DETECTION OF MIGRATING SALMON IN THE FRASER-RIVER USING 100-KHZ SIDESCAN SONARS

During a 10-day period in September 1995, field trials were conducted in the Fraser River near Mission, B.C., to evaluate 100-kHz side-looki ng sonars for salmon detection. A two-element sidescan installation lo oking transverse to the river flow detected echo traces due to salmon at ranges up to 250 m in water 6-13 m deep. The salmon target trajecto ries were identified and counted against a background of acoustic reve rberation from the river surface and bottom sediments. Occasional, str ong interference from boat wakes and wind-induced surface waves and bu bbles was observed. Fish echo trajectory analyses allowed extraction o f upstream and cross-stream swimming speeds. For some targets acoustic multipaths from surface and bottom reflections were distinguishable f rom direct echoes at ranges up to 150 m. Acoustic models of surface an d bottom boundary backscattering, combined with bubble layer scatterin g and attenuation under breaking wave conditions, were able to match t he observed background echo levels. Comparisons of model and data pred ict that under ideal conditions fish detection should be possible at r anges up to 250 m, but under windy conditions detection was limited to less than 70 m.