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Target strengths of brown trout, rainbow trout, roach, perch, dace and chub, crucian and common carp, bleak and bream were determined for two ultrasonic frequencies of 200 kHz and 420 kHz using dual-beam sonar directed horizontally at tethered individual stunned fish of known size whilst being rotated through 360° by means of a carousel structure. This provided a data base for each fish of real size in length or weight and target strength for side body aspect, head/tail aspect and mean all-aspect (the average acoustic size of an individual fish rotated through 360°). In all, 182 fish were insonified in this way with more than 800 sonar runs (=replicated rotations). In all individuals, the largest target strength was for side-aspect, the lowest for head/tail-aspect and the mean all-aspect was intermediate. Statistically significant linear regressions of target strength on log length or log weight ( Y = aX + c ) were fitted to the data for individual species with an adequate number of individuals of sufficiently wide size range. Regressions incorporating all three kinds of target strengths, three variants of length (standard, fork and total) and weight have been calculated for each of two frequencies. In addition to individual species regressions, significant pooled all-species regressions ( n =180) were obtained but these were less useful than were family regressions for salmonids ( n =41) and cyprinids plus perch ( n =114). The comparable family regressions differed significantly, either in slope or in elevation (where slopes were in parallel) and the salmonid regression lay lower than the cyprinid plus perch one. In some but not all species, comparable regressions at different frequencies were similar. This study demonstrates the importance of body aspect for describing relations between acoustic size and real size in freshwater fish species. |
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