Temporary Hearing Threshold Shift at Ecologically Relevant Frequencies in a Harbor Porpoise (Phocoena phocoena) Due to Exposure to a Noise Band Centered at 88.4 kHz

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Abstract: Susceptibility to temporary hearing threshold shift (TTS) in harbor porpoises (Phocoena phocoena) depends on the frequency of the fatiguing sound causing the shift. TTS in harbor porpoises has been tested for sounds within the 1 to 63 kHz frequency range. Susceptibility to TTS caused by sounds of ~88 kHz is ecologically relevant since these sounds are expected to affect hearing in the frequency range of harbor porpoise echolocation signals. TTS was quantified in a female porpoise after exposure for 1 h to a continuous one-sixth-octave noise band centered at 88.4 kHz, at average received sound pressure levels of 137 to 161 dB re 1 µPa (resulting sound exposure levels [SELs]: 173 to 197 dB re 1 µPa2s). To quantify TTS and recovery, hearing thresholds for 88.4, 100, and 125 kHz tonal signals were determined before and after exposure. Control trials were used as a baseline and to determine which exposure levels resulted in statistically significant TTS in the 4 min after the fatiguing sound stopped (TTS1-4). At 88.4 kHz, the lowest SEL that resulted in significant TTS1-4 (3.6 dB) was 185 dB re 1µPa2s; at 100 kHz, the lowest SEL that resulted in significant TTS1-4 (5.2 dB) was 191 dB re 1 µPa2s; and at 125 kHz, the lowest SEL that resulted in significant TTS1-4 (5.4 dB) was 191 dB re 1 µPa2s. At higher SELs, the TTS at this frequency remained similar. The highest TTS1-4 (13.1 dB) occurred at 100 kHz after exposure to an SEL of 197 dB re 1 µPa2s. In most cases, hearing recovered within 12 min after the fatiguing sound stopped; in the remaining cases, recovery took less than 1 h. TTS onset (defined as 6 dB TTS; Southall et al., 2019) occurred after exposures to SELs of ~191 dB re 1 µPa2s (when hearing was measured at 100 kHz, one third of an octave above the center frequency of the fatiguing sound).
Key Words: anthropogenic noise, audiogram, frequency weighting, harbor porpoise, Phocoena phocoena, hearing, hearing damage, hearing loss, hearing sensitivity, odontocete, temporary threshold shift, TTS
DOI: https://doi.org/10.1578/AM.46.5.2020.444
Page Numbers: 444-453

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