Prey choice of great cormorants (Phalacrocorax carbo) in a marine protected area : potential impact on collapsed fish stocks and implications for future monitoring

Abstract: Great cormorant (Phalacrocorax carbo) populations have experienced a rapid growth in Europe since the 1980s, resulting in conflicts with fisheries over local fish resources. The 8-fjords area is a marine protected area on the Swedish west coast, where long term overfishing has led to a collapse of several local demersal fish stocks. Subsequently, fishing regulations have been implemented to protect and re-establish the stocks of cod- and flatfish species, at the same time as cormorants have increased largely in numbers. The increased predation pressure has raised concerns about the recovery of local fish stocks, although knowledge about cormorant feeding habits are lacking. Here, cormorant prey choice in two subareas of the 8-fjords area is investigated, using morphological identification of prey remains in regurgitated pellets. An estimated 15 077 fishes were identified from a total of 480 pellets. The most important prey families based on relative abundance were flatfish (Pleuronectidae 48%), gobies (Gobiidae 21%) and codfish (Gadidae 13%). Pairwise ANOSIM analyses showed extremely small differences in diet composition between sites (p≤0.003, R<0.08). In comparison to available monitoring fishery data, the diet did not correspond to prey abundance. Estimated fish sizes of a subsample of flat- and codfish showed that juveniles seemed to be preferred. Pellet analysis provided limited information about consumed species and should be combined with other methods for more detailed assessments of cormorant prey choice in the 8-fjords area.