Measurement of Fish School Backscattering Strength Directivity Using Omnidirectional Scanning Sonar

Abstract

The horizontal acoustic backscattering strength of a fish school is an important factor for fisheries and for estimation of the size of a fish school using omnidirectional scanning sonar. However, the backscattering strength of a fish school is influenced by their behavior and orientation because the individual fish in the fish school have strong directivity of target strength. This paper discusses a method for estimating the directivity of the backscattering strength of the fish school by analyzing fish school behavior. Sonar data were collected by a purse seine fishing boat equipped with the quantitative scanning sonar FSV-30R in Barents Sea and on the Pacific coast of northern Japan. Swimming speed, swimming direction, volume backscattering strength (SV_–SCH), and the target strength of the fish school (NTS) were analyzed for several swimming fish schools. The results showed strong directivity in the backscattering strength at the direction perpendicular to the swimming course; this result is similar to that of the directivity of the target strength. The directivity varied in relation to the distribution of the orientation of the fish, resulting from both the swimming speed and the direction of the fish school. These findings will enable the estimation of fish school abundance more precisely.