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February 12, 2025
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Temporary Hearing Threshold Shift in California Sea Lions (Zalophus californianus) Due to a Noise Band Centered at 40 kHz and Comparison with Shifts Due to Lower-Frequency Sounds

Author(s):

Ronald A. Kastelein, Lean Helder-Hoek, Laura Van Acoleyen, Linde N. Defillet, John M. Terhune, and Nancy Jennings

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Document: Article

Abstract: 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-sixth-octave 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.

Key Words: anthropogenic noise, audiogram, auditory weighting, fatiguing sound, hearing damage, hearing recovery, hearing sensitivity, Otariidae, pinniped, temporary hearing threshold shift, TTS

DOI: https://doi.org/10.1578/AM.51.1.2025.13

Page Numbers: 13-30

Kastelein et al. is Open Access: Click here for PDF

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