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California sea lions (Zalophus californianus) exposed to anthropogenic noise may experience temporary hearing threshold shift (TTS). The function used in regulations to protect their hearing from such damage in the Pacific Ocean is based on only one datapoint, so more data are needed. To determine their frequency-dependent susceptibility to noise-induced TTS, two California sea lions were exposed for 60 minutes to a continuous one-sixthoctave noise band (NB) centered at 40 kHz as the fatiguing sound, at sound pressure levels of 119 to 143 dB re 1 µPa, resulting in sound exposure levels (SELs) of 155 to 179 dB re 1 µPa2s. TTSs were quantified at the center frequency of the fatiguing sound and up to one octave above that frequency (at 40, 50, 56.5, 63, and 80 kHz). Statistically significant TTS occurred at all hearing test frequencies; higher SELs caused greater TTSs. Significant onset of TTS(1-4 min) occurred after exposure to a minimum SEL of 167 dB re 1 µPa2s--a shift of 5.2 dB at hearing frequency 56.5 kHz. At other hearing frequencies, onset of TTS1-4 occurred at SEL 173 dB re 1 µPa2s. TTSs1-4 = 8 dB recovered within 12 min, and TTSs1-4 of > 8 dB recovered within 60 min. TTSs and hearing recovery patterns were similar in both subjects. Comparison with TTS data for the species' hearing frequency range (0.6 to 40 kHz) shows that after exposure to fatiguing sound frequencies of 0.6, 1, 4, 8, and 16 kHz, the largest TTS1-4 occurred half an octave above the frequency of each of the fatiguing sounds. After exposure to fatiguing sound frequencies 2, 32, and 40 kHz, the largest TTS occurred at the frequency of the fatiguing sounds. Recovery patterns after exposure to the NB at 40 kHz were similar to those after exposure to NBs at 0.6, 1, 2, 4, 8, 16, and 32 kHz. Over almost the entire hearing range, the shape of the audiogram is a poor predictor of the shape of the TTS-onset function. The low TTS-onset SELs show that the hearing of California sea lions is more vulnerable to injury by anthropogenic sound in the oceans than was previously thought. [ABSTRACT FROM AUTHOR]

Susceptibility to temporary hearing threshold shifts (TTS) in harbor porpoises (Phocoena phocoena) depends in part on the frequency of the fatiguing sound (the sound causing the shift). The TTS induced and the pattern of recovery were documented in a female porpoise after exposure for one hour to a continuous one-sixth-octave noise band centered at 8 kHz. This fatiguing sound was emitted at average received sound pressure levels (SPLs) between 126 and 144 dB re 1 µPa, resulting in average sound exposure levels (SELs) of 162 to 180 dB re 1 µPa2s. Hearing thresholds for narrow-band sweeps centered at 8, 11.3, and 16 kHz were determined before and after exposure. Control sessions were used to determine which SELs resulted in statistically significant TTS in the first four minutes after the fatiguing sound stopped (TTS1-4). At 8 kHz, the lowest SEL that resulted in significant TTS (4.4 dB) was 174 dB re 1 µPa2s; at 11.3 kHz, the lowest SEL 1-4 that resulted in significant TTS1-4 (4.9 dB) was 168 dB re 1 µPa2s; and at 16 kHz, the lowest SEL that resulted in significant TTS1-4 (1.3 dB) was 174 dB re 1 µPa2s. The hearing frequency that was most affected was 11.3 kHz, half an octave above the fatiguing sound's center frequency. The equal-energy hypothesis was tested by exposing the porpoise to the same noise band with SPLs of 137 to 153 dB re 1 µPa and exposure durations between two and 80 minutes; all seven combinations resulted in the same fatiguing SEL of 174 dB re 1 µPa2s; and for these combinations, the equal-energy hypothesis was upheld. The results add to the body of data on TTS-onset SELs that were used to generate a revised auditory weighting function and, thus, enhance regulatory protection of wild harbor porpoises that are exposed to anthropogenic noise at sea. [ABSTRACT FROM AUTHOR]

Susceptibility to temporary hearing threshold shift (TTS) in harbor porpoises (Phocoena phocoena) depends on the frequency of the fatiguing sound causing the shift. This study is part of a larger project in which TTS in harbor porpoises was tested after exposure to fatiguing sounds within the 0.5 to 88.4 kHz frequency range. Herein, we report on the TTS induced and hearing recovery in a female harbor porpoise after exposure to 1, 2, and 4 h of continuous one-sixth-octave noise band centered at 0.5 kHz, which is within the frequency range of many high-amplitude anthropogenic sounds. This fatiguing sound was emitted at an average received sound pressure level of 163 dB re 1 µPa, resulting in sound exposure levels (SELs) of 199 to 206 dB re 1 µPa2s. Hearing thresholds for 0.5, 0.71, and 1 kHz tonal signals were determined before and after exposure. Control sessions were used as a baseline and to determine which SELs resulted in statistically significant TTS in the first 4 min after the fatiguing sound stopped (TTS1-4). At 0.5 kHz, the lowest SEL that resulted in significant TTS1-4 (4.3 dB) was 202 dB re 1 µPa2s. At 0.71 and 1 kHz, the lowest SEL that resulted in significant TTS1-4 (5.8 dB and 3.8 dB, respectively) was 205 dB re 1 µPa2s. Hearing always recovered within 60 min after the fatiguing sound stopped. Within the SEL range that was tested, the greatest mean TTS1-4 (7.6 dB) occurred at 0.5 kHz, the center frequency of the fatiguing sound, after exposure to an SEL of 205 dB re 1 µPa2s (4-h exposure); 0.5 kHz is close to the lower bound of porpoise hearing, and ecological impacts of reduced hearing at this frequency are unclear. Results will be used in a future study of this series to generate an auditory weighting curve and to enhance regulatory protection of the harbor porpoise. [ABSTRACT FROM AUTHOR]