Journal Article

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

Xianyong Zhao and Egil Ona

in ICES Journal of Marine Science

Volume 60, issue 1, pages 155-163
Published in print January 2003 | ISSN: 1054-3139
Published online January 2003 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1006/jmsc.2002.1319
Estimation and compensation models for the shadowing effect in dense fish aggregations

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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.

Keywords: acoustics; compensation; dense fish aggregation; estimation; model; shadowing effect

Journal Article.  0 words. 

Subjects: Environmental Science ; Marine and Estuarine Biology

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