Carss D.N. & Ekins G.R. (2002) Further European integration: Mixed sub-species colonies of Great Cormorants Phalacrocorax carbo in Britain - Colony establishment, diet, and implications for fisheries management. ARDEA 90 (1): 23-41

Inland colonies of Great Cormorants Phalacrocorax carbo in Britain contain individuals of both European subspecies the Atlantic Great Cormorant P.c. carbo and the Continental Great Cormorant P.c. sinensis. Not only are the two breeding together, they are probably hybridising. Colony size and diet were studied in the three oldest and largest of these colonies: Abberton Reservoir, Little Paxton and Besthorpe. Abberton was founded in 1981 and increased steadily to 526 nests in 1993, thereafter numbers varied and averaged 464 nests per annum with a peak count of 551 in 1996. This colony, the largest inland one in Britain, is 7 km from the sea and breeding birds foraged mostly in estuarine habitat. Diet was diverse (at least 26 marine and freshwater fishes) but dominated by Eel Anguilla anguilla and flatfishes Pleuronectidae. Calculations suggest that breeding birds here consumed 70.3 tonnes of fish in 1993. Colony-size trajectories for the other two colonies were similar to Abberton but with maximum counts of around 200 pairs. Diet at these colonies, both > 60 km from the coast, comprised exclusively freshwater fish, mostly cyprinids and particularly Roach Rutilus rutilus. Many of the English inland colonies seem to have formed as separate 'colonisation' events rather than following Van Eerden & Gregersen's (1995) mother-satellite colony formation model for the expansion of Continental Great Cormorants in mainland Europe. Presumably such events in Britain were, at least in part, facilitated because birds were already roosting in many different places, and by the large number of non-breeding birds of continental origin. We speculate on the implications of inland mixed subspecies Great Cormorant colonies in Britain for the future management of both Great Cormorants and fisheries