Size selection and competition for mussels,Mytilus edulis, by oystercatchers,Haematopus ostralegus, herring gulls,Larus argentatus, and common eiders,Somateria mollissima

Abstract

Abstract Investigations on size selection of blue mussels, Mylilus edulis, and exploitation competition by ventral hammering oystercatchers, Haematopus ostralegus, herring gulls, Larus argentatus, and common eiders, Somateria mollissima, were carried out on various blue mussel beds on the tidal flats of Spiekeroog (Niedersachsen/Germany). The size-class was considered to be the ideal one, which would be selected if all size-classes were available. On one mussel bed the mussel sizes selected by oystercatchers were the most abundant mussel sizes, the median being 51 mm. It was concluded that the most abundant size-class corresponded to the ideal size-class which was much bigger than predicted for ventral hammering oystercatchers. If all mussels available were smaller than the ideal size, they selected the largest ones. Unlike oyster-catchers, eiders did not select the largest mussels present when they were all smaller than the ideal size. Instead, they selected mussels that were only slightly longer than the median of the available mussels. As eiders dive for several mussels when they are feeding on small mussels, they are time-restricted and less selective than if diving for single large mussels. For herring gulls, the ideal mussel size seemed to be 20 mm. Mussel sizes selected by different bird species on the same mussel bed differed significantly. Each species chose other size-classes if the ideal size was not available. The species never competed for the same size-class on a mussel bed.