Perch (Perca fluviatilis) and pikeperch (Sander lucioperca) in the diet of the great cormorant (Phalacrocorax carbo) and effects on catches in the Archipelago Sea, Southwest coast of Finland

We investigated cormorant diet in the Archipelago Sea, Southwest coast of Finland. Samples of cormorant pellets, regurgitates and stomachs were collected in 2010-2011. During these years, pikeperch and perch were found to make up 0.04-10% and 21-43% of the diet respectively, by mass. The length distribution of perch eaten by cormorants was more similar to the length distribution in purse seine samples rather than to that in commercial catches. The size structure of pikeperch in the cormorants' diet was intermediate between those in purse seine samples and commercial catches. Combined commercial and recreational catches have varied between 500-2000 t for perch and 225-525 t for pikeperch in the 2000s. Potential yield losses to fisheries caused by cormorant feeding in 2010 were 340-420 t for perch and 110-140 t for pikeperch, thus cormorant predation may affect catches and the profitability of fishing.