ACOUSTIC MONITORING OF SALMONID DENSITY, TARGET STRENGTH, AND TRAJECTORIES AT 2 DAMS ON THE COLUMBIA RIVER, USING A SPLIT-BEAM SCANNING SYSTEM

Horizontal acoustic scanning was used at two hydropower dams on the Co lumbia River to monitor downstream migrating juvenile salmonids. Two s plit-beam systems with computer-controlled rotating capabilities monit ored the fish movement patterns in the forebay areas of the Rocky Reac h and Rock Island dams. Scanning consisted of 15 or 17 horizontal aimi ng angles, separated in 10 degrees increments each monitored for 4 min , and tilted downwards from the horizontal at either 10 degrees or 15 degrees. Scanning was carried out for 24 h/day for periods of five day s in May 1995 and was part of a larger study which lasted from 4-6 wee ks. Estimates of mean fish density, target strength and trajectory wer e made for 5-m range cells within each horizontal scanning angle. Resu lts show that fish movement patterns differed in the two dams. At Rock y Reach Dam, fish followed a circular trajectory pattern which was sim ilar to the observed pattern of river flow and fish target strengths w ere higher in areas of low water velocity. At Rock Island Dam, fish be havior differed according to whether turbine units were operating or n ot. When turbines were in operation, fish moved towards the flow which contrasted with their milling behavior when the turbines were not ope rating. The fish target strengths were also greater in the latter situ ation. The difference in body aspect being insonified may account for this random acoustic aspect of milling fish compared with primarily he ad/dorsal aspect of fish oriented to the operational turbines. (C) 199 8 Elsevier Science B.V.