Loss of hearing from exposure to sound may be a temporary or permanent condition. When hearing sensitivity recovers to normal after a temporary loss, it is referred to as a temporary threshold shift (TTS). When hearing loss is permanent, it is called a permanent threshold shift (PTS). The extent to which temporary hearing loss occurs depends on a number of variables including the frequency and level of the sound, duration of exposure, the health of the organism, etc. (see How do you determine if a sound affects a marine animal?) Exposure to high enough levels of sound for a long enough period may also result in hearing loss because the sound mechanically destroys the sensory hair cells of the inner ear (1).

Several studies have shown that exposure to loud sounds for a few minutes to a few hours will cause (TTS) in fishes (2). For example, fathead minnows experienced temporary threshold shift after the playback of boat engine noise at 142 underwater dB for 2 hours (3), whereas goldfish exhibited a threshold shift after 10 minutes of exposure to 166-170 underwater dB of white noise (4). In both studies, the hearing returned to normal over time, but it appears that recovery varies by the frequency and duration of exposure (5). The amount of hearing loss appears to relate to how loud the noise is compared to the threshold of hearing at that frequency (6). At frequencies where a fish is more sensitive (i.e., has a lower threshold), TTS produced by constant white noise was greater. This hypothesis was also supported when three fish species of varying hearing sensitivity were exposed to the sounds of an airgun (7). A predictable linear relationship was found between TTS and the difference between the sound pressure of the noise and the baseline hearing threshold of the fishes.

Loss of sensory hair cells due to exposure to sound has been observed in oscars, a species of cichlid, four days after exposure to 1 hour of 300Hz continuous tones at 180 underwater dB (8). Similarly, the ears of pink snappers exposed to an operating airgun showed damage, with the most extensive damage being found 58 days after exposure (9).

A number of fishes continue to produce hair cells throughout the duration of their lives and hair cells have regenerated after being damaged by drugs (10, 11, 12). Are fishes able to regenerate hair cells damaged or destroyed from exposure to sound as well? This, along with continued research on the effects of different sounds and exposure times, are important research topics for the future.

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