| Summary: | Modern echosounders produce very accurate and versatile information on observed fish, but applicability of hydroacoustics in fish stock assessment is still largely determined by the behaviour of aquatic animals. A valid assessment is possible only when the target species avoid the inaccessible zones of echo sounder: surface blind zone, bottom dead zone and shallow areas. In addition, it requires an applicable method for eliminating echoes from other targets, especially phantom midge (Chaoborus) larvae, which are abundant in clay-turbid lakes. In Fennoscandia, acoustics has been widely used in vendace (Coregonus albula (L.)) stock monitoring due to its high economic value but other species have been less frequently assessed. In the present study, the applicability of hydroacoustics in fish stock assessment was investigated in two contrasting environments, in eutrophic, clay-turbid lakes dominated by smelt (Osmerus eperlanus (L.)) located in southern Finland and in oligotrophic, clear-water lakes dominated by whitefish (Coregonus lavaretus (L.)) located in northern Lapland. In both lake groups, the suitable diel periods and seasons for acoustic assessment were sought. In southern lakes, a new method was developed for eliminating the disturbance of Chaoborus. In northern lakes, the differences of the applicability of acoustics between whitefish populations were explored. The developed method enabled valid smelt density estimation also in lakes with abundant Chaoborus population. Without the new method, the smelt density would have been seriously overestimated. In southern lakes, the pelagic occurrence of smelt favours acoustic assessment. Both day and night surveys can be used and the suitable seasonal sampling window lasts from late July to October. However, because young-of-the-year smelt may occasionally inhabit shallow areas, acoustics should be supplemented by trawling in these areas. In highly turbid lakes, the surface blind zone may be a considerable source of bias and acoustics should be supplemented by ...
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