Temporary Hearing Threshold Shift in Harbor Porpoises (Phocoena phocoena) Due to One-Sixth-Octave Noise Bands at 63 kHz

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Abstract: Susceptibility to temporary hearing threshold shift (TTS) depends on the frequency of the fatiguing sound. So far, TTS in harbor porpoises (Phocoena phocoena) has been tested for sounds in the 1 to 32 kHz range. It is, however, unclear if TTS growth functions differ across frequencies. To assess impacts of anthropogenic sound on porpoise hearing, full insight into the TTS growth functions across all ecologically relevant hearing frequencies for the harbor porpoise is required. In the present study, which is part of a series studying the susceptibility of harbor porpoises to TTS over their entire hearing range, TTS growth functions were quantified after two harbor porpoises (M06 and F05) were exposed for one hour to a continuous one-sixth-octave noise band centered at 63 kHz, at average received sound pressure levels (SPLs) of 106 to 156 dB re 1 µPa, resulting in a sound exposure level (SEL) range of 142 to 192 dB re 1 µPa2s. Hearing thresholds for 63, 88.4, 100, and 125 kHz signals were determined before and after exposure to quantify TTS and recovery. Porpoise M06’s hearing was tested 1 to 4 minutes after exposure. At 63 kHz, the lowest SEL which resulted in significant TTS1-4 (6.5 dB) was 154 dB re 1 µPa2s, and the highest TTS1-4 (7.8 dB) occurred at 175 dB SEL. Porpoise F05’s hearing was tested both 1 to 4 and 12 to 16 minutes after exposure. At 63 kHz, the only SEL which resulted in significant TTS1-4 (3.2 dB) was 181 dB re 1 µPa2s; no TTS12-16 occurred. At 88.4 kHz, the lowest SEL which resulted in significant TTS1-4 (6.6 dB) and TTS12-16 (3.2 dB) was 192 dB re 1 µPa2s. No evidence was found for TTS at 100 or 125 kHz in either animal, but sample sizes were very small for those frequencies. Based on the seven available TTS studies conducted with harbor porpoises and 60-minute continuous sound, it appears that susceptibility to TTS increases with increasing frequency of the fatiguing sound below ~6.5 kHz but decreases with increasing frequency of the sound above ~6.5 kHz. For the frequency used in the present study (63 kHz), harbor porpoises are less susceptible to TTS than formerly assumed.
Key Words: anthropogenic noise, audiogram, frequency weighting, hearing, hearing damage, hearing sensitivity, odontocete, temporary threshold shift, TTS
DOI: https://doi.org/10.1578/AM.46.2.2020.167
Page Numbers: 167-182

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