Lateral-aspect target strength measurements of in situ herring (Clupea harengus)

Abstract

Surveys of schooling herring with the new multi-beam sonar (Simrad MS70) pose new challenges when converting the echo energy to estimates of biomass. Since the sonar projects horizontally, data and models of lateral-aspect herring target strength TS are needed. In this study, TS of herring are measured with a horizontally-projecting split-beam echosounder (Simrad EK60). Target tracking methods are used to estimate swimming angles relative to the horizontal θ of individual herring within schools and layers, and to evaluate how θ and TS change vs. depth z. Measurements of θ and TS are input to a model describing TS as a function of θ and z. The results indicate that the mean lateral-aspect TS of in situ herring depends on z. Moreover, the mean lateral-aspect TS is more sensitive to z than is the mean dorsal-aspect TS predicted by a published model. At z = 50 m, the mean lateral-aspect TS is nearly 2.5 dB higher than the mean dorsalaspect TS. Conversely, at z = 350 m, the lateral-aspect TS is 5 dB lower. These results suggest that herring swimbladders do not compress uniformly with increasing pressure, but compress dorso-ventrally more than laterally.