Birds, mussels, cockles and shellfish fishery in the Dutch Wadden Sea: How to deal with low food stocks for eiders and oystercatchers?

Abstract

Intensive fishing in the Dutch Wadden Sea in the late 1980s and 1990, in combination with the effects of storms and ice and low spatfall, has led to an almost complete disappearance of intertidal mussel beds in 1990. Since then, this habitat type has re-established in only very few places, covering some hundreds hectares at the most. Lack of good spatfall in the late 1980s also resulted in low cockle stocks. These stocks were further reduced due to heavy mortality in the 1995/96 winter. These developments yielded very low food stocks for shellfish eating birds in some winters. This paper describes the fishery policy which came into force since 1993 and the effects the low food stocks have had on Eiders and Oystercatchers which are dependent on cockles and mussels. Eiders suffered higher mortality in 1991 and partly emigrated to alternative feeding areas on the North Sea and German Wadden Sea. Oystercatchers suffered higher mortality as well, were found in lower numbers in the Dutch Wadden Sea and in higher numbers in inland feeding areas. This information, together with a comparison of quantitative data on available food stocks and food consumption by birds, indicate that in some winters the number of shellfish eating birds has exceeded the carrying capacity of the Dutch Wadden Sea. Alternative food sources, such asMacoma balthica, Mya arenaria, Scrobicularia plana, Ensis americanus, Nereis spp. andArenicola marina cannot compensate for low mussel and cockle stocks. Before 1990, intertidal mussel beds must have acted as an important alternative food source. In the current situation, with hardly any intertidal mussel beds in the Dutch Wadden Sea, cockles represent the major food source, especially in winter. Since cockle stocks can be very low in some years they are an unreliable food source at the same time. Intertidal mussel beds represent a winter resistant and a more reliable food source. These considerations are important viewpoints in the discussion on a future fishery policy for the Dutch Wadden Sea. The most important conclusions from the present analysis is that re-establishment of intertidal mussel beds in the Dutch Wadden Sea is essential to allow the area to play its role as vitally important wetland in the East Atlantic flyway. In order to achieve this goal future fishery on intertidal mussel beds should be highly restricted.