BUBBLE DENSITY IN THE OPEN WATER OF FRESHWATER RESERVOIRS: CONSEQUENCE FOR FISH STOCK STUDIES

摘要

New method of assessment the "acoustic biomass" of bubbles from individual size groups of total "acoustic biomass" of the mixture offish stock and bubbles in freshwaters has been developed. The method is based on the comparison of horizontal and vertical hydroacoustic surveys. Horizontal survey enable to estimate bulk of the fish biomass concentrated near the water surface in freshwaters. In the comparasion, vertical survey enables to investigate bubbles emerging from the bottom (spatial distribution of bubbles, their changes during the course through water column and size distribution). The density of emerging bubbles was much higher in the zones of former river bed compared with the surrounding reservoir areas. The errors in assessment of total fish biomass is negligible (2-3%), but the bubbles can cause significant problems in assessment of small fish sizes (1-15 cm length; up to 20% of biomass).