Blue mussels Mytilus edulis in the Baltic: good news for foraging eiders Sornateria mollissima

Abstract

We analysed the food of eiders Somateria mollissima at Hanko, Finland, in the northern Baltic, by examining the contents of mussels in the faeces and digestive system of eiders. The aim of our study was to determine whether size selection of the main food, blue mussels Mytilus edulis, occurs and if so why. Generally, diving ducks seem to choose mussels smaller than the average found on mussel beds. One hypothesis states that size selection of mussels occurs because eiders choose to minimise the daily intake of shell material. We did not find unambiguous evidence of size selective feeding on blue mussels in our study area. Although eiders selected mussels somewhat smaller than the average found on mussel beds when the entire data set was analysed, this minor discrepancy can probably be attributed to the fact that benthic sampling was conducted in areas which were partly different from the areas in which eider specimens and faeces were collected. Individual differences among eiders were significant. The meat content of northern Baltic mussels was generally higher than that of Atlantic blue mussels, with only a slightly decreasing trend with increasing mussel size. Shell weight increased more than meat weight with increasing mussel size. However, the shell weight that birds have to ingest to obtain their daily requirement of food is many times greater in the Atlantic than in our study area for all size categories of mussels. Thus, shell mass minimisation is probably not of crucial importance to eiders in the northern Baltic.