SALMON AND HERRING SCHOOL DETECTION IN SHALLOW WATERS USING SIDESCAN SONARS

This work describes results from two recent field programs exploring t he feasibility of high-frequency, side-looking sonar systems for fish detection in shallow waters. The first project in September 1995 was a imed at detection of migratory salmon in the Fraser River near Mission , British Columbia. Using a fixed installation of 100-kHz sidescans lo oking transverse to the river flow from one bank, individual salmon ta rgets were observed at ranges up to 200 m in waters 4-12 m deep. A sec ond experiment in May 1996 sought to image spawning herring schools us ing towed and azimuthally scanning 100 and 330 kHz sidescans in a shal low marine region near Escuminac, New Brunswick. Herring schools were successfully imaged at ranges up to 150 m in waters only 3-4 m deep. I n both cases, the fish targets were identified and quantified against a background of acoustic reverberation using manual recognition techni ques. Example sonograms from both experiments will be discussed. In bo th experiments the detection efficiency diminished with range due to i nterference from surface and bottom boundary back-scattering. Occasion al, strong interference from boat traffic and breaking-wave induced bu bble layers drastically reduced fish detection capability. (C) 1998 El sevier Science B.V.