The effects of the structure of a fish school on the backscattering signal are reported. A numerical model has been employed to calculate the effective scattering function of the small fish schools, incorporating all orders of multiple scattering between fish. It is shown that when the fish school has certain order, regular scattering patterns may appear resulting from the interference rather than the multiple scattering. The interference peaks are due to in-phase scattering and can be explained by the Bragg reflection mechanism. Such regular interference patterns are gradually degraded as the random deviation in each individual fish location from its mean position increases. When the random deviation is strong enough the widely used random phase approximation (RPA) method is shown to be valid. Furthermore, a validity criterion has been found for the RPA approach, that is, k sigma greater than or equal to 1, with k being the wavenumber, and sigma the standard deviation of the fish position distribution. (C) 1999 International Council for the Exploration of the Sea.
