Acoustic cross sections and resonance frequencies of large fish schools

Abstract

A prerequisite for stable inversion of bioacoustic parameters of fish schools from large-scale broadband acoustic observations is a theoretical model that permits fast and accurate calculations of acoustic cross sections and school resonance frequencies based on realistic geometrical models of fish schools. The schools of commercially important species, such as sardines, anchovies, and herring, may be characterized by dense nuclei which contain tens of thousands of individuals (N) with separations (S) on the order of one fish length, and diffuse “fuzz” regions with fish at significantly larger separations. Numerical computations of cross sections and school resonances based on the fundamental equations of multiple scattering for point scatterers for these fish school geometries will be presented. Initial results indicate that bubble cloud frequencies of large schools depend primarily on the average spacing between fish in the nuclei and are essentially independent of school size and shape. It will be shown that theoretical calculations of bubble cloud frequencies (based on previously reported values of N and S of sardine schools) are consistent with Diachok's (1999) observations of the average resonance frequency of sardine schools which were derived from broadband transmission loss measurements. [Work is supported by the Office of Naval Research.]