DETECTION OF ATLANTIC HERRING (CLUPEA-HARENGUS) SCHOOLS IN SHALLOW WATERS USING HIGH-FREQUENCY SIDESCAN SONARS

This work examines results from field trials in April and May 1996 exp loring the feasibility for estimating spawning Atlantic herring (Clupe a harengus) abundance using a 330-kHz sidescan sonar in a shallow coas tal region near Escuminac, New Brunswick. Herring schools were observe d using both towed and fixed-location, azimuthally scanning techniques at ranges up to 100 m in water depths of 3-6 m. The schools were typi cally linear in shape, with lengths of 100-280 m, thickness of 4-14 m, and peak acoustic target strength near -12 dB. Using acoustic ray-tra cing analysis, surface-and bottom-reflected multipaths from an ensembl e of targets an shown to increase the echo amplitude and reverberation time, producing large overestimates (4-27 times actual) in herring de nsities when using echo-integration. Using the acoustic simulations to compensate for this reflection focusing, average population densities near 11.m(-3) were estimated, with the schools comprising 10 900 - 10 3 000 individuals. These total school sizes and densities were consist ent with expectations for spawning herring.