization of vocalizing animals is a relatively unexploited technique that can compensate for some of the weaknesses of the alternative approaches. The U.S. Navy is currently encouraging the use of its integrated undersea surveillance system (IUSS) for locating and triangulating points of origin of marine mammal vocalizations (see Amato, 1993; Gagnon and Clark, 1993). IUSS is an integrated system of hydrophone arrays mounted on the sea floor in many widely separated areas allowing coverage of large parts of the North Atlantic and North Pacific. The IUSS has been used to locate calling whales at apparent ranges of hundreds of kilometers. The IUSS might be helpful in evaluating the potential effects of intense noise sources on whales distributed over large areas of ocean. Unlike vessels, which may disturb whales before ship-based observers can even detect them, monitoring of whales with IUSS does not disturb the animals. It may be possible to use IUSS to map the density of whale calls and even to track whales, under ideal conditions. This mapping could provide a synoptic view of the vocal behavior of hundreds or thousands of animals. If a sound source were activated in the center of such a map, one could evaluate several potential disturbance responses. When disturbed, whales may either cease vocalizing, change vocalizations, or move away from or toward the disturbing sound. These responses would show up on IUSS as a lower density and/or altered distribution of whale calls. An IUSS study would help to evaluate the range over which whales may respond to noise and would also help to integrate responses of large numbers of animals. Both kinds of data would complement intensive studies of how individual whales in one place respond to the same noise source.
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--> SWAT Team for Studying Hearing by Ensnared or Beached Animals Aim: To develop procedures for rapid determinations of hearing capabilities (and perhaps other physiological studies) on beached or ensnared marine mammals. Although it is possible to study some species of smaller marine mammals in captivity, it is not feasible to study larger species under such conditions using behavioral methods. Data from larger species (especially baleen whales) are particularly important for developing an understanding of the physiological effects of high-intensity, low-frequency sounds on these animals. One approach to determining some aspects of the hearing capabilities of such species would be to record physiological evoked potentials in response to presentation of acoustic signals (see subsection Measurements on Ensnared or Beached Marine Mammals in the section on the Auditory System). Physiological evoked potentials include, for example, averaged brain-wave responses such as cortical evoked potentials and averaged brainstem responses, heart rate changes, otoacoustic emissions, and galvanic skin responses. A major potential source of animals for such studies would be those species that are ensnared in nets for short periods of time or animals that are beached. However, since ensnarement and beaching are generally unpredictable, both in time and in place, it would be valuable to have a group of investigators prepared to depart for such sites on very short notice. Therefore the committee proposes development of a SWAT (Stranded Whale Auditory Test) team of investigators having portable equipment and highly developed techniques that would enable them to go to sites of ensnarement or beaching to obtain data on the hearing capabilities of animals. These teams should also have the training, equipment, and permits needed to collect auditory structures and inner ears from those animals that do not survive the ensnarement or beaching. Warning Signals Aim: To investigate the possibility of protecting marine mammals from some of the adverse effects of intense, low-frequency sounds by capitalizing on any normal avoidance reactions these animals might have to certain sounds. Some of the negative effects of intense, low-frequency sound on marine mammals could obviously be avoided if there were no mammals
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--> in the immediate vicinity of the sound at the time of its generation. This might be accomplished through the use of stimuli that particular species naturally find offensive, alarming, or noxious. If it were possible to transmit an avoidance or warning signal for a period of time prior to producing a sound that might be dangerous to the auditory systems of some of the animals in the vicinity, considerable protection might be realized for that species as well as for the individual animals involved. Numerous reports have appeared over the years of whales fleeing from sounds of killer whales or certain human activities [see reviews by Reeves (1992) and Richardson et al. (1991)]. If these reports could be confirmed, and/or additional warning sounds found for other species, it might be possible to reduce the effects of various intense sounds on marine mammals greatly. Indeed, for some species it is possible that effective warning signals could be developed over time through the process of classical conditioning. Alternatively, the potentially damaging sounds might initially be projected at a low intensity, allowing behavioral avoidance to occur before the level is increased to high intensity. A broader application would be to develop effective warning signals to protect marine mammals from other potentially harmful activities such as entanglement with fishing gear or collision with ships. References Allen, A.R., and A. Starr. 1978. Auditory brain stem potentials in monkey (M. mulatta) and man. Electroenceph. Clin. Neurophysiol. 45:53–63. Amato, I. 1993. A sub surveillance network becomes a window on whales. Science 261(5121):549–550. Anon. 1968. Report of the Ad Hoc Committee of the Harvard Medical School to Examine the Definition of Brain Death. A definition of irreversible coma. J.A.M.A. 205(6):337–340. Awbrey, F.T., J.A. Thomas, and R.A. Kastelein. 1988. Low-frequency underwater hearing sensitivity in belugas, Delphinapterus leucas. J. Acoust. Soc. Am. 84:2273–2275. Banner, A., and M. Hyatt. 1973. Effects of noise on eggs and larvae of 2 estuarine fish. Trans. Am. Fish. Soc. 102(1):134–136. Beamish, P. 1973. Behavior and significance of entrapped baleen whales. In: H.E. Winn and B.L. Olla (eds.), Behavior of Marine Animals: Current Perspectives in Research. Plenum, New York. pp. 291–309. Budelmann, B-U. 1992. Hearing in crustacea. In: D.B. Webster, R.R. Fay, and A.N. Popper (eds). Evolutionary Biology of Hearing. Springer-Verlag, New York. pp. 131–139. Budelmann, B-U., and H. Bleckmann. 1988. A lateral line analog in cephalopods: Water waves generate microphonic potentials in the epidermal head lines of Sepia officinalis and Lolligunculus brevis. J. Comp. Physiol. A 164:1–5. Bullock, T.H., and S.H. Ridgway. 1972. Evoked potentials in the central auditory system of alert porpoises to their own and artificial sounds. J. Neurobiol. 3:79–99.
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