Fish Finding Sonar

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. Different species of fish actually have different shaped and sized swim bladders. These differences cause sound to reflect differently from each fish. Therefore, by studying the return echoes, scientists can now find 'signature echoes' for specific species of fish.

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