Diel habitat shifts are a common phenomenon for marine and freshwater fish species. The aim of this paper was to investigate the influence of fish migration pattern, i.e. vertical versus horizontal, on the acoustical estimates of fish stocks. The Biosonics 101 dual beam echosounder was used to study fish spatial distribution during the day and night in two reservoirs differing by depth and littoral coverage. In the Solina reservoir with no littoral vegetation diel vertical migrations of fish were observed which led to the day estimates of fish stocks up to 50% higher than those done at night. The difference between day and night estimates depended on the seasonal changes of food availability and was larger when the food was scarce. In the Dobczyve reservoir with extended littoral apart from vertical also horizontal migrations of fish were observed which led to the night estimates being higher than the day estimates. The larger was the littoral area covered by vegetation the larger was the difference between day and night estimates. Hydroacoustics provides the most precise and reliable tool for estimating fish stocks, however, in order to make a proper use of it, a preliminary information on fish behaviour is required.
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