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The contents of 174 stomachs of great cormorants (Phalacrocorax carbo) inhabiting northeastern Poland were analyzed. Fish and hard parts of fish were found in most of the stomachs. (87.4%). Moreover, nematode and tapeworm parasites were observed in 76.4% and 4.6% of stomachs, respectively. Only 1.7% of stomachs were entirely empty. The fish identified in the stomachs were represented by 14 species. The most common species (72.2% frequency) in the diet of cormorants was European perch (Perca fluviatilis L.). Roach, Rutilus rutilus (L.) was the second most abundant prey (30.8%), followed by bream, Abramis brama (L.) (22.6%), bleak, Alburnus alburnus (L.) (16.5%), and vendace, Coregonus albula (L.) (13.5%), while nine other species occurred sporadically (frequencies < 7%). The number of fish species per stomach ranged from 1 to 5 (most often one species), while the number of individuals varied between 1 and 43 (most often 5–10 species). The length (SL) and weight of prey ranged from 4.0 to 62.0 cm and from 0.4 to 721.5 g, respectively. The most specimens were in the size range of 7–9 cm. The smallest specimens were perch and bitterling, Rhodeus amarus (Bloch), while the largest were pike and European eel, Anguilla anguilla (L.). Statistical analysis revealed significant differences in the length and weight of the fish consumed among the most abundant species.
Piotr et al. (Wed,) studied this question.