Fish schooling behavior inferred from differences between backscatter levels in Doppler sonar beams

Abstract

The general availability of Doppler profilers on survey ships provides a convenient source of acoustic backscatter data. Aside from calibration issues, caution must be exercised when analyzing this data because each of the diverging beams has a different interaction angle with the scatterers. In particular, for targets with directional scattering characteristics a different backscatter strength will be seen in each beam. The availability of data from multiple beam directions can however provide information on scatterer orientation when groups of such scatterers undertake coherent motion. This effect is demonstrated in observations of Norwegian spring spawning herring (Clupea harengus). In data collected while these fish are actively migrating with mean swimming speeds of 20 cm<th>s−1, a difference of 5 dB is seen in volume backscatter strength depending on the direction of fish movement with respect to the acoustic beams. In contrast, in data collected when these fish schools have less well defined movements with a mean swimming speed of less than 10 cm<th>s−1, a difference of less than 1 dB is seen.