Harbor porpoises may suffer hearing loss when exposed to intense sounds. After exposure to playbacks of broadband pile driving sounds (rate: 2760 strikes/hr, inter-pulse interval: 1.3 s) at one average received single strike (124 ms) unweighted sound exposure level (SEL) of 146 dB re 1μPa2s for 60 min (cumulative SEL: 180 dB re 1μPa2s), the temporary hearing threshold shift (TTS) of a porpoise was quantified at 0.5, 1, 2, 4, 8, 16, 32, 63 and 125 kHz with a psychoacoustic technique. Statistically significant TTS occurred at 4 and 8 kHz; mean TTS (1-4 min. after sound exposure stopped) was ~2.3 dB at 4 kHz, and ~3.6 dB at 8 kHz; recovery occurred within 48 min. Thus, exposure to multiple impulsive sounds can cause reduced hearing in a specific frequency band. Ecological effects of TTS depend not only on the magnitude of the TTS, its duration (which is related to the exposure duration), and the recovery time after the exposure stopped, but also on the hearing frequencies that are affected by the fatiguing noise. The hearing thresholds of harbor porpoises for the frequencies of their echolocation signals are not affected by pile driving sounds.
This report was presented at a symposium at Naturalis in Leiden on September 8, 2015. A complete report containing brief abstracts of all studies presented at the symposium can be found here.