Seasonal and size-related changes in the diet of perch,Perca fluviatilis L., in the shallows of an Australian river,and their implications for the conservation of indigenous teleosts

1. Perca fluviatilis was sampled monthly over 2 years from the shallow waters (<2 m deep) of the Collie River in south-western Australia to determine the diet of this introduced species and the extent to which it comprised indigenous fish species. 2. As P. fluviatilis increased in size, its diet shifted from predominantly planktonic crustaceans to benthic invertebrates, with decapod crustaceans constituting between 55% and 88% of the volume of the food ingested by larger fish (> 120 mm long) in spring, summer and autumn. 3. All size groups of P. fluviatilis fed on the small indigenous teleosts Edelia vittata and Bostockia porosa. 4. The prevalence with which the different size groups of P. fluviatilis ingested indigenous fish species in the main channel of the river was as high as 14% in fish < 50 mm long in summer, 25% in fish 50–120 mm long in spring and 17% of fish > 120 mm long in autumn. 5. Despite appreciable predation and a relatively high prevalence of P. fluviatilis, indigenous fish species have coexisted with perch in the Collie River since the early 1900s. Such coexistence has apparently been facilitated by a high density of invertebrate prey (as a result of eutrophication) and limited interspecific dietary overlap. 6. The absence of E. vittata in those parts of a nearby river system now occupied by P. fluviatilis and the extremely high prevalence of fish that is sometimes found in P. fluviatilis stomachs, suggest that under certain extreme conditions, such as when a marked depletion in alternative food sources occurs, the presence of perch could pose a threat to the conservation of indigenous fish species in certain river systems in south-western Australia.