INCREASED MORTALITY IN ALTERNATIVE BIVALVE PREY DURING A PERIOD WHEN THE TIDAL FLATS OF THE DUTCH WADDEN SEA WERE DEVOID OF MUSSELS

In the course of 1990, stocks of mussels (Mytilus edulis) declined to unprecedentedly low levels in the Dutch Wadden Sea. Hardly a wild mussel bed was left on the tidal flats as a consequence of three years (1988, 1989, and 1990) with failing recruitment and intensive fishing for seed mussels. During these three years, recruitment of cockles (Cerastoderma edule) also failed, whereas fishing was continued. Bird species taking these bivalves as staple food, the oystercatcher (Haematopus ostralegus) and the eider (Somateria mollissima), experienced food shortage. Significant numbers of eiders left the Dutch Wadden Sea area or died, whereas oystercatchers remained abundant throughout the winter in most of the Dutch Wadden Sea. Alternative prey species of oystercatchers experienced unusually high mortality rates in the appropriate size classes. This was so in all other common species of bivalves, viz. first-year and older cockles, adult Macoma balthica, and juvenile Mya arenaria. This led to minimal stocks of food for oystercatchers in the late winter of 1991. In March 1991, cockles were depleted and the combined stocks of Mya and Macoma would soon have run out of food supply to the overwintering oyster-catcher population. Apparently, oystercatchers are able to reduce the stocks of their various bivalve prey species to very low levels.