Comparisons between the Diets of Distant Taxa (Teleost and Cormorant) in an Australian Estuary

Abstract

While most studies of the utilisation of food resources in aquatic ecosystems have focused on comparing the diets of closely related taxa, the present study was aimed at elucidating whether the diets of five species, representing two distant taxa, were correlated and overlapped. We have thus compared the diets of a fish species (the flathead, Platycephalus speculator) with those of two small species of cormorant (little pied, Phalocrocorax melanoleucos, and little black, P. sulcirostris) and two larger species of cormorant (pied, P. varius, and black, P. carbo) in a temperate southwestern Australian estuary. The flathead and the two smaller cormorants consumed predominantly shrimp and small, short-lived teleosts, whereas the two larger cormorants fed almost exclusively on larger species of teleost. Although the diet of flathead were correlated with and overlapped those of the two smaller cormorant species, there was no correlation or overlap between the diets of these three species and those of either of the larger species of cormorant. Our observations of P. speculator are consistent with the results of previous studies which have shown that platycephalids are ambush predators, whereas cormorants are pursuit surface plungers, with the little pied cormorant feeding solitarily and the little black cormorant feeding communally. Flathead preyed on both demersal and mid-water teleosts. The little pied cormorant fed almost exclusively on demersal teleosts and the little black cormorant exploited predominantly mid-water schooling fish. Thus, differences in the fish components of the diets of these three predators can be related to differences in the mode of foraging and the level in the water column in which feeding takes place. The respective daily consumption of shrimp and teleosts in the estuary was estimated as 1,000 kg and 450 kg by flathead and as 180 kg and 150kg by the four species of cormorants combined. The results of this study show that, in estuaries, the degree to which diets were correlated and overlapped amongst predatory vertebrates can be greater between representatives of distant taxa than between species within the same genus.