Consideration on acoustic scattering by individual fish schools

The echo-integration (EI) method is the primary method for fisheries resource surveys by acoustics. Application of EI results to estimation of individual school abundance and/or to school descriptor for species identification, however, has not been well developed. This paper first considers, based on the basic theory of the EI method, the raw volume backscattering strength of an individual school and the school section scattering strength which are observed by the vertical beam of an echo sounder. Next, approximation methods to estimate the total school scattering strength of an individual school are proposed. The methods are verified by exact computer simulation of school echoes. The methods are also applied to actual echo data obtained by a quantitative echo sounder to confirm applicability. The section scattering strength is appropriate as an index of school abundance, but the school scattering strength, especially the strength combined echo sounder data with sonar 3D distribution data, is most appropriate for the abundance index of an individual school.