Continuous monitoring of fish in a shallow channel using a fixed horizontal sonar

An acoustic monitoring program of herring migration in Drogden channel, near Copenhagen, Denmark was conducted from June 1996 until the end of May 1997. Fixed 100-kHz side-looking sonars provided nearly continuous surveillance in a 1-km-wide by 12-m-deep navigation channel. Water temperature, salinity, and current profiles were simultaneously monitored at this site. The sonars were positioned to insonify regions near the seabed at ranges up to 800 m, such that the typical reverberation was due to low-grazing angle seabed backscatter. It was found that under normal, weakly stratified flow conditions, fish schools attributable to herring (Clupea harengus) were observed from the 50- to 500-m range. This could be done despite interference from the dense vessel traffic, specifically direct echoes from hulls, propeller cavitation noise, and bubbly wakes. At close ranges (<150 m) hyperbolic trajectories attributable to individual herring were observed, with horizontal advection speeds in close agreement with measured current magnitudes. It was further observed that occasional intrusions of saline bottom waters created strong upward-refracting conditions that significantly limited the range for fish school detection. Ray-tracing analysis is used to define the insonified areas and describe the backscattered reverberation under normal and stratified flow conditions. It is shown using simulations of fish-school echoes that seabed-reflected multipaths can create an upward bias in fish-school densities calculated using echo-integration techniques.