Estimation and compensation models for the shadowing effect in dense fish aggregations
Zhao, X.; Ona, E. (2003). Estimation and compensation models for the shadowing effect in dense fish aggregations. ICES J. Mar. Sci./J. Cons. int. Explor. Mer 60: 155-163
In: ICES Journal of Marine Science. Academic Press: London. ISSN 1054-3139; e-ISSN 1095-9289, more
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| 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. |
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