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Dietary studies on carp and other fish species in western Victorian lakes

Summary In Australia, carp have been implicated in the degradation of aquatic ecosystems. However, Australian studies have been inconclusive or absent. The present study assessed whether carp had direct dietary overlap with native fish species and whether carp feeding could be implicated in modifying the abundance or diversity of invertebrate communities through active selection or avoidance. Small carp showed a high preference for microcrustacea and a similar diet to native galaxid and smelt (proportional similarity indices 0.7-0.8). As carp size increased the proportion of macroinvertebrates in gut contents increased and carp diets became more similar to those of native eels and less similar to those of the smaller galaxid and smelt. Dietary overlap indices (Hurlbert's niche overlap) for macroinvertebrate foods were much greater than 1 for all combinations of carp and native species, indicating that the combined utilization of some macroinvertebrate groups is far more intense than others and well above that expected if all species were perfect generalists. Carp showed a selective preference for Diptera and a balanced preference for Amphipoda. Small native species (galaxid and smelt) showed a selective preference for Diptera in one lake and avoidance in the other, suggesting firstly that strong predatory control on Diptera may be occurring in Lake Colac and, secondly, that the intensity of dietary overlap between carp and small native fish for Diptera is site specific. Native eels showed a selective preference for Trichoptera and Amphipoda and lower dietary overlap with carp than that observed between carp and the small native fish, galaxid and smelt.