Black rockfish in Monterey Bay National Marine Sanctuary, California. Photo courtesy of NOAA Photo Library.

Fishes use sound to find prey, avoid predators, and find and communicate with mates (see How do fish communicate using sound? and How do fish use or make sounds when feeding?). Studies that investigate the effects of sound on fishes have been limited. Most fish experiments have been conducted in the laboratory where conditions can be controlled and fishes can be observed. However, results obtained in a laboratory setting may not reflect natural responses in the wild ^[1] and some fish are too large or difficult to keep in captivity. In the wild, fish are known to behave differently depending on location, temperature, physiological state, age, body size, and school size ^[2]. Different fish species will also react differently to sound. The behavioral effects of sound on fish, in general, should be made with caution.

Responses by fish to sounds might range from the fish exhibiting an awareness of the sound, to small movements, or escape responses. Depending on the degree of the behavioral response, there may not be a significant impact . An example of a significant impact to a population would be if sound causes fish to move away from foraging or breeding grounds ^[1].

Several studies demonstrated an effect on fish behavior from sounds produced by airguns used for seismic exploration. In two studies, catch rates of commercially important species including cod, haddock, blue whiting, and Norwegian herring, declined in areas where airguns were used ^{[3] [4]}. These studies also showed that catch rates of these species increased 30-50 km away from the sound source, suggesting that fishes avoided the areas where the airguns were operating. A study of caged pink snapper suggested that fish would have fled when they heard an approaching airgun array if they had been able to swim away ^[5]. In another study, rockfishes began to exhibit behavioral changes at 161 underwater dB. When the sound level was 180 underwater dB, blue rockfish swam in tight circles and schools of black rockfish moved to the seafloor ^[6].

Some studies have found no biologically significant effect of airgun sounds on fish behavior. In one study ^[7], coral reef fishes contained in field enclosures temporarily increased swimming speed and changed swimming direction when exposed to an airgun (peak source level of 196 underwater dB at 1 m) at distances of 0 to 7 m. However, the fish returned to normal swimming activities shortly thereafter. Repeated exposure to airgun sounds appeared to result in a lessening of these responses, suggesting potential habituation to the disturbance. In another study, free-swimming marine fishes on an inshore, rocky reef, including pollock, cod, and whiting, also showed no significant behavioral response to sounds produced by an airgun. At 16 m from the source, the fish received peak sound levels of 210 underwater dB, and at 109 m from the source, they received 195 underwater dB ^[8]. Although fishes displayed an initial startle response (a sudden bending of the body, which is a characteristic escape response), there was no movement away from the sound source. Only when the airgun was moved within 5 m of the fish (received level of 218 underwater dB) and the explosion of the airgun was visible to the fish, did the fish react by moving away.

Noise generated by boating activities has been shown to affect fish behavior. Noise-reduced vessels designed to minimize fish avoidance were found to detect more Alaskan walleye pollock under survey conditions than conventional vessels ^[9]. Sounds produced by ferryboats, hydrofoils, and the outboard motors of small boats were shown to affect blue-fin tuna in large oceanic pens ^[10]. Boating noise significantly influenced the swimming behavior of tuna, their position in the water column, and their schooling structure. Schooling in tuna has been identified as a strategy to enhance the accuracy of migration routes and foraging efficiency, and a departure from normal schooling patterns could pose a significant threat to fish survival in this species.

A school of bluefin tuna (Thunnus thynnus). Boating noise was shown to impact schooling structure in this fish species, a behavior that is important during migration and foraging. Image credit: NOAA.

There has been concern over the potential effect of the Acoustic Thermometry of Ocean Climate (ATOC) project, in which sounds transmitted underwater were used to measure ocean temperature. The ATOC sounds were very different from the impulses produced by airguns. The signals lasted much longer (20 minutes) and sounded like low rumbles. Its potential effect on the distribution and behavior of fishes prompted a study on three species of rockfish, yellowtail rockfish, kelp rockfish, and blue rockfish ^[11]. Rockfishes were kept in an enclosure and observed before being exposed to sound (a "control" period), during ramp-up of transmissions similar to those of the ATOC source, and during a 20-minute ATOC-like signal. The fishes did not move towards or away from the sound source despite receiving levels of sound of 145-153 underwater dB. They also did not change their behavior between the control period and the exposure periods.