BROWN TROUT POPULATIONS OF 3 SCOTTISH LOCHS ESTIMATED BY HORIZONTAL SONAR AND MULTIMESH GILL NETS

Brown trout (Salmo trutta) populations of the three Orkney freshwater lochs-Boardhouse, Harray and Swannay-were studied in late September 19 91 using horizontally orientated sonar. Dual-beam echo counting during mobile surveys gave the following densities of fish per 100 m3: 2.94 in Boardhouse, 1.94 in Harray and 1.08 in Swannay. Multiple fish targe ts and reflections from submerged macrophytes were filtered out. The r anking of the estimated densities corresponds with the anglers' yield which was the highest in Boardhouse and the lowest in Swannay. The aco ustic signal size distribution obtained by mobile survey was interpret ed by a deconvolution procedure analogous to analysis of target streng ths by single-beam sonar. This revealed different fish length frequenc y distributions between the lochs. The sonar technique was sampling th e same length range of fish as the multimesh gill nets earlier in the year but it estimated a higher proportion of the smallest fish. Result s from the application of the deconvolution procedure to mobile survey data were compared with measurement of the target strengths of side-a spect tracked fish from fixed location data.