Comparison of fish size distribution and fish abundance estimates obtained with hydroacoustics and gill netting in the open water of a large shallow lake

Abstract

The distribution and overall fish abundance, and biomass of fish populations in the open water of meso-eutrophic Lake Balaton were studied in 2003-2007 using hydroacoustics and gill nets. Gillnet catches revealed that bleak (53-76 %) common bream (8-42 %), white bream and razor fish (2-18 %) dominated in the open water of the lake. Comparisons of target strength distribution from hydroacoustics and length distribution from the gillnets showed that the fishing gear did not quantitatively catch the smallest and largest size groups of fish. Mean acoustic fish abundance and biomass fluctuated significantly between both basins and years. The biomass of fish populations reflects the trophic gradient along the longitudinal axis of the lake (min-max.: 89-181 kg ha-1) showing a decreasing trend with years. The mean acoustic fish biomass was significantly higher in the eutrophic western basin, compared with mesotrophic eastern and middle ones.