Dietary studies on exotic carp (Cyprinus carpio L.) from two lakes of western Victoria, Australia

There is little published information on the impacts of exotic fishes in the shallow slightly saline, closed catchment lakes of western Victoria. The present study assessed the diet of exotic carp (Cyprinus carpio) from two lakes of western Victoria (Lake Colac and Lake Modewarre) between November 1999-September 2001, using a numerical and volumetric method of gut contents analysis. Carp larvae <2 cm total length fed exclusively on microcrustacea (Cladocera and Copepoda). At a mean total length of >2 cm carp larvae shifted their diet to include benthic food resources, but microcrustacea still dominated the larval diet. Small carp (≤15 cm) showed a high preference for microcrustacea and tended to avoid benthic macroinvertebrate food resources. As carp size increased the proportion of macroinvertebrates in gut contents increased. Medium (15–40 cm) and large sized carp (>40 cm) displayed broad food assemblages in their guts with microcrustacea, Gastropoda, Ostracoda, Amphipoda and detritus being dominant. Carp showed a selective preference for Diptera and a balanced preference for Amphipoda. Schoener's dietary overlap was high between small and medium carp (0.55–0.65) and medium and large carp (0.72–0.74) but was low between small and large carp (0.36–0.44). When considering only benthic macroinvertebrate food resources, Hurlbert's dietary overlap between size classes of carp remained reasonably low. There were no observable seasonal patterns in the diet of carp in the two lakes but the proportion of the food items in the gut varied between sampling occasions. This study identified the potential of exotic carp in altering lake function by feeding predominantly on microcrustacea, thus causing a 'top-down' trophic cascade.