Find Fish

Fish finding sonar systems assist fishermen and scientists when trying to locate and identify fish underwater. These sonar units operate very similar to other types of sonar. A transducer, attached or towed by a boat, sends out an acoustic signal. This signal will reflect off the air in the swim bladder of a fish, if it has one, or the fish itself. A computer will pick up the return signal and convert it into fish images on a screen. The images on the screen appear as arches because of the movement of the fish through the beam of acoustic energy.

Image showing the arch formed by a fish as it passes through the sonar beam.
Example of how an arch is formed as a fish passes through the sonar beam. Courtesy of Lowrance Electronics, Inc.

Fish finders operate at high frequencies, around 20-200 kHz. The higher end of this frequency range gives detail of the target and can even separate two fish as separate arches. The lower end of the frequency range give a greater depth range; however, less detail can be displayed.

A fish finder display showing the seafloor and individual fishes.
Fish finder display shows the seafloor gently sloping down to 33.9 feet (black horizontal line with gray beneath). Individual fishes appear as arches on the display. Courtesy of Lowrance Electronics, Inc.
New fish finders have the ability to not only locate fish, but also differentiate between species of fish.

Scientists are developing new and improved methods to differentiate between the marks, also called echo signatures. Each species of fish has an unique size and shape of its swim bladder. The differences in swim bladders cause differences in the return echo of a sonar signal. Echo signatures for specific species can then be determined and used to identify fish.

Various fish species and their acoustic signatures
The unique echo signatures of 3 different species of Hawaiian snapper. Echo signatures on the left were taken from an anesthetized fish under controlled conditions at the surface. Echoes on the right were taken from free-swimming fish at 250m deep. Differences in echo signature structure are observed between species, but differences between control and free-swimming measurements are minimal for each species. Echoes measured at the surface, under controlled conditions, can therefore be applied to identify different fish species at depth. This type of acoustic data is important for fisheries surveys. Courtesy of Kelly Benoit-Bird

This method is especially useful when studying deep-sea bottom fish. Many fish are found below diving depth, and therefore, can only be studied using submersible vehicles or fishing gear. Acoustics provide an additional means for scientists to identify bottom fish and monitor them in their natural environment.