Gillnet catch in estimating the density and structure of fish community—Comparison of gillnet and trawl samples in a eutrophic lake

To improve the accuracy of fish population estimates, understanding the deficiencies of fish sampling methods is important. Gillnets are a passive and selective gear and give biased estimates of abundance and distribution of species and their size structure. By comparing gillnet results with those of active gear (e.g. trawl), it is possible to account for the bias in the gillnet data. The aim of this study is to compare fish-community data collected with two different methods: gillnet and trawl, and to consider possible reasons for differences in the results. The fish community in the two basins of a shallow, eutrophic lake in southern Finland was sampled diurnally with gillnets and trawl in different years. The differences in abundance estimates, species and length distributions were considered. The gillnet NPUE (number per unit effort) of ≥6.0 cm fish was correlated with the trawl-abundance estimate. The most abundant species in the trawl catch, smelt, was almost totally missing from the gillnet catch. The proportion of bream was lower, while perch, roach and white bream were higher in the gillnet catch. Gillnets regularly underestimated the proportion of small (<10 cm) individuals in size distributions. Due to many confounding factors, caution is recommended when making deductions of fish density from gillnet NPUE.