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.
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.
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.
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.