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 exploring the feasibility for estimating spawning Atlantic herring (Clupea harengus) abundance using a 330-kHz sidescan sonar in a shallow coastal region near Escuminac, New Brunswick. Herring schools were observed 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 typically 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-tracing analysis, surface- and bottom-reflected multipaths from an ensemble of targets are shown to increase the echo amplitude and reverberation time, producing large overestimates (4-27 times actual) in herring densities 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 - 103 000 individuals. These total school sizes and densities were consistent with expectations for spawning herring.