Can stationary bottom split-beam hydroacoustics be used to measure fish swimming speed in situ?

We used split-beam hydroacoustics (Simrad EY500, 70 kHz) as a method for estimating fish swimming speed in situ. The method was first evaluated in the field using underwater video cameras (stereocinematographic method, SCG) to estimate accurate fish swimming speeds. The mean and distribution of swimming speeds of 15-cm brook trout (Salvelinus fontinalis) obtained by the two methods were not statistically different (average 17.8 cm s(-1) with split-beam and 18.6 cm s(-1) with SCG). We then used the split-beam technique to measure swimming performance in situ for fish assumed to be yellow perch (Perca flavescens) and alewives (Alosa pseudoharengus) in two lakes in New York State, USA. The measured swimming speeds ranged from 0.5 to 6 body length (BL) per second for juvenile and adult fish. Other laboratory studies on swimming speeds have reported values in the same range. However, measured swimming speeds for smaller fish were unrealistically high (2-32 BL s(-1)) Advantages of the split-beam method are the ability of measuring swimming speed independently of visibility, with minimal disturbance and at large distances. Disadvantages are the inability to distinguish species observed and some variance in target location, which results in calculated average swimming speeds of 2.6 cm s(-1) even for a stationary target. (C) 2000 Elsevier Science B.V, All rights reserved.