Alfågelns Clangula hyemalis födoekologi under övervintringen i Gdanskbukten i södra Östersjön

Abstract

The feeding ecology of the Long-tailed Duck Clangula hyemalis was studied in the Gulf of Gdańsk during eight wintering seasons (November–April) 1972–1976 and 1986–1990 (423 food samples). The number of food taxa consumed was at least 28 (23 animal and 5 plant). Long-tailed Ducks fed mainly on bivalves of which Macoma baltica and Mya arenaria were the most important, then Mytilus edulis and Cardium glaucum. Fish, mostly Ammodytidae, and fish eggs were present in ca. every fourth sample. Crustaceans, polychaetes and gastropods constituted the remaining significant prey. Plant food was incidental. Birds selected main food items (3 bivalves) of shell length ca. 11.0–11.6 mm regardless of their different average size found in zoobenthos. Seasonal and some age and sex related differences in Long-tailed Duck diet were found. They consumed proportionally more Mytilus and Cardium in December and January and less later in the season. Crustaceans, polychaetes and especially fish (fish eggs) increased their percent share in the diet in the spring. Adult birds tended to eat more Mytilus and Cardium while immatures ingested more crustaceans. Birds caught in nets set deep (>20 m) fed largely on Mesidothea entomon. Long-tailed Ducks function as first to fourth-order carnivores in the food web of the Gulf of Gdansk. Due to their high numbers and long period of staying they consume considerable biomass of main food prey. Estimated yearly consumption of bivalves amounts to 6 350 tonnes (Macoma baltica–2,100, Mya arenaria–1,700, Mytilus edulis–1,600, Cardium glaucum–950), fish–1,120, crustaceans–400, polychaetes–200, and gastropods–125 tonnes. At least locally, they prey significantly on certain age classes of the most important food taxa.