Opportunistic foraging by three species of cormorants in an Australian estuary

Aspects of the feeding biology of three species of cormorant (Phalacrocorax melanoleucos, P. sulcirostris and P. varius) were investigated in the large Peel-Harvey Estuary in south-western Australia in April and October 1986 and January 1987. The duration of dives by each species increased with depth and at each depth was greater in P. varius than in the two smaller species. Differences in the ranges of depths in which the similarly sized P. melanoleucos and P. sulcirostris fed would have reduced any potential interspecific competition for feeding space and food. The dive-rest ratio was similar (2.6–2.8) in all three species. Comparisons between the diet of each cormorant species and the potential prey where foraging occurred demonstrated that all three cormorant species were opportunistic feeders, ingesting predominantly small, slow-swimming, benthic-feeding fish and crustaceans. The small size of the prey was limited by the small dimensions of the gullet. There was no evidence that any of the three cormorant species were likely to exert a pronounced adverse effect on the abundance of the major commercial fish in the estuary or on the food of these teleosts