Olieslachtoffers op de Nederlandse kust, 2009/2010

This paper presents the proportion of dead oiled birds washed ashore in The Netherlands of the total number of birds washing ashore as a result of beached bird surveys conducted by volunteers of the Dutch Seabird Group (NZG/NSO). Apart from the survey results for winter 2009/10, a summary is provided of data collected in summer 2009. The results are presented in a context of data collected in nearly 40 earlier seasons (1970-2009). The oil rate (fraction of oiled corpses of all birds found dead) is considered an indicator of levels of (chronic) oil pollution in the Southern Bight with mineral oil and other lipophilic substances. These species-specific oil rates are calculated on the basis of hundreds of beached bird surveys between November and April, carefully checking all dead birds found. The results of winter 2009/10 are compared with long-term trends calculated over 1975/76-2009/10. Along the North Sea coast, over the years, downward trends in oil rates were found in all species and species groups. Most of the recent data fit this pattern. In the past few years, however, it seemed that the initial decline in oil rates of Common Guillemots Uria aalge (the international indicator species for oil pollution in the Oiled-Guillemot-EcoQO) had more or less come to a halt. New data show that this is probably an artifact, caused be a change in agecomposition of the Guillemots at sea. The higher oil rates in these auks point at illegal oil spills offshore, some distance away from the coast: close enough to provide numerous fresh corpses littering Dutch beaches, but distant enough to prevent the nearshore seabird population to be hit by the oil. In winter 2009/10. Oil rates in the Wadden Sea area are rather lower than oil rates on North Sea beaches. The results obtained in 2009/10 did fit that picture, at least for birds numerous enough to provide a reliable sample. Winter 2009/10 was the first cold winter for years. Unexpectedly, however, densities of species that are sensitive to cold weather (waders and waterfowl) hardly increased. Entanglements in fishing gear and plastics and mortality from oil pollution are currently almost equally important as a cause of death for Northern Gannets. In March 2010, numerous seabirds were affected by a spill of Polyisobutylene. Common Guillemots and Northern Fulmars were primarily affected, indicating the offshore nature of the spill. Some of the Fulmars were also oilcontaminated. The event was also noted in the German Bight (Mellum). In April 2010, colonies of Lesser Black-backed Gulls at Texel and Vlieland were hit by an illegal spill of oil somewhere at sea. Beached bird surveys at the time did not produce unusual numbers of oiled dead seabirds on Dutch beached. The European Commission wrote a 'Commission Decision', in which "Occurrence, origin (where possible) and scale of significant spills (for example of oil or oil products) and the effect on marine biota" is described as an indicator. The beached bird surveys are an important contribution to this indicator.