The Diet of Two Sympatric Great Cormorant Phalacrocorax carbo Subspecies Wintering at Freshwater Fishery Sites in England and Wales

Abstract

The recent increase in Cormorant numbers in the UK and the increasing use of inland feeding sites by the birds have created conflicts at many freshwater fisheries. Information on the prey composition of Cormorants foraging at such sites is important in understanding these conflicts and in providing a sound basis for management. Two subspecies of Great Cormorant, the continental subspecies Phalacrocorax carbo sinensis and the Atlantic subspecies Phalacrocorax carbo carbo occur in the UK and forage sympatrically. This study presents information on the winter diet of over 1400 Cormorants shot at inland fishery sites over an eleven-year period. A wide range of prey species were identified, but five fish species compromised almost 80% of the prey items recorded by number, and most prey items were small (≤10 cm). Dietary differences, in terms of the species, sizes and number of fish consumed, were apparent between birds of different sex and subspecies, consistent with the differing sizes of the birds. The largest birds (male P. c. carbo) tended to consume fewer, larger fish and the smallest birds (female P. c. sinensis) more, smaller items. Bird size also appears to influence foraging site selection, with male P. c. carbo more likely to forage on fisheries, such as put-and-take trout fisheries, where larger prey items were available.