Measurement of scattering directivity and behavior of fish schools using multibeam sonar

Abstract

Fish dispersion and abundance are commonly estimated from volume backscattering coefficients (sv) of animals located directly beneath the survey vessel, inside a narrow transducer beamwidth. Fish density (♯/m2) is estimated from a ratio of integrated sv and a representative backscattering cross-sectional area. Fish biomass is estimated from the product of fish density and the area sampled. Uncertainty in biomass estimates can arise from small sampling volumes, and the scattering directivities of the fish coupled with their orientations relative to the acoustic beam. Multibeam sonars can be used to increase the observational volumes, but add the complexity of a wider range of incident angles. If fish size and packing density are constant throughout a school, then the variation in sv versus aspect angle can be attributed to fish orientation. With modeling and multibeam sonar measurements, the following null hypotheses are explored: (1) The variability of sv versus roll angle is nearly uniform; (2) For measurements made with beams at or near vertical, sv variability is primarily due to the pitch angle; (3) For measurements with nonvertical beams, such as the outer beams of multibeam echosounders, sv variability is primarily due to the yaw angle.