The development and evaluation of a three-dimensional, echo-integration method for estimating fish-school abundance

Abstract

A three-dimensional, echo-integration method (3DEI) which uses scanning-sonar observations of a fish school to estimate its backscattering cross section (σbss = Nσbs) was developed. Coupled with a modelled estimate of the average backscattering cross section of individual fish (σbs), the 3DEI theoretically allows estimation of the number of fish in a school (N). To test the practicality of the method, measurements were made of a metal sphere simulating fish, and several spheres simulating a fish school. The 3DEI correctly measured the σbss of each target. Next, the 3DEI was applied to echo data from a herring school in the Norwegian Sea, to estimate its σbss. Several values of σbs were estimated with a prolate-spheroid model, each assuming different distributions of fish orientations relative to the sonar beam. Dividing the 3DEI-estimated σbss by these modelled σbs shows that the resulting estimates of N were closer to the skipper's estimate than those estimated using the apparent school volume. The 3DEI measurements of σbss, modelled σbs, and resulting accuracy of N depend largely on the assumed orientations of the fish relative to the acoustic beam. © United States Government, NOAA/NMFS/AFSC 2009.