Diet of the Great Cormorant (Phalacrocorax carbo sinensis) at two areas at L?vstabukten, South Bothnian Sea, Sweden, based on otolith size-correction factors

摘要

This study determines Cormorant (Phalacrocorax carbo sinensis) diet composition and prey size at two areas 6 kmapart, atBay L?vstabukten, Sweden, in 2005. The study entails the examination of 333 pellets and 2,669 regurgitated fish. To determine the size of the predated fish based onworn otoliths found in pellets, size-correction factorswere applied. Regression slopes between fish size and otolith size were applied to estimate the original size of the predated fish. Both pellet-based and regurgitated-fish-based analyses suggested the same dominating prey species by biomass. Based on pellets, Herring (Clupea harengus; 32.5%), European Perch (Perca fluviatilis; 20.9%) and Eelpout (Zoarces viviparus; 18.5%) dominated the diet. Regurgitated-fish samples contained species that were not found in pellets. These were mostly species with small and easily-eroding otoliths, such as Three-spined Stickleback (Gasterosteus aculeatus) and Greater Sand Eel (Hyperoplus lanceolatus). Cormorant diet between the two areas differed, and within both areas there were gradual changes in diet composition between incubation, nestling, chick and fledgling phases.