Shallow-water salmon and herring detection using 100-kHz sidescan sonars

Abstract

Acoustic results of fish detection using fixed 100-kHz sidescan sonar installations in the Fraser River (British Columbia) and in the Oresund (near Copenhagen, Denmark) are reviewed. Looking transverse to the river flow, echo traces due to migratory salmon were observed at ranges up to 250 m in water 6 to 13 m deep. The salmon trajectories were readily identified and counted against the background of reverberation from the river surface and bottom sediments. Similarly in the Oresund, long-term monitoring detected migrating herring schools above background reverberation at ranges up to 500 m in water 10 to 13 m deep. In both cases, occasional strong backscatter interference from vessel wakes and wind-induced surface bubble layers was observed. In the Oresund, changes in acoustic propagation conditions due to bottom inflow of more saline oceanic water drastically reduced fish detectability. For salmon in the river, acoustic multipaths due to boundary reflections were distinguishable from direct echoes at ranges up to 150 m. The general problems of shallow-water boundary reverberation and multipath reflection focusing will be reviewed, with discussion on how to compensate for these waveguide effects.