Consequences of the Sudden Removal of Nearly All Mussels and Cockles from the Dutch Wadden Sea

Abstract. Three successive years of recruitment failure, intensive fishery, and high rates of natural mortality eliminated all intertidal mussel beds and nearly all cockle beds from the Dutch Wadden Sea in the course of 1990. In late 1990 and early 1991, zoobenthic biomass and particularly bivalve stocks were unprecedentedly low. The following unusual events were noted:

• 1)

  Relatively high diatom and chlorophyll concentrations in the autumn of 1990 and an exceptionally early and dense spring bloom of diatoms in March 1991;
• 2)

  Exceptionally early onset of rapid growth in the bivalve Macoma balthica in March 1991;
• 3)

  Above-average weights of the soft parts of bivalves in the 1990/91 winter;
• 4)

  Elevated mortality rates in bivalves that could serve as alternative prey for birds specialised on big bivalves (oystercatchers and eiders);
• 5)

  Signs of food shortage in such birds: high mortality rates and emigration to other feeding areas;
• 6)

  A rapid recovery of the benthos started in the summer of 1991, accelerated by an exceptionally high recruitment in some of the affected species.

It is concluded that the Wadden Sea ecosystem responded in an elastic way to the removal of two of its key species. The events are not seen as merely coincidental but are regarded as a chain of causes and effects.