Low-Frequency Sound and Marine Mammals Current Knowledge and Research Needs (1994) / Chapter Skim
Currently Skimming:
1 A REVIEW OF CURRENT KNOWLEDGE
1 A REVIEW OF CURRENT KNOWLEDGE
Pages 8-24
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From page 8... ...
It then describes the state of knowledge regarding components that must be determined to evaluate a potential source of noise interference affecting marine mammals. The committee then discusses the gaps in knowledge, including those on auditory sensitivity and behavioral responsiveness of marine mammals, that would require research before predictions can be made of the effects of low-frequency sound on marine mammals.
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From page 9... ...
Consequently, the hearing abilities of marine mammals undoubtedly evolved to deal with the ambient noise levels in the aquatic environment prior to the industrial age. Terrestrial mammals depend on sound to analyze and interact with their environment and to communicate among themselves.
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From page 10... ...
Acoustic oceanographers use intense sounds, especially low-frequency sounds, to study the physical properties of the ocean (Spindel and Worcester, 1990; Baggeroer and Munk, 1992; Worcester et al., 1993~. Such human-made sounds combine with the natural sounds of the ocean, and elevate the ambient noise level, mainly in the frequency region below 1,000 Hz (Figure 1~.
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From page 11... ...
FIGURE 1 Ambient noise spectra. SOURCE: Adapted from Wenz (1962)
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From page 12... ...
Recently, concern has also arisen about the potential effects of strong low-frequency sounds used by acoustic oceanographers, underwater acousticians, and operational naval activities (Mulroy, 1991; Simmonds and Lopez-Jurado, 1991~. EVALUATING NOISE INTERFERENCE For a sound signal to be detected by a marine mammal or its prey, the signal must contain enough energy to exceed the ambient (background)
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From page 13... ...
These levels and the details of their spectra and variation over time establish the conditions for detecting extraneous signals either by a physical receiver or by any biological organism. Extensive data and models concerning both acoustic propagation and the ambient noise level can be used to help evaluate the potential impact on marine mammals of sound from any human-made source.
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From page 14... ...
Recordings from a variety of sources were used in these studies, including continuous sounds such as those from oil drilling and production platforms, as well as impulsive sounds from air guns. The broadband levels of low-frequency sounds causing avoidance by about 50 percent of the gray and bowhead whales in the area of the observations occurred when the received levels were around 115 to 120 dB re 1 Spa-water standard for the continuous sounds and about 160 to 170 dB for the pulsed sounds.
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From page 15... ...
15 FIGURE 2 Underwater audiograms of selected toothed whales (A) and pinnipeds (B)
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From page 16... ...
In the low-frequency region, it is uncertain that the toothed whale could even detect continuous sounds with received levels near 115 dB re 1 Spa water standard. The implications of this paucity of audiometric information include profound uncertainty about the interfering effect of any potential sound source on baleen whales.
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From page 17... ...
The committee is not aware of any specific data on the hearing sensitivity of sea otters or polar bears, both of which are considered marine mammals. There is little information on the reactions of these mammals to underwater sound (see, however, Riedman, 1984~.
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In reviewing plans for activities that produce underwater noise, it appears to the committee that the National Marine Fisheries Service of the U.S. Department of Commerce considers that marine mammals exposed to broadband received levels above 120 dB (re 1 ,uPa water standard)
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From page 19... ...
to produce the same change in the animal's behavior as did a steadystate or continuous sound at 120 dB (Malme et al., 1984; Richardson et al., 1986; Ljungblad et al., 1988~. In contrast, bowhead whales sometimes react to increasing noise levels from an approaching boat when the broadband level is well below 120 dB (Miles et al., 1987:225/f; Richardson and Malme, 1993~.
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From page 20... ...
Furthermore, only two species of whales were involved, and the results of the same experiment would very likely be different for other species. Because of their apparent lack of sensitivity at these low frequencies, some toothed whales, for example, may not detect sound at the levels that affected the gray whales and bowhead whales (see Figure 2A)
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From page 21... ...
The limits were subsequently confirmed in epidemiological studies of permanent hearing loss. No data on temporary threshold shifts (that is, the occurrence of temporary hearing loss)
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1983. Investigations of the potential effects of underwater noise from petroleum industry activities on migrating gray whale behavior.
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1986. Reactions of bowhead whales, Balaena mysticetus, to seismic exploration in the Canadian Beaufort Sea.
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From page 24... ...
1975. Underwater hearing sensitivity of two ringed seals (Pusa hispida.
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