During November 1982 through April 1983 abnormally large numbers of seabirds washed ashore in the Netherlands. This paper reviews the numbers and composition of birds found during this period, discusses information on beached oil, oil at sea and reports on results from chemical analysis of various oil-samples. Following Anker-Ni1ssen & Røstad (1981), a number of seabirds was collected to obtain an impression of sex and age compositions. Some of the results are included in this paper but ageing methods and anatomical investigations are discussed elsewhere within this issue. (2) surveys During this period, 373 counts were made by NSO (Dutch Oil-victims project) members, which covered 3,056 km of beach (in table 2.1 some of the counts are excluded). All seabirds, divers, grebes, swans, geese, ducks and waders are included in the analysis, although only the Kittiwake, Razorbill and Guillemot are discussed in detail, survey-area Four major survey areas are described: Delta area (Zeeuwsche & Zuidhollandse eilanden), south of Hoek van Holland; the mainland coast of Noord- and Zuid Holland; the Wadden Sea Isles; the mainland coast of the Wadden Sea district. collection A sample of just over 400 birds were taken for closer examination (table 2.2). Data collected from this sample were: age (using external characters, if available), plumage, degree of oiling, weight, measurements, presence of subcutaneous fat and deposited fat, description of pectoral emaciation, sex, presence of bursa Fabricii and notes concerning all pecularities of stomach, gut, lungs, kidneys and other organs. oil-samples Eighteen oil-samples were collected from various coastal locations and from oiled birds (figure 2.2). Apart from these samples two were taken by the North Sea Directorate from the North Sea (June 1982, M.V. Katina) and from M.V. Cast Heron (Rotterdam Harbor, December 1982). (3) Major weather conditions during the period are briefly discussed. Strong winds were recorded throughout November, December and January, and only in February were low temperatures recorded. (4) This chapter reports on oil-sightings at sea during surveys by the North Sea Directorate (table 4.1), oil recorded by NSO-members at the beach (during regular counts)(table 4.2) and results of chemical analysis of oil-samples. At sea the North Sea Directorate conducted systematic surveys using remote sensing equipment only after December 1982. The origin of most spills was not succesfully determined. However, in the following cases oil spills originated from: oildumps from ships, recorded 32 times, oil leaks from (nearby) platforms/rigs, recorded 28 times and minor shipping accident, recorded once (Westerschelde, December’82). Beached oil occurred regularly throughout the period (frequency 39-53%, table 4.2). Chemical analysis of oil-samples could determine specifically the type and composition of oil and could be correlated to certain sources such as a particular oil-rig or ship. Most samples contained fuel-oil, all different in chemical composition, thus leading to the conclusion that numerous oil-dumps were mainly responsible for the oiling of birds and pollution of the sea. Oil types from beached birds did not match oil types from samples taken from the beach. (5) The number of stranded birds increased slowly in November and December, with major peaks in January and February and then slowly decreasing in March and April (tables 5.1-5.6). The Kittiwake, Razorbill and Guillemot were responsible for 70% of all the records (n= 12,000). The amount of oil on the major species is shown in the above tables. A large number of Razorbills was recorded (primarily at the mainland coast of Noord- and Zuid Holland), which is atypical for beached auks in the Netherlands. The most common species recorded, especially at the mainland coast of Noord- and Zuid-Holland were the Kittiwake, Razorbill and Guillemot which comprized 85% (n= 2,004) in January and even 88% (n= 1,888) in February (table 5.4). Besides those species only a few others were found there. Many more different bird-species were found in the Wadden Sea district (both the islands and mainland coast). Average of birds per kilometre for some frequently visited sections is found in figure 5.1. (6) This chapter discusses the most common species found, including the results from dissections. Diagrams, presenting age- and sex-compositions, are drawn after Anker-Nilssen & Røstad (1981) for reasons of comparison but more detailed ageing systems (discussed in this issue) were used and results are given in the text. The majority of Kittiwakes, Razorbills and Guillemots was slightly oiled and many emaciated. Kittiwakes were also frequently reported in inland areas (not included in the figures), but very few were received at coastal rehabilitation centres. At these centres 1,000 to 2,000 Guillemots were received, also these numbers are not included in the total figures, tables 5.1-5.6. Using the ageing method, contrast/bursa-system, discussed by De Wijs (in this issue), we determined that in Razorbills 13% were juvenile, 20% immature and 66% adult. However, only 44-45% was estimated to be sexually mature (see Van Franeker in this issue). For Guillemots 25% were juvenile, 35% immature and 40% adult; 32-40% were considered sexually mature. Chapter 6.4 reviews some important species not already discussed. The number of cases for Little Auks and Puffins were small in a global sense, however, in a respect to the total numbers as compared to other years, their numbers were abnormally high. The total for the Larus-group is rather low (n= 1,399, table 6.1), as were the numbers for ocher shore-bound species such as Common Scoter, Eider Duck, divers and grebes. (7) This chapter offers a discussion on causes that may have contributed to the demise of the victims. Two major causes are suggested: one, bad weather conditions; another, fouling of plumage by oil. Under good environmental conditions a bird may be successful in removing minor oil contaminations of its plumage, however, under bad environmental conditions (e.g. adverse weather conditions) oil plays a greater role in affecting the condition of a bird. In the 1970’s bad weather conditions were also recorded but no similar beaching incidents were witnessed. Even though most victims were only lightly oil fouled, oil on the plumage is considered an important factor in this beaching-incident. It is suggested that oil acts as a trigger-factor in a complex of hostilable factors and therefore is responsible for mortality. In the Netherlands 12,000 corpses were found. It can be estimated that these birds represent a 20 to 30,000 individuals actually washed ashore. At the German coast ca. 11,000 birds were found (Reineking 1983), so the final figure of birds beached in the eastern North Sea may be at least twice as high. An auk-wreck was witnessed on the coast of Britain in February (Underwood & Harbard 1983), however, there seems to be no evidence linking it with the high mortality found from the Dutch and German coasts.