Low frequency scattering from fish schools: A comparison of two models

A theoretical comparison between two scattering models for fish schools was performed. The effective medium approach, based on the work of Foldy [Phys. Rev. 67, 107–109 (1945)], which has previously been used to describe scattering from bubble clouds, was used to calculate the variations of the scattering length of a fish school with frequency, and with azimuth. Calculations were also performed with a model used previously to study collective back scattering from schools of swim bladder fish, which incorporates both multiple scattering effects between fish, and coherent interactions of their individual scattered fields [J. Acoust. Soc. Am., 99(1), 196–208 (1996)]. Two different packing algorithms were used, in order to investigate the influence of the spatial distribution of fish on the scattering response of the school. Comparison of the two models shows good agreement in the forward scattering direction, where no frequency dependent interference effects are observed. The models indicate divergent results in the back scattering direction, where the arrangement of fish in the school strongly affects the scattering amplitude. The upper frequency limit of the effective medium approach is also discussed, and the effect of the depth of the school in the water column. [Research supported by ONR.]