Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys

Cutter, G. R. Jr and Demer, D. A. 2007. Accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys. – ICES Journal of Marine Science, 64. Multibeam echosounders can improve the efficiency and the precision of acoustic-survey estimates by providing greater sampling volumes than single-beam echosounders. For a multibeam echosounder, the target strength of fish can vary with its pitch, roll, and yaw. Here, normalized, acoustic backscatter patterns from fish schools are modelled by beam-incidence angle,0–180°, considering the scattering-directivity patterns of each fish. Variation of pitch angle causes a decrease in the mean and an increase in the variance of the backscatter in the vertical beam, but has no effect on the backscatter in the outer beams. Conversely, variation of the yaw causes a decrease in the mean and an increase in the variance of the backscatter in only the outer beams. Because the fish-scattering model predicts different backscatter at dorsal- vs. lateral-incidence angles, backscatter did vary with roll angle. In the hypothetical case of fish avoiding a vessel, the backscatter decreases strongly the outer beams. The results of the model were compared with multibeam measurements of fish schools. In general, the measured mean backscatter vs. beam-incidence angle was nearly uniform. The methods described here provide an approach to accounting for scattering directivity and fish behaviour in multibeam-echosounder surveys.