Abstract: 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.
Key Words: anthropogenic noise, audiogram, frequency weighting, harbor porpoise, Phocoena phocoena, hearing, hearing damage, hearing loss, hearing sensitivity, low frequency, odontocete, temporary hearing threshold shift, TTS
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