ROOMEN M VAN, TURNHOUT C VAN, WINDEN E VAN, KOKS B, GOEDHART P, LEOPOLD M & SMIT C (2005) Trends in bentivorous waterbirds in the Dutch Wadden Sea 1975-2002: large differences between shellfish-eaters and worm-eaters. LIMOSA 78 (1): 21-38.

In this paper trends in the numerical presence of 18 benthivorous waterbird species in the Dutch Wadden Sea (Figure 1) in the period 1975-2002 are analysed, using all available counts (Figure 2) and U-Index to impute missing counts. Three species have decreased in numbers, ten species have increased, and five species have fluctuated around a stable mean (Table 1, Figure 3). However, for ten species the trend in the period 1975/76- 1990/91 differed significantly from that during 1991/92-2001/02. This concerns especially species that mainly feed on either worms or shellfish. Furthermore, worm-eating species and shellfish-eating species show opposite trends, a difference that is significant for the period 1991/92-2001/02. All four species which depend on bivalves have decreased (Common Eider, Eurasian Oystercatcher, Red Knot and European Herring Gull), whereas five out of six worm-eating species have increased. The larger the contribution of shellfish in the diet, the larger the population decrease in 1991/92-2001/02. Other studies have shown that the recent decrease in Common Eider and Eurasian Oystercatcher numbers are at least partly caused by deteriorating feeding opportunities. Intertidal mussel beds have largely disappeared in the early nineties (due to overfishing, poor spatfall and possibly winter storms) and cockles were heavily exploited by commercial mechanised fisheries. For Red Knot especially the disturbance of the sediment by mechanised cockle fisheries is important with a range of effects on the quality and quantity of their food. The recent, broad increase of numbers of worm-eating species may be linked to a shift in the ecosystem (reduced shellfish stocks, increased worm stocks) but the mechanisms underlying this change remain to be identified fully. Common Shelduck Tadorna tadorna Common Eider Somateria mollissima Eurasian Oystercatcher Haematopus ostralegus Pied Avocet Recurvirostra avosetta Common Ringed Plover Charadrius hiaticula Red Knot Calidris canutus Bar-tailed Godwit Limosa lapponica Eurasian Curlew Numenius arquata Spotted Redshank Tringa erythropus Common Redshank Tringa totanus Common Greenshank Tringa nebularia Ruddy Turnstone Arenaria interpres Black-headed Gull Chroicocephalus ridibundus = Larus ridibundus Common Gull Larus canus canus European Herring Gull Larus argentatus

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