The effect of lakes on the stream fish fauna

The effect of natural lakes on the fish species distribution in surrounding streams was studied using electrofishing survey data from 3281 localities in streams throughout Sweden. The occurrence of lake fish (northern pike, European perch, roach and burbot) was significantly higher close to lakes, both up‐ and downstream, as compared to localities further up‐ or downstream of lakes. Correspondingly, the occurrences of stream fish (brown trout, grayling, European minnow and bullheads) were lower close to lakes. It is suggested that the lower occurrences of the latter close to lakes be due to biotic interaction with lake fish, especially predation from pike and burbot. The results strongly indicate that the effect of lakes on water temperature, drift of plankton and invertebrates or the moderating effect on water fluctuations and quality did not influence the stream fish fauna to the same extent as the presence of lake fish in a zone up‐ as downstream of the lakes. Lakes function as a reservoir of strong predators and competitors.