Landelijke Olieslachtoffertellingen in Februari in Nederland, 1965 – 84

(1-3) Since 1965 Beached Bird Surveys are conducted almost annualy in February in the Netherlands. This paper reviews the results in detail and discusses certain trends found in the material. Results are given in appendices (1a, 1b) and are compared with older data (published by Mörzer Bruyns, 1959, and Tanis & Mörzer Bruyns, 1962) and with results from other countries (West Germany, Belgium, Britain) as published by Stowe (1982). Results are given as numbers per kilometer and proportions oiled (% of total number found) and details are worked out for seabirds, waders and wildfowl only. (4) All counts are discussed briefly, giving the proportion of coastline surveyed in % (n= 670 km), the number of dead birds per kilometer, the three most numerous species found (% of total number of birds found) and some remarks are made on oiling of the birds. Exceptional numbers were found thrice. Twice due to severe weather (1976, 1979), once due to a local oiling incident in de Wadden Sea (1969). A series of years (1981-84) showed large numbers of mainly oiled seabirds. Guillemots and Common Scoters were the most often beaching oil victims, however, the numbers of Common Scoters were quite small in recent years. Tables 1 and 2 show averages per kilometers for 1947-58 and 1958-62 (table 2) and for 5-years periods in recent years (table 1). Three different trends are easily found: decreasing numbers of, coastal bound, divers and scoters, an increase of gulls (excluding Kittiwake) and a sudden increase of specific seabirds such as Kittiwake, Guillemot and Razorbill in recent years. A general trend is given in figure 1. The increase of seabirdnumbers coming ashore as oil victims is quite alarming. Similar increases can be found in German and Belgian material, but nothing is to be found in results of East Britain (apart from a wreck in 1983 but small numbers were oiled in this case). (5) An explanation for those trends is hard to give. Decreasing numbers of divers may be explained by decreasing numbers wintering off the Dutch coast. In recent years (1970-1984) numbers are definitely smaller as compared to earlier this century, but at least during 1975-84 increasing numbers are reported migrating both in autumn and spring (and possibly also partly wintering). Numbers of scoters may have become smaller but the Common Scoter still is a very numerous bird as a winter visitor and migrant. A better control of oil pollution in, especially the direct coastal zone may have reduced the risk to become oiled for coastal bound species. An Increase of gulls, with not very many oiled among them, may be easily explained by a fast growing breeding (and wintering ?) population. An Increase of Kittiwake (sudden, hardly any found before), Razorbill and Guillemot (both after a marked decrease in 1947-70 and rather small numbers in 1971-80) may be explained by larger numbers wintering in the southern North Sea, rather than by any exceptional increase in offshore oil pollution. Further research is necessary and recommended. (6) We should know a lot more about the distribution of seabirds at sea, in particular in the southern North Sea. Seawatching data do not fit in this case. It is very difficult to interpret data of Beached Bird Surveys without information on presence (or absence) of the species involved in the area. New drifting experiments also could throw new light on the matter.