OYSTERCATCHER HAEMATOPUS-OSTRALEGUS WINTER MORTALITY IN THE NETHERLANDS - THE EFFECT OF SEVERE WEATHER AND FOOD-SUPPLY

Wintering Oystercatchers in The Netherlands are concentrated in the Wa dden Sea (c. 200 000), with substantial numbers in the Delta area (c. 90 000). Only 1% of the total wintering population is normally found a long the North Sea coast. Cold-rushes under severe winter conditions l ead to a reduction of wintering numbers in the Wadden Sea, and to incr eases ir,the Delta and along the North Sea coast. The mortality of win tering Oystercatchers in The Netherlands was studied on the basis of b eached bird surveys along the coast of the North Sea and the Wadden Se a between 1969 and 1996. On the whole, the pattern corresponded well t o annual mortality estimates from population studies on the Wadden Sea islands of Schiermonnikoog and Texel. Peak numbers of dead Oystercatc hers were found in most severe winters, but also in some moderate wint ers such as in 1976 and 1991. In mild winters, rather small numbers of dead Oystercatchers were recorded. For 1986-1996, annual mortality al ong the coast ranged from a c. 500 in the mild winters of 1989 to c. 1 0 000 individuals in 1987 and 1996. Multiple regression analysis of th e number of dead Oystercatchers in winter 1975-1996 on the number of c old days and the biomass of benthic prey revealed an explained varianc e of 66% (n = 21). Both the number of cold days and prey biomass had a significant effect. The results explained the relatively high mortali ty among Oystercatchers in the moderate winters of 1976 and 1991 (very low food stock), and the low mortality in the severe winter of 1982 ( large food stock). The extremely high mortality in 1987 was due to a c ombination of low food stock and severe weather. The depletion of Cock les and Mussels in the Wadden Sea in the early 1990s, due to a combina tion of overfishing and failing reproduction, triggered a southward ma ss movement of Oystercatchers and substantial mortality in the Delta a rea in the winter of 1990-1991. The absence of a peak in the mortality among adult breeders in the population studies for that particular wi nter is the major discrepancy between these population studies and the beached bird surveys. It is an indication that primarily non-local, a nd therefore probably less dominant, Oystercatchers suffered from the food shortage in the Wadden Sea. The results suggest that for Oysterca tchers the scarcity of their principal prey due to overfishing has tak en its toll. Had this season also been a cold winter, the results sugg est that the mortality would have been unprecedented.