Seasonal changes in the diets and relative abundances of perch and roach in the littoral and pelagic zones of a large lake

In the Enonselka and Laitialanselka basins of Lake Vesijarvi, perch Perca f luviatilis and roach Rutilus rutilus were abundant in the littoral and in t he pelagic zones throughout the summer. In the littoral zone, roach was alw ays more numerous than perch, while perch dominated in the open water. Intr aspecific diet overlap values were higher than interspecific values. In the pelagic zone, perch <155 mm fed mainly on the cladoceran Leptodora kindtii , while small bosminids were most important food items for roach. Large per ch were piscivorous, feeding mainly on smelt Osmerus eperelanus. In the lit toral zone small perch foraged on zooplankton and chironomid larvae and lar ge perch on chironomids and fish (small perch). Small roach fed mainly on b osminids and detritus, while for roach >185 mm macrophytes (Elodea canadens is, Lemna trisulca) were also of importance. Detritus was more common in th e food of roach in Laitialanselka than in Enonselka. The slower growth rate of roach in Laitialanselka compared with Enonselka was probably connected with this. However, considering the latitude of the lake, the growth rate o f both roach and perch was relatively fast in both basins. The results indi cated that in a large lake both perch and roach are able to utilize effecti vely the different habitats and diverse food resources. By segregation in f ood resource utilization they are able to co-exist in large quantities, at the same time maintaining a relatively fast growth rate. (C) 2000 The Fishe ries Society ui the British Isles.