Vertical distribution patterns of zooplanktivorous fish in a shallow, eutrophic lake, mediated by water transparency

The vertical distribution pattern (VDP) of fish at shallow sites in eutrophic lake - Lake IJssel, the Netherlands - as affected by water transparency, was examined. The pattern was assessed by pair trawling at three depths and by hydroacoustics from June to August. Water transparency was estimated by light intensity and Secchi depth measurements. Hydroacoustics showed that fish were more dispersed at night than during the day. European smelt, (Osmerus eperlanus) and young-of-the-year Eurasian perch (Perca fluviatilis) comprised 97% of the fish in numbers. European smelt concentrated near the surface, at light intensities of >200 muW . cm(-2) when the water was turbid, and near the bottom when the water was clear (Secchi depth > 1.2 m). We concluded that low water transparency over-ruled the light sensitivity of European smelt, and caused European smelt to stay higher in the water column.