Marine Mammals and Low-Frequency Sound Progress Since 1994 (2000) / Chapter Skim
Currently Skimming:
3 Assessment of Continuing Research Needs
3 Assessment of Continuing Research Needs
Pages 41-64
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From page 41... ...
Computational models based on the anatomical parameters of marine mammal cochleae have been developed, and predictions from such models have correlated well with behaviorally determined audiograms in several species of toothed whales (Ketten, 1997~. This kind of modeling provides an important new tool for assessing the auditory sensitivity and frequency range in whales that are not amenable to experimental measurements.
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From page 42... ...
anatomical studies of species with known audiograms to validate the use of anatomical features to predict auditory capabilities and (2) studies using both behavioral and auditory evoked potentials techniques to determine auditory capabilities of marine mammals (especially of baleen whales)
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From page 43... ...
; and (2) known or suspected to hear and communicate using low-frequency sound (e.g., baleen whales, sperm whales, elephant seals)
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From page 44... ...
The recent availability of the Navy's Integrated Undersea Surveillance System (IUSS) and other hydrophore arrays to scientific researchers has permitted the use of passive acoustic tracking as a method for documenting migration routes and critical habitats where baleen whales are seasonal visitors or residents (e.g., Stafford and Fox, 1996; Stafford et al., 1998, 1999; Watkins et al., 2000~.
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From page 45... ...
This innovative protocol may be useful for determining masking effects in other toothed whales using a variety of human-generated noises, including the ATOC signal and LEA sonar. A logistically challenging field study was conducted in the St.
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From page 46... ...
Calling rates declined in five of the six sessions as noise levels increased, the occurrence of certain call types increased, repetition rates of certain calls increased when a vessel was within 1 km of the white whale, and the mean frequencies produced were higher, presumably to move the frequency of the call outside the frequency band of the masking noise produced by the vessels. These two studies provide evidence that specific human-generated noises can affect the vocal activity of white whales in the short term.
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From page 47... ...
This study confirmed that whales change their response as source level is changed, which demonstrates that they are responding to received level, not just distance or sound gradient. When the source was located on the offshore side of the migratory path, there was little evidence of any diversion in the individual migratory paths for source levels of both 185 and 200 dB (Tyack and Clark, 1998~.
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From page 48... ...
states that received levels as high as 150-160 dB were estimated to occur at ranges of 50 km. Sperm whales were
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From page 49... ...
. Determine the response of deep-diving marine mammals to lowfrequency sounds whose characteristics (source level, frequency bandwidth, duty cycle)
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From page 50... ...
They presented the argument that in water sound pressure is a more biologically appropriate measure than sound intensity.6 Using sound pressure sensitivity as the critical parameter, they found consistent correlations between hearing sensitivities and the environment in which the animal spends the greater portion of its time. Thus, the elephant seal, which spends a greater proportion of time in water, has better underwater hearing sensitivity than the sea lion, which hears better in air than in water, and the harbor seal, which has almost equal sensitivity in air and in water.
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From page 51... ...
They also concluded that the ATOC signal is unlikely to harm baleen whales or to mask their vocalizations, but these conclusions are based on calculations and interpretations of source levels of whale vocalizations without using actual behavioral data. Source levels for baleen whale vocalizations have been calculated from measurements for single animals at considerable distances from the hydrophore.
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From page 52... ...
lower-range ultrasonic toothed whales, and (3) baleen whales.
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From page 53... ...
The sperm whale cochlea has characteristics of both the higher and the lower ultrasonic toothed whale cochlea, but the critical measurements of the cochlea and the vestibular system that could indicate whether the ear may be sensitive to the low frequencies of ATOC or LEA sounds have not yet been made. When investigators are collecting data on hearing capabilities, they should be alert to the possibility of differences in hearing sensitivity as a function of the sex and age of the animal.
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From page 54... ...
The fact that some hearing losses last only minutes, hours, or days suggests that some cochlear structures have the ability to recover from whatever damage is inflicted by the exposure stimulus. In terrestrial mammals the receptor cells known as outer hair cells are known to be far more susceptible to acoustic damage than are the less numerous inner hair cells (Saunders et al., 1991)
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From page 55... ...
An interesting characteristic of TTS and PTS is that for terrestrial mammals the maximum hearing loss typically occurs in a frequency region above the exposure frequency (McFadden, 1986~. Investigators studying TTS in marine mammals should design their experiments to obtain information about any upward shifts in maximum hearing loss that exist in marine mammal ears.
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From page 56... ...
They used test frequencies ranging from 100 to 2,000 Hz and "octave-band noise exposure levels that were approximately 60-75 dB SL (sensation level at center frequency) ." They found TTS averaging 4.8 dB for one harbor seal, 4.9 dB for two California sea lions, and 4.6 dB for one northern elephant seal.
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From page 57... ...
. Determine condition of important cochlear structures in wild marine mammals using postmortem examinations (this topic did not appear in earlier reports)
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From page 58... ...
, meaning that the cochlear physiologists have good expectations about what to look for, and where, in the cochleae of marine mammals. Damage of particular types and extents will be definitive evidence of permanent extensive hearing loss, and less severe damage will be evidence of less extensive hearing loss.
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From page 59... ...
These data suggest that this species may be disproportionately insensitive to very short sounds; that is, their temporal integration function appears to be much steeper or displaced from that of typical terrestrial mammals. Data on the hearing capabilities of other baleen whales may suggest that long temporal integration functions, with a corresponding relative insensitivity to transient sounds,
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From page 60... ...
In contrast to baleen whales, characterized by their sensitivity to lowfrequency sounds, the toothed whales, characterized by high-frequency hearing, do appear to exhibit short temporal integration functions. Measures of temporal resolution capability and temporal integration times were obtained in several species of toothed whales by Dolphin (1995, 1996)
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From page 61... ...
suggests that fish and perhaps some shark and amphibian species have the capacity to regenerate damaged hair cells in their auditory and balance organs (see also Lombarte et al., 1993~. To the extent that this ability is widespread in those species, they may be at lesser long-term risk from exposure to intense sounds than are marine mammals, which are presumably like terrestrial mammals in being incapable of regenerating new receptor cells to replace damaged ones (e.g., Hastings et al., 1996; Corwin and Oberholtzer, 1997~.
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From page 62... ...
This is unlikely to be a problem with the ATOC sources because they are so far offshore from dive sites and in deep water, but it may be a problem with more powerful shallow-water sources, such as SURTASS-LFA. Although marine mammals do not carry a tank of pressurized breathing gas as human divers do, they make frequent dives to depths greater than 100 m, which may produce over 200 percent supersaturation of nitrogen in muscle tissue after repetitive dives (Ridgway and Howard, 1979, 1982~.
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From page 63... ...
This monitoring optimally should also include or be coordinated with other assessments of habitat quality such as prey fields and chemical pollutants. Coordination of noise monitoring with stranding networks would enable more systematic and controlled evaluation of whether noise influences strandings and whether cochlear damage in stranded animals is associated with acute noise exposure.
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From page 64... ...
Evaluation of the appropriateness of the IUSS should determine whether the bandwidth and geographic coverage of the IUSS is adequate for the task of monitoring ambient noise and marine mammals or whether it is necessary to design and build an array of sensors specifically to monitor marine mammals. Such a system could be automated to activate when significant sounds are detected.
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