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 fluviatilis and roach Rutilus rutilus were abundant in the littoral and in the pelagic zones throughout the summer. In the littoral zone, roach was always more numerous than perch, while perch dominated in the open water. Intraspecific 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 perch were piscivorous, feeding mainly on smelt Osmerus eperelanus. In the littoral zone small perch foraged on zooplankton and chironomid larvae and large perch on chironomids and fish (small perch). Small roach fed mainly on bosminids and detritus, while for roach >185 mm macrophytes (Elodea canadensis, Lemna trisulca) were also of importance. Detritus was more common in the 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 of both roach and perch was relatively fast in both basins. The results indicated that in a large lake both perch and roach are able to utilize effectively the different habitats and diverse food resources. By segregation in food resource utilization they are able to co-exist in large quantities, at the same time maintaining a relatively fast growth rate. (C) 2000 The Fisheries Society ui the British Isles.