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

An acoustic monitoring program of herring migration in Drogden channel, nea r Copenhagen, Denmark was conducted from June 1996 until the end of May 199 7. Fixed 100-kHz side-looking sonars provided nearly continuous surveillanc e in a 1-km-wide by 12-m-deep navigation channel. Water temperature, salini ty, and current profiles were simultaneously monitored at this site. The so nars were positioned to insonify regions near the seabed at ranges up to 80 0 m, such that the typical reverberation was due to low-grazing angle seabe d backscatter. It was found that under normal, weakly stratified flow condi tions, 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 tra jectories attributable to individual herring were observed, with horizontal advection speeds in close agreement with measured current magnitudes. It w as further observed that;occasional intrusions of saline bottom waters crea ted strong upward-refracting conditions that significantly limited the rang e for fish school detection. Ray-tracing analysis is used to define the ins onified areas and describe the backscattered reverberation under normal and stratified flow conditions. It is shown using simulations of fish-school e choes that seabed-reflected multipaths can create an upward bias in fish-sc hool densities calculated using echo-integration techniques. (C) 1999 Acous tical Society of America. [S0001-4966(99)01806].