ACOUSTIC SIZE VS. REAL SIZE RELATIONSHIPS FOR COMMON SPECIES OF RIVERINE FISH

Target strengths of brown trout, rainbow trout, roach, perch, dace and chub, crucian and common carp, bleak and bream were determined for tw o ultrasonic frequencies of 200 kHz and 420 kHz using dual-beam sonar directed horizontally at tethered individual stunned fish of known siz e whilst being rotated through 360 degrees by means of a carousel stru cture. 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 a nd mean all-aspect (the average acoustic size of an individual fish ro tated through 360 degrees). In all, 182 fish were insonified in this w ay with more than 800 sonar runs (= replicated rotations). In all indi viduals, the largest target strength was for side-aspect, the lowest f or head/tail-aspect and the mean all-aspect was intermediate. Statisti cally significant linear regressions of target strength on log length or log weight (Y = aX + c) were fitted to the data for individual spec ies with an adequate number of individuals of sufficiently wide size r ange. Regressions incorporating all three kinds of target strengths, t hree variants of length (standard, fork and total) and weight have bee n calculated for each of two frequencies. In addition to individual sp ecies regressions, significant pooled all-species regressions (n = 180 ) were obtained but these were less useful than were family regression s for salmonids (n = 41) and cyprinids plus perch (n = 114). The compa rable 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 speci es, comparable regressions at different frequencies were similar. This study demonstrates the importance of body aspect for describing relat ions between acoustic size and real size in freshwater fish species. ( C) 1998 Elsevier Science B.V.