Fine scale foraging behaviour of cormorants

摘要

Human activities, like commercial fisheries, produce major changes in the structure of marine food webs. With declining fish stocks prey availability to seabirds and other top predators might be reduced, forcing these species to spend an increased amount of time and energy to locate and capture their prey. A threshold fish density might exist, below which foraging is no longer sustainable in terms of time and energy expended, with knock-on effects for reproductive success and survival. Very little is known about the functional link between prey density and predator performance in the upper trophic levels of marine ecosystems and threshold prey densities are therefore extremely difficult to define. We observed the fine scale foraging behaviour of Double-crested Cormorants (Phalacrocorax auritus) foraging on live Ainbow Trout (Oncorhynchus mykiss). We experimentally investigated the effect of prey availability, prey size, light conditions, and fish behaviour on cormorant prey capture behaviour. Foraging success of cormorants depended critically on prey availability, with search time increasing and prey capture rate drastically decreasing when fish density was below 2-3 g fish m super(-3). Fish behaviour (shoaling vs. individual fish) had important consequences for cormorant predatory success. Birds spent an increased amount of time in pursuit when attacking shoaling fish and overall success was significantly reduced. Our results highlight the complexity of predator prey interactions on a fine scale and illustrate the effects of biotic and abiotic factors on seabird foraging tactics and energetics.