From acoustic scattering models of zooplankton to acoustic surveys of large regions

Underwater acoustic sampling techniques provide an advantage over traditional net-sampling for zooplankton research. The research presents a methodology for extracting both biological and physical information from high frequency sonar. These methods can easily provide the information that will improve our understanding of the spatial and temporal distribution of zooplankton. Measured acoustic data converted into biological organisms and numerical physics-based scattering models were used in this research. The numerical backscattering process was modeled using the Distorted-Wave Born Approximation (DWBA) to predict the amount of sound scattered by a weakly scattering animal. Both acoustic measurement and DWBA modeled scattering patterns showed that acoustic scattering levels are highly dependent on zooplankton orientation. The acoustic backscattering from zooplankton depends on the material properties (i.e. the sound speed and density of the zooplankton), the shape and size, and the orientation relative to the incident acoustic wave. DWBA model significantly improve the accuracy and precision of zooplankton acoustic surveys. Zooplankton data measurement and DWBA model analysis provide a basis for future acoustical studies.

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Bias in Hydroacoustic Estimates of Fish Abundance due to Acoustic Shadowing: Evidence from Day–Night Surveys of Vertically Migrating Fish

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f92-240 ◽

1992 ◽

Vol 49 (10) ◽

pp. 2179-2189 ◽

Cited By ~ 42

Author(s):

A. R. Appenzeller ◽

W. C. Leggett

Keyword(s):

Fish Community ◽

Acoustic Scattering ◽

Fish Abundance ◽

Diel Changes ◽

Light Levels ◽

Abundance Estimates ◽

Acoustic Quantification ◽

Acoustic Methods ◽

Acoustic Surveys ◽

Acoustic Shadowing

We investigated the hypothesis that fish schooling behavior leads to underestimation of fish abundance when assessed by acoustic methods. Current methods for acoustic quantification of fish abundance rely on the assumption that fish biomass and numerical fish abundance are linearly related to acoustic scattering under all natural fish densities. However, cage experiments as well as field observations have indicated that acoustic shadowing effects occur at very dense and large aggregations of fish. Acoustic surveys of the pelagic fish community of Lake Memphremagog, Quebec, were conducted when fish were aggregated and dispersed. These differences in aggregation were related to diel responses to light levels. We found that estimates of fish abundance, as measured by echo integration, were consistently and significantly lower when fish were aggregated in dense schools. This bias was not due to diel changes in average echo level per fish, which exhibited no relationship to diel changes in vertical and hortizontal distributions. We conclude that the reduced abundance estimates obtained when fish were aggregated resulted from acoustic shadowing. Our data suggest that this bias may be as large as 50%.

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