Salmon and herring school detection in shallow waters using sidescan sonars

Abstract

This work describes results from two recent field programs exploring the feasibility of high-frequency, side-looking sonar systems for fish detection in shallow waters. The first project in September 1995 was aimed at detection of migratory salmon in the Fraser River near Mission, British Columbia. Using a fixed installation of 100-kHz sidescans looking transverse to the river flow from one bank, individual salmon targets were observed at ranges up to 200 m in waters 4–12 m deep. A second experiment in May 1996 sought to image spawning herring schools using towed and azimuthally scanning 100 and 330 kHz sidescans in a shallow marine region near Escuminac, New Brunswick. Herring schools were successfully imaged at ranges up to 150 m in waters only 3–4 m deep. In both cases, the fish targets were identified and quantified against a background of acoustic reverberation using manual recognition techniques. Example sonograms from both experiments will be discussed. In both experiments the detection efficiency diminished with range due to interference from surface and bottom boundary back-scattering. Occasional, strong interference from boat traffic and breaking-wave induced bubble layers drastically reduced fish detection capability.