Estimation and compensation models for the shadowing effect in dense fish aggregations

Abstract

This paper addresses the problem of acoustic "shadowing" (i.e. first-order scattering) using an heuristic approach. It is shown that the relationship between the shadow coefficient - the proportional reduction of the acoustic energy due to the shadowing effect of fish (Zhao et al., 1993) - and the apparent area backscattering coefficient of the fish is practically linear, and that the linear relationship will hold true even with inhomogeneous fish distributions. Based on this finding a simple linear model for the estimation of the σe/σ (extinction cross-section/acoustic cross-section) ratio is developed. The model applies the reference target method of Foote et al. (1992), which allows the shadow coefficient to be determined. A model is also developed for compensation of the shadowing effect. This model can also be used to deduce the maximum-detectable fish density when the σe/σ ratio is known; and the maximum-detected apparent fish density may, in turn, suggest an upper limit of the σe/σ ratio under the specific survey condition. A correction table is provided to serve as an approximate reference and a registration from a typical herring survey is shown as an example. © 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.