Target strength spectra of tracked individual fish in schools

Broadband sonar echoes received from individual fish in schools contain information that might be useful for species discrimination. The echo characteristics are closely related to the incident angle of the sound to fish body, corresponding to the apparent tilt angle of a fish from a viewpoint of a transducer. Therefore, it is necessary to isolate individual fish echoes and estimate the incident angles. In this study, a broadband split-beam echo sounder system was used to extract features from free-swimming fish schools. Echoes from individual fish in dense schools were measured and their position was estimated using the system with a downward-oriented acoustic beam. Individual fish were tracked in schools of Japanese jack mackerel Trachurus japonicus, chub mackerel Scomber japonicus, and red sea bream Pagrus major with the system. The target strength (TS) spectra of echoes ranging from 80 to 120 kHz were calculated and sorted on the basis of the incident angle, which was estimated by the tracking result. Using this approach, TS spectra were obtained from entire schools of free-swimming fish, and the obtained TS spectra were found to be dependent on fish species and the incident angles of the individual fish involved.