Lake Ostrowite is a mesotrophic lake in Northern Poland 280.7 ha in area and 43 m deep at its deepest point. To study vendace (Coregonus albula L.) diet in the lake, fish and zooplankton were sampled at two stations on 28-29 April, 15-16 May (fish only), 28-29 July and 15-16 August 2004. Fish were caught by gill netting, while zooplankton was collected from the whole water column by means of a Ruttner sampler (5 x 2 dm3). In total 91 vendace specimens were analysed, of which 64.8% had non-empty alimentary tracts. In the April and May samples Copepoda were the most numerous prey, while Daphnidae dominated the diet in the July- and September-caught fish. Planktonic Chironomidae pupae were also an important dietary item in the April-caught fish. Cladocerans, especially from Daphnidae family, were positively selected by vendace whilst copepods were not. Large predatory cladocerans, Leptodora kindtii, and the non-indigenous Bythotrephes longimanus were consumed in substantial quantities by the July- and September-sampled fish, with the vendace feeding more intensively on the L. kindtii than on the B. longimanus.
