An evaluation of a steerable sidescan sonar for surveys of near-surface fish

A towed 330 kHz sector scanning sonar was evaluated for surveys of near-sur face migratory salmonids (Oncorhynchus sp.) and other fish in the Strait of Georgia near the mouth of the Fraser River near Vancouver, BC (Canada). De tection of large near-surface fish presents a serious challenge to conventi onal net trawls or echo-sounder surveys due to vessel avoidance behaviour b y the fish. The sonar was towed beside a research vessel on a custom-built towfish at a nominal depth of 18 m. The optimum configuration was determine d to be with the sonar scanning a 30 degrees vertical sector oriented perpe ndicular to the tow direction, at a ping rate of 5 Hz and a sector scan per iod of 5 s. Fish were detected up to a range of 100 m, Limited by systemic noise and backscattered reverberation from the sea surface. Some evidence o f vessel-avoidance behaviour by the fish was observed. Near surface tempera ture and salinity gradients are shown to induce downwards refraction of the sonar beam, resulting in biases in apparent depth and target strength of t he fish. A simple model of the sonar performance in this scanning mode, inc luding beam-pattern effects, is used to generate curves of fish target dete ctability and beam-pattern induced bias as a function of range and fish tar get strength. After correction for refraction effects, beam-pattern bias, a nd detectability, the survey results show reasonable agreement with net tra wl and riverine escapement data. (C) 2001 Elsevier Science B.V. All rights reserved.