Similar Hearing Thresholds at 8 kHz, but Dissimilar Susceptibility of Hearing to Damage by 8 kHz Sounds in a Harbor Porpoise, a Harbor Seal, and a California Sea Lion

For each mammalian species, the pattern of susceptibility to temporary hearing threshold shift (TTS) with changing frequency has been assumed to be related to the pattern of hearing thresholds with changing frequency (the basic audiogram). This relationship was tested with one individual of each of three marine mammal species: one odontocete (harbor porpoise [Phocoena phocoena]) and two pinniped species (harbor seal [Phoca vitulina] and California sea lion [Zalophus californianus]). Audiograms showed that they had similar basic hearing thresholds at 8 kHz (61, 59, and 60 dB re 1 µPa, respectively). Hearing thresholds for narrow-band sweeps at 11.3 kHz (the hearing frequency at which the greatest TTS occurred in all three species) before and after exposure for 1 h to a continuous, constant-amplitude, one-sixth-octave noise band centered at 8 kHz at several sound exposure levels (SELs) showed that the TTS experienced by the three individuals differed. The 6 dB TTS-onset SELs were for the harbor porpoise: 169 dB re 1 µPa2s; for the harbor seal: 182 dB re 1 µPa2s; and for the California sea lion: 176 dB re 1 µPa2s. The pattern of increase in TTS with increasing SEL was steeper for the harbor porpoise and harbor seal than for the California sea lion. This means that TTS onset occurs at a different sensation level (i.e., at a different number of dB above the basic hearing threshold) in each of the three marine mammal species. Therefore, different policies, recommendations, or regulations for permissible sound exposure are needed for each species.