Pluszne Lake, with an area of 903 ha and a maximal depth of 52 m, is located in northeastern Poland. It is a typical mesotrophic lake, and its resources of pelagic fish are dominated by vendace and smelt. A Simrad EY-M research echosounder (70 kHz, single beam) and a complementary HADAS computer data analysis system were used for fish distribution and density estimation. Control catches were carried out to identify species and to verify hydroacoustic data. A pelagic trawl with an entrance area of S = 10.8 m² and a filtration coefficient of F = 903.4 m³ min⁻¹ was used for these catches. In August, thirteen acoustic surveys were conducted at one hour intervals along the same transect of the depth profile. Surveying was begun at 19.00, one hour before sunset, and was concluded at 07.00, two hours after sunrise. Thirteen maps were obtained illustrating the hourly variations of fish dispersion and aggregation. Three indices were used to analyze these changes: mean fish densities; number of echoes from single fish; % of the smallest fish (TS < -44 dB). The optimal night period for estimating pelagic fish resources with the echocounting method at TVG 40 log R was determined. In August, this period fell between 22.00 and 04.00, i.e. approximately two hours after sunset and before sunrise. The results of control catches made with the pelagic trawl, fish distribution and behavior are discussed.
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