ENS BJ, KATS R & CAMPHUYSEN CJ (2006) Why was there no mass-starvation of Eiders Somateria mollissima in The Netherlands in winter 2005/06?. LIMOSA 79 (3): 95-106.

Common Eiders have suffered several episodes of mass-mortality in the Dutch Wadden Sea in recent years (Fig. 2). These mass-mortalities could be linked to severe food shortages. Eiders swallow their prey whole and need shellfish with a favourable shell: flesh ratio, i.e. with thin shells and high flesh content. It turns out that Mussels Mytilus edulis from the sublittoral zone have the most favourable ratio. When such Mussels are in short supply, the probability of mass-mortality among Common Eiders increases (Fig. 3), especially when there is also a shortage of alternative food stocks, namely the Cut Through Shell Spisula subtruncata, which occurs in the coastal zone of the North Sea. In the winter of 2005/06 stocks of sublittoral Mussels were again very low and stocks of the Cut Through Shell were negligible. Yet, mortality among Common Eiders was not exceptional. One reason that few Eiders died was that the wintering number was exceptionally low (Fig. 4). In 1993-2005 aerial midwinter counts fluctuated between 91 000 and 168 000 with an average of about 120 000. In the winter of 2005/06 only 82 000 ducks were counted. Another reason for the low mortality is that many Eiders apparently fed on the American Razor Clam Ensis directus. Since the disappearance of the Cut Through Shell, this species has become the most numerous shellfish in the North Sea coastal zone. It is conceivable that this species can only profitably be harvested by the ducks when they are not above a certain size, but very little is actually known.
Although mechanized fishing for Cockles Cerastoderma edule in the Dutch Wadden Sea was stopped in January 2005, this offers no guarantee against future mass-mortalities of Common Eiders. Eiders regularly feed on Cockles, but there is as yet no evidence that Cockles, which mainly occur in the littoral zone, are important as an alternative food source when sublittoral Mussels are scarce. The stocks of sublittoral Mussels in the Dutch Wadden Sea may well decline further due to declining eutrophication, declining recruitment due to climate change and overfishing by the mussel farmers.

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