Fish-Finding

a method of locating (for commercial purposes) and studying the behavior of fish, marine mammals, and other aquatic organisms by means of active and passive underwater sonar detection. Fish-finding systems generally are of an active type that can determine the species, size, and number of fish, as well as the position of the schools.

The basic difference between fish-finding and underwater sonar detection (hydrolocation) lies in the sound reflection and scattering characteristics of aquatic organisms. Since the acoustic properties of fish tissues and seawater are similar, the scattering and absorption characteristics are determined mainly by the swim bladder. There is a very substantial difference between the acoustic impedances of water and of the gas within the bladder. According to calculations, the swim bladder should reflect approximately 50 times more sound energy than the body. However, measurements show that this energy is two to five times less. The apparatus used in fish-finding includes both underwater sonar detectors and echometers.

The effectiveness of fish-finding can be reduced by the presence of scattering layers that are created, for example, by plank-tonic organisms. The layers may also be created by reverberation (surface, bottom, and volume), which is often difficult to distinguish from the reflections of schools; by temperature non-uniformities (horizontal and vertical), which register like scattering layers; by wake echoes that are frequently encountered when a large number of vessels are operating in an area, and by interference from beams emitted by sonar detectors and echometers on various vessels.