Influence of annual variation in food supply on abundance of wintering common eiders Somateria mollissima

Abstract

We investigated how the annual variation in the abundance of wintering common eiders Somateria mollissima was related to the abundance of benthos over 4 yr. The study was conducted at 2 sites, Tuno Knob and Ringebjerg Sand, Denmark, from 1994 to 1998. Benthos was sampled along permanent transects in November, and common eiders counted and localized from observation towers during the winter. The benthic community was dominated by the blue mussel Mytilus edulis, with sizes varying with depth. Small and intermediary-sized individuals were primarily found in 0 to 6 m depth, and large individuals exclusively in 6 to 12 m depth. The pattern of variation in abundance of common eiders matched that of the benthos in 0 to 6 m depth, where they were mostly found, but not in 6 to 12 m depth. Based on a bioenergetic model of food consumption, we estimated that com- mon eiders during the 3rd and 4th years exploited between 22 and 64% of the entire benthic com- munity biomass in 0 to 6 m depth, compared to  3% in 6 to 12 m depth. We conclude that common eiders tracked annual variation in food abundance, primarily related to the presence of small and intermediary-sized blue mussels.