Abstract
Bottom trawl surveys are carried out every autumn to estimate the relative abundance of six major fish species, especially that of pikeperch (Stizostedion lucioperca) and Eurasian perch (Perca fluviatilis), in the 182 000-ha Lake IJssel, The Netherlands. The catchability of these species is influenced by light intensity at the bottom and therefore by water clarity and water depth. In autumn, water clarity can differ greatly from day to day because of wind-mediated resuspension of sediments. Catchability of ruffe (Gymnocephalus cernua) and age 0 pikeperch showed a significant inverse relationship with light intensity at the bottom, and therefore, a correction should be made when catch data for these species are used to estimate population size or year-class strength. Results were not consistent for perch, while for smelt (Osmerus eperlanus), roach (Rutilus rutilus), and bream (Abramis brama) the influence of light intensity on catchability was not significant. Corrected and uncorrected estimates of the abundance of age 0 pikeperch, based on trawl samples, were compared to demonstrate the effect of water clarity on the estimation of year-class strength. Increased water clarity can lead to zero catches and consequently diminish the effectiveness of sampling programmes.
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