Analysis and Ranking of the Acoustic Disturbance Potential of ...

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Report No. 6945 BBN Systems and Technologies Corporation The increase in thresholds is more abrupt at high frequencies, at least when frequencies are shown on a logarithmic scale as in normal. This upper frequency cutoff was at about 31 kHz for the one killer whale tested, 120 kHz for white whales, and somewhere above 140 kHz for the harbor porpoise. Johnson ( 1980) has suggested that, above 50 kHz, the hearing of odontocetes may be limited by water molecule motion known as thermal noise (Urick 1975). Bullock et al. (1968) and several subsequent investigations have obtained electrophysiological audiograms from several specie9 of dolphins and porpoises. Electrophysiological audiograms are based on neural responses (evoked potentials) received from electrodes implanted in the animal's brain or, in some more recent studies, applied outside the skull. The shapes of the electrophysiological audiograms are generally comparable to those obtained behaviorally. In the case of the harbor porpoise, however, the lowest threshold determined by the evoked potential method was at a much higher frequency than that determined behaviorally (about 125 kHz vs. 8-32 kHz, Voronov and Stosman 1983; Popov et al. 1986 vs. Andersen 1970). Bullock et al. (1968) were not able to accurately record absolute intensities, but some of the subsequent electrophysiological studies may have provided absolute audiograms. Popper (1980) indicates, however, that thresholds obtained by these methods may be higher than those obtained behaviorally. In any case, invoked potential methods based on external electrodes hold particular promise for examining the hearing abilities of marine mammals such as baleen whales that are very difficult to hold in captivity (Ridgway et al. 1981; Ridgway and Carder 1983 ; Popov et a1 . 1986) . 2.3.3 Pinnipeds Behavioral audiograms have been obtained for three species of hair (phocid) seals--ringed, harbor and harp seals. Also, the grey seal has been studied by the evoked potential method. Ringed and harbor seals occur in Alaska. Phocid seals can apparently detect very high frequencies of underwater sound--up to 180 kHz in the case of the harbor seal (Fig. 2.26). However, above 60 kHz sensitivity is poor and different frequencies cannot be discriminated (Mdhl 1968a,b). 'The functional high frequency cutoff is thus around 60 kHz for the species tested (Schusterman 1981 ) . Below about 50 kHz, the hearing threshold of phocids is quite flat down at least to 1 kHz, ranging between 65 and 85 dB re 1 pPa (Mdhl 1968a; Terhune 1981 ; Terhune and Ronald 1972, 1975a; Fig. 2.26). The lower limit of phocid hearing has not been clearly delineated since frequencies below 1 kHz have not been tested. The two species for which more than one individual has been tested (ringed and grey seals) exhibit some audiogram variability within species (Terhune and Ronald 1974; Ridgway and Joyce 1975). The high frequency cutoff of eared seals (otariids) for underwater sound is lower than that of phocids (Schusterman 1981); however sensitivity in the range of best hearing is not substantially different from that of phocids (Fig. 2.26). The high frequency cutoff of both qpecies of otariids that have been tested (California sea lion and northern fur seal) is between 36 and 40 kHz based on behavioral techniques (Schusterman 1981). Tke fur seal has a peak sensitivity of about 60 dB re 1 pPa between 4 and 28 kHz (Moore and Schusterman 1987), whereas the California sea lion has a peak sensitivity of
Report No. 6945 BBN Systems and Technologies corporation 160 I pinnipeds In Water 0 Cal. Sea Lion 0 Fur Seal r Harbor Seal e Rtnged Seal A Rarp Seal, 40 142 "io3 . "io4 ' ' 'ios . ' 'io6 Frequency (Hz) Figure 2.26. Underwater Audiograms of Several Pinnipeds: California Sea Lion (Schusterman et al. 1972); Average of Two Fur Seals (Moore and Schusterman 1987) ; Harbor Seal (MQhl 1968a) ; Average of Two Ringed Seals (Terhune and Ronald 1975a); Harp Seal (~erhune and Ronald 1972 ) . 80 dB re 1 pPa at about 2 and 16 kHz (Schusterman et a1 . 1972). The hearing threshold of the California sea lion rises from about 87 dB re 1 pPa at 1 kHz to about 116 dB at 250 Hz. These low frequency hearing thresholds are probably valid since Schusterman et al. (1972) made very careful measurements of echoes and ambient noise in the test tank, and rigidly positioned the subject sea lion in a position where the signal level was measured at its maximum. As amphibious animals, pinnipeds need to respond to in-air sound as well as to underwater sound. Aerial audiograms have been determined behaviorally for two fur seals and a California sea lion (Moore and Schusterman 1987), a harbor seal (Mdhl 1968a), and a harp seal (Terhune and Ronald 197 1 ) . An earlier determination for another sea lion (Schusterman 1974) is now considered to be artefactual, and the reliability of the harp seal data for 1- 8 kHz has also been questioned (Moore and Schusterman 1987). Besides these behaviorally-determined results, relative thresholds of in-air hearing at different frequencies have been determined by the evoked potential method for California sea lions and a harbor seal (Bullock et al. 1971). In air,
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FIG- 3-12 STATISTICAL ANALYSIS OF C
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