CAMPHUYSEN C.J. (1993) Zeevogelstrandingen op de Nederlandse kust: 26 jaar een vinger aan de pols. LIMOSA 66 (1): 1-16.

National beached bird surveys in February has been conducted annually in The Netherlands since 1965, with the exception of 1974. Between 1965 and 1978, the surveys were organized by The Netherlands Youth organisation for Nature studies (NJN). Since 1979, the working group beached bird surveys (NSO) was responsible. Slight differences in methodology between the two schemes have made that the oil rates were different (generally slightly lower since 1979). Volunteers were asked to search for corpses at the high water mark. During 1965-78, the main attempt was to cover as many kilometres as possible, regardless where. Since 1979, 6 subregions (fig. I) were sampled (at least I 0% surveyed was considered a reliable sample). The usual output of the national surveys, a national density (N/km) was considered unreliable, particularly because some of the subregions were not visited every year (tab. I). A re-analysis of the data was performed in order to assess densities in each of the subregions and to provide baseline data for a future monitoring programme for beached birds in The Netherlands. Proportions oiled were calculated using all birds in a survey, to avoid small samples (national index): oil rates were calculated only when samples contained at least 10 corpses of a species or group. Trends in oil rates were tested using Spearmans Rank Correlation Coefficient for 1965-78 and for 1979-91 separately (tab.2). Correlations between oil rate and density, and between an index of winter severity (IJnsen index) and density were tested using linear regression analysis, differences in species composition were assessed using the G-test (Sokal & Rohlf 1981). Of 531 divers found dead, 8.1% remained unidentified. Red-throated divers predominated (75.6%, N=488). Ca. 90% ofthe divers were oiled, while densities were slightly lower in recent years than previously (fig. 2). Great Crested Grebes were the most numerous grebes (84.6%, N=1929), followed by Red-necked Grebes (9.0%). Massive mortality was recorded in most cold and all severe winters (fig. 3), when oil rates were comparatively low. Fulmars were found in small numbers in most years (N=473), but an increase was found in recent years (fig. 4, appendix 1,2). Mass strandings in 1981 (95), 1984 (54), and 1988 (83) included 91.7, 90.2 and 67.2% with oil on the feathers. Gannets were found in small numbers, and generally, over 75% were oiled. Several oil incidents involving Eiders and scoters were recorded (fig. 5, 6). In Eider, oil rates declined, while other mortality factors became more prominent. Particularly so in the 1990s, when acute food shortages due to overfishing of ashellfish in the Wadden Sea led to starvation and mass mortality. Oil was the most important cause of death in scoters (tab. 2). Scaup, Goldeneye, Goosander and Red-breasted Merganser were the most common species in a group named other sea-duck. Severe winter conditions can force these birds to leave the IJsselmeer and Wadden Sea to enter the coastal marine environment and mass mortality occurred particularly in cold and severe winters (tab. 3). Other waterfowl and waders also were mainly encountered in beached bird surveys in cold winters (appendix I, 2). Relatively few were oiled, and this proportion declined (tab. 2). Herring, Common, Black-headed and Great Black-backed were the commonest of the Lams-gulls. Some 50% of these gulls were oiled and the proportion declined slightly, but not significantly (fig. 7, tab. 2). Slightly higher numbers were found in cold winters. Of Kittiwakes, a common species only in the early 1980s (fig. 8), the majoritywere oiled. Wrecks were recorded during 1981-84, with oil rates above average. The Guillemot was the most numerous oiled bird on Dutch beaches in the 1980s and early 1990s. A sudden increase in numbers was found from 1981 onwards (fig. 9), remarkably few were found in 1987. Oil rates were always high (tab. 2). As in the next species, very few were found in the 1970s. Also the Razorbill was found in large numbers in the early 1980s, but numbers fell after 1984 (fig. 10). A wreck was recorded in 1990. Very high numbers were oiled (tab. 2). The value of national surveys is discussed. Important features to be derived from these data are: (1) winter mortality, particularly amongst species not normally occurring in Dutch coastal waters (grebes, seaduck, waterfowl, waders), (2) local oil incidents, (3) a decline in the oil rate in several of the coastal species, (4) very large proportions oiled in pelagic species, and (5) mass strandings of pelagic species in the 1980s. The auk wrecks are discussed in more detail. Patterns found in The Netherlands were in fact found all along the North Sea, including very small numbers in 1987 (and 1992). The availability of food in winter is probably the key factor, leading to changes in winter distribution. The oil-induced mortality in our waters, clearly shows that pollution of the North Sea by oil is still at an unacceptable level.

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