Diel variations in the vertical distribution and density of vendace Coregonus albula (L.) in Pluszne Lake

Abstract

Lake Pluszne, 903 ha in area, maximal depth 52 m is located in North-East Poland. It is a typical mesotrophic lake, in which the resources of pelagic fish are predominated by vendace and smelt. A research echosounder Simrad EY-M (70 kHz, single beam) and a coopereting system HADAS of computer data analysis were used. Species identification of the fish and verification of the hydroacoustic data were performed using control catches with a pelagic trawl having entrance area S = 10.8 m² and filtration F = 903.4 m³ min^-1. In August, thirteen acoustic surveys were made every hour, along the same transect of the depth profile. Surveying started at 19.00 h, one hour before the sunset, and ended at 7.00 hours, two hours after the sunrise. Thirteen maps were obtained illustrating hourly variations of the fish dispersing and aggregation. Three indices were used to analyse these changes: mean fish densities, number of echoes from single fish, % of the smallest (TS < -44 dB) fish. Optimal period of the night was determined in which the pelagic fish resources should be estimated with the method of echocounting at TVG 40 log R. In August this period falls between hours 22.00 and 4.00, viz about 2 h after the sunset and before the sunrise. Discussion is presented on the results of control catches with a pelagic trawl, fish distribution and behaviour.