Jj. Beukema, INCREASED MORTALITY IN ALTERNATIVE BIVALVE PREY DURING A PERIOD WHEN THE TIDAL FLATS OF THE DUTCH WADDEN SEA WERE DEVOID OF MUSSELS, Netherlands journal of sea research, 31(4), 1993, pp. 395-406
In the course of 1990, stocks of mussels (Mytilus edulis) declined to
unprecedentedly low levels in the Dutch Wadden Sea. Hardly a wild muss
el bed was left on the tidal flats as a consequence of three years (19
88, 1989, and 1990) with failing recruitment and intensive fishing for
seed mussels. During these three years, recruitment of cockles (Ceras
toderma edule) also failed, whereas fishing was continued. Bird specie
s taking these bivalves as staple food, the oystercatcher (Haematopus
ostralegus) and the eider (Somateria mollissima), experienced food sho
rtage. 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 oystercat
chers experienced unusually high mortality rates in the appropriate si
ze 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 com
bined stocks of Mya and Macoma would soon have run out of food supply
to the overwintering oyster-catcher population. Apparently, oystercatc
hers are able to reduce the stocks of their various bivalve prey speci
es to very low levels.