Roach rules: shoaling fish are a constant factor in the diet of Cormorants Phalacrocorax carbo in Switzerland

摘要: The spatial and temporal variation in the diet of Cormorants Phalacrocorax carbo visiting Switzerland non-breeding season was studied from 1985-1992, by means regurgitated pellets stomach contents. re­ sults, combined with data most other dietary studies Switzer­ land since 1974, rest on 4810 samples representing 24 122 fish ten lakes nine river stretches. Of 31 species occurring at feeding si­ tes, 23 were found diet, yet 5-7 accounted for 85-95% numbers. Roach Rutilus rutilus 58% all and, together Perch Perea fiuviatilis, 65% Diet habitat­ specific, three main types could be discerned canonical variate anal­ ysis: (1) strongly dominated Perch, typical eu­ trophic empounded rivers; (2) containing a high percentage whitefish Coregonus sp. Tench Tinea tinca, two ex­ tremely low biomass; (3) Grayling Thymallus thymallus, Trout Salmo trutta or riverine cyprinids, free-running rivers. Seasonal to small Roach-dominated diets but strong some sites. Prey size ranged fry adult up 800 g average larger rivers than lakes. Small « 12 cm) mainly taken reservoirs. At landscape scale, proportion associated preference eutrophic that support densities Perch. estimated provide > 50% biomass ingested Swiss waters. local possible preferences less conclusive, apparently still overrepresented compared abundance while underrepresented. are both shoaling hunted which themselves congregate large flocks. A similar highly gregarious is apparent continental European inland