Some anthropogenic sounds may cause a partial hearing loss (temporary threshold shift (TTS)) depending on a number of variables including the frequency and intensity of the sound, duration of exposure, etc. (see How do you determine if a sound affects a marine animal?) In addition to fatigue of the sensory hair cells of the inner ear, exposure to high enough levels of sound for a long enough period of time may also result in a hearing loss because the sound causes mechanical damage to the hair cells.

Studies have shown that exposure to loud sounds for a few minutes to a few hours may cause TTS in some (but not all) fish species^[1]. For example, fathead minnows experienced TTS after the playback of boat engine noise at 142 underwater dB for 2 hours^[2], whereas goldfish exhibited a TTS after exposure to 166-170 underwater dB of white noise for 10 minutes. In both studies, hearing returned to normal over time, but it appeared that recovery varied as a function of the frequency and duration of exposure^[3].

Sensory hair cell damage due to exposure to sound has been observed in several fish species, but not in others exposed to the same sound levels. Loss of sensory hair cells was observed in the oscar (a cichlid species) four days after exposure to 300 Hz continuous tones at 180 underwater dB for 1 hour^[4]. Similarly, the sensory hair cells of pink snapper exposed to an operating airgun showed significant damage up to 58 days after air-gun exposure^[5].

The inner ear and other tissues of some fish species, including the rainbow trout (shown here), showed no physiological damage when exposed to intense sounds associated with low-frequency (170-320Hz) and mid-frequency (2.8-3.8 kHz) sonar. Image credit: U.S. Environmental Protection Agency.

All fish species studied to date continuously produce hair cells throughout their lives, and hair cells in several species are known to regenerate after being damaged by ototoxic drugs^[6][7][8]. Regeneration is correlated with a functional recovery of hearing ability^[9]. As a consequence of the ability to repair and regenerate hair cells, permanent threshold shift (PTS) is not thought to occur in fish.

In contrast to the effects seen in the above studies, caged rainbow trout, channel catfish, and hybrid sunfish exposed to a US Navy SURTASS LFA sound source (maximum received level of 193 underwater dB) for 324 or 628 seconds did not show exposure-related damage in the inner ear and other tissues^[10]. Additionally, exposure-related damage was not observed when these fish were exposed to mid-frequency signals (maximum received level 210 underwater dB) for 15 seconds. It is important to note that the duration and level of exposure in this study are much longer than would be encountered by fish in the ocean.