Harbor seals may suffer hearing loss due to intense sounds. After exposure for 60-min to a continuous 6.5kHz tone at sound pressure levels of 123–159 dB re 1 µPa, resulting in sound exposure levels (SELs) of 159–195dB re 1 µPa2s, temporary threshold shifts (TTSs) in two harbor seals were quantified at the center frequency of the fatiguing sound (6.5kHz) and at 0.5 and 1.0 octaves above that frequency (9.2 and 13.0kHz) by means of a psychoacoustic technique. Taking into account the different timing of post-exposure hearing tests, susceptibility to TTS was similar in both animals. The higher the SEL, the higher the TTS induced at frequencies above the fatiguing sound’s center frequency. Below 179 dB re 1 µPa2s, the maximum TTS was at the center frequency (6.5 kHz); above 179 dB re 1 µPa2s, the maximum TTS was at half an octave above the center frequency (9.2 kHz). These results should be considered when interpreting previous TTS studies, and when estimating ecological impacts of anthropogenic sound on the hearing and ecology of harbor seals. Based on the results of the present study and previous studies, harbor seal hearing, in the frequency range 2.5–6.5kHz, appears to be approximately equally susceptible to TTS.

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