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-looking sonars for salmon detection. A two-element sidescan installation looking 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 trajectories were identified and counted against a background of acoustic reverberation from the river surface and bottom sediments. Occasional, strong interference from boat wakes and wind-induced surface waves and bubbles was observed. Fish echo trajectory analyses allowed extraction of upstream and cross-stream swimming speeds. For some targets acoustic multipaths from surface and bottom reflections were distinguishable from direct echoes at ranges up to 150 m. Acoustic models of surface and bottom boundary backscattering, combined with bubble layer scattering and attenuation under breaking wave conditions, were able to match the observed background echo levels. Comparisons of model and data predict that under ideal conditions fish detection should be possible at ranges up to 250 m, but under windy conditions detection was limited to less than 70 m.