Roach rules: Shoaling fish are a constant factor in the diet of Cormorants Phalacrocorax carbox in Switzerland

The spatial and temporal variation in the diet of Cormorants Phalacrocorax carbo visiting Switzerland in the non-breeding season was studied from 1985-1992, by means of regurgitated pellets and stomach contents. The results, combined with the data of most other dietary studies from Switzerland since 1974, rest on 4810 samples representing 24122 fish from ten lakes and nine river stretches. Of 31 fish species occurring at the feeding sites, 23 were found in the diet, yet 5-7 accounted for 85-95% by numbers. Roach Rutilus rutilus was found in 58% of all samples and, together with Perch Perca fluviatilis, accounted for 65% by numbers. Diet was habitat-specific, and three main types could be discerned by canonical variate analysis: (1) diet strongly dominated by Roach and Perch, typical for most eutrophic lakes and empounded rivers; (2) diet containing a high percentage of whitefish Coregonus sp. and Tench Tinca tinca, on two lakes with extremely low Roach biomass; (3) diet dominated by Grayling Thymallus thymallus, Trout Salmo trutta or riverine cyprinids, in free-running rivers. Seasonal variation was found to be small in Roach-dominated diets but strong at some other sites.

Suter W., ARDEA 85 (1): 9-27.