| Summary: | Acoustic abundance estimation of dense, vertically extended aggregations of fish requires knowledge of the extinction cross section. An established method based on a comparison of the area backscattering coefficients of fish and underlying flat bottom is exercised for herring in Ofotfjorden, January 1994, as observed with the SIMRAD EK500/38 kHz scientific echo sounder and Bergen Echo Integrator. Variability in the measurement results both for the present period and with respect to earlier years is viewed in the light of the optical literature. This suggests that the extinction cross section of finite but complicated physical scatterers may be as sensitive to details of orientation as is the backscattering cross section. The possibility of exploiting this property is considered.
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