Temporary hearing threshold shifts (TTSs) were investigated in two adult female harbor seals after exposure for 60min to a continuous one-sixth-octave noise band centered at 16kHz (the fatiguing sound) at sound pressure levels of 128–149dB re 1 µPa, resulting in sound exposure levels (SELs) of 164–185dB re 1 µPa2s. TTSs were quantified at the center frequency of the fatiguing sound (16kHz) and at half an octave above that frequency (22.4kHz) by means of a psychoacoustic hearing test method. Susceptibility to TTS was similar in both animals when measured 8–12 and 12–16min after cessation of the fatiguing sound. TTS increased with increasing SEL at both frequencies, but above an SEL of 174dB re 1 µPa2s, TTS was greater at 22.4kHz than at 16kHz for the same SELs. Recovery was rapid: the greatest TTS, measured at 22.4kHz 1–4min after cessation of the sound, was 17dB, but dropped to 3dB in 1h, and hearing recovered fully within 2h. The affected hearing frequency should be considered when estimating ecological impacts of anthropogenic sound on seals. Between 2.5 and 16kHz the species appears equally susceptible to TTS.

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