Brown trout (Salmo trutta) populations of the three Orkney freshwater lochs-Boardhouse, Harray and Swannay-were studied in late September 1991 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 targets and reflections from submerged macrophytes were filtered out. The ranking of the estimated densities corresponds with the anglers’ yield which was the highest in Boardhouse and the lowest in Swannay. The acoustic signal size distribution obtained by mobile survey was interpreted by a deconvolution procedure analogous to analysis of target strengths by single-beam sonar. This revealed different fish length frequency distributions between the lochs. The sonar technique was sampling the same length range of fish as the multimesh gill nets earlier in the year but it estimated a higher proportion of the smallest fish. Results from the application of the deconvolution procedure to mobile survey data were compared with measurement of the target strengths of side-aspect tracked fish from fixed location data.