Temporary Hearing Threshold Shift in Harbor Porpoises (Phocoena phocoena) Due to One-Sixth-Octave Noise Band at 32 kHz

Abstract

Temporary hearing threshold shift (TTS) caused by fatiguing sounds in the 1.5 to 16 kHz range has been documented in harbor porpoises (Phocoena phocoena). To assess impacts of anthropogenic noise on porpoise hearing, TTS needs to be investigated for other frequencies, as susceptibility appears to depend on the frequency of the fatiguing sound. TTS was quantified after two porpoises (Porpoises F05 and M06) were exposed for 1 hour to a continuous one-sixth-octave noise band centered at 32 kHz, at average received sound pressure levels of 118 to 148 dB re 1 µPa, and at a sound exposure level (SEL) range of 154 to 184 dB re 1 µPa2s. Hearing thresholds for 32, 44.8, and 63 kHz tonal signals were determined before and after exposure to quantify initial TTS and recovery. Porpoise M06’s hearing was tested 1 to 4 min after exposure. At 32 kHz, the lowest SEL that resulted in significant TTS1-4 (3.4 dB) was 166 dB re 1 µPa2s. At 44.8 kHz, the lowest SEL that resulted in significant TTS1-4 (5.2 dB) was 178 dB re 1 µPa2s. The highest TTS1-4 (18.3 dB) occurred at 44.8 kHz after exposure to 184 dB SEL. Porpoise F05’s hearing was tested 12 to 16 min after exposure. At 32 kHz, the lowest SEL that resulted in significant TTS12-16 (3.5 dB) was 184 dB re 1 µPa2s. At 44.8 kHz, the lowest SEL that resulted in significant TTS12-16 (1.2 dB) was 178 dB re 1 µPa2s. The highest TTS12-16 (8.2 dB) occurred in Porpoise F05 at 44.8 kHz after exposure to 184 dB SEL. At 63 kHz, no TTS could be elicited in either animal. Considering that Porpoise F05 had more time than Porpoise M06 for recovery, the susceptibility of the two porpoises to TTS after exposure to sounds of 32 kHz was similar. In the range investigated so far (1.5 to 32 kHz), susceptibility to TTS appears to increase with increasing frequency below ~6.5 kHz, and to decrease with increasing frequency above ~6.5 kHz.