Olievervuiling en olieslachtoffers langs de Nederlandse kust, 1969-97: signalen van een schonere zee

This report presents the results of beached bird surveys along the Dutch coast between I969 and 1997. with emphasis on data collected since winter 1976/77. Changes in oil rates of beached birds are presented as changes in the risk for (corpses of) seabirds, coastal birds and land birds to become oil contaminated at sea due to chronic (oil) pollution. Beached bird surveys were conducted along the entire Dutch North Sea coast (beaches and dikes) and in the Wadden Sea area (dikes and saltmarshes: Figure 1, Table I). To the date of publication, the NZG/NSO database contained the results of 5178 surveys over 38 138 km of shoreline. At least 578 volunteers have contributed to these surveys and the corpses of 184 808 (sea-) birds and (marine) mammals were recorded. The results were split into ’winter’ (November-April) and 'summer' (May-October) surveys (Table 2). The North Sea coast has been treated separately from the Wadden Sea area. With the period 1969-97, a total of 16 winters were recorded as being rather ’mild’ (20 or less cold days between December and March), 7 winters were listed as ’moderate’ (21-45 cold days) and 6 as severe (>45 cold days; Table 3). Severe winters often caused considerable extramortality among grebes, wildfowl and waders, which resulted in comparatively low oil-rates in these species. Mass-mortality as a result of certain illegal discharges of oil or shipping accidents were recorded in 1969, 1973, 1974, 1985, 1987, 1988, 1990 and 1992. Countless smaller or less conspicuous incidents took place. In the 1980s and early 1990s, massive seabird wrecks (with many seabirds starved to death) have been recorded nearly annually. The most numerous species/groups of species found in the Netherlands during 1969-97 were Guillemot (30 766), Larus-gulls (29 999), waders (23 465), Eider (18 648), scoters (15 724), Kittiwake (11 483), land birds (10 364), other wildfowl (8729), and Razorbill (7234). In the species accounts, effort-corrected seasonal patterns are given for each group of birds, showing their relative abundance as beached birds throughout the year (based on data collected between 1977 and 1997). In the heading of each account, a table shows the total number of birds found in three main periods (1969-76, 1977-85, and 1986-97, plus the proportion of oiled birds <%) if at least 10 complete corpses were available in the sample. The assumption is made that the fraction of all beached birds that is oil contaminated is in some way related to oil pollution at sea. The percentage of oiled birds along a shore is thought to mirror the specific risk of species or groups of birds to become oil contaminated at sea, before or after death. In most land birds and waders, post-mortal oil contamination will be more common than in often swimming seabirds such as divers and auks. If we assume that all factors that influence the risk for birds to become oiled are constant, than the oil rate will mirror fluctuations in the risk to become oiled at sea and, hence, in the amount of oil present in the marine environment. While an absolute measure of the quantities of oil released into the sea cannot be derived from beached bird surveys, it is believed that changes in oil rates will follow the level of marine oil pollution. A decline in oil rate is thus believed to be indicative of a decline in oil pollution at sea. Spatial differences in marine oil pollution are clearly represented in differences in species specific oil rates between countries around the North Sea, while several examples have shown the sensitivity of beached bird surveys in time when radical measures to reduce oil pollution come into effect (Camphuysen & Van Franeker 1992). Presumably, the fraction of oiled birds (y) has some S-shaped relation with an index of oil pollution (x; Appendix Table I, Figure A). A widely used mathematical representation of such a S-shaped curve is the logit function; ex y= l+«x (li- ne analysis focuses on this index of oil pollution, which equals (as follows from (I)): '""■fe) (2). Trends in oil rates were now calculated after logit-transformation of the data by means of linear regression (by least-squares estimation). Trends are shown in diagrams (Figures 28-43), whereas Tables 6-8 give all the relevant statistics accompanying the calculations (n, a, b, seh r2, rms, t and the level of significance). In the trend analysis of oil rates, samples of less than 25 corpses were considered insufficient and were therefore omitted. From the seasonal patterns and the relative abundance as beached birds within the two main areas (North Sea, Wadden Sea), data sets were selected to be of use in the trend analysis of oil rates (Table 5). For example, for divers, only eight winters of beached bird surveys along the North Sea coast provided a sufficient sample (>25 individuals each winter) to calculate a trend in oil rates (Figure 28, Table 6). Summer surveys in this area, and the surveys within the Wadden Sea (winter and summer) provided insufficient information for a trend analysis in oil rates of divers. In contrast, the Eider gave valuable results for both winter (19 years: Table 6, Figure 30) and summer (9 years, Table 8, Figure 39) along the North Sea coast and also for both winter (17 years; Table 7. Figure 34) and summer (8 years: Table 8, Figure 42) within the Wadden Sea. From beached bird survey results, there is strong evidence that the risk to become oil contaminated (for birds or for corpses of birds floating in the water) is substantially less within the Wadden Sea than it is in the North Sea (Figure 22). Several coastal species which occur both within the Wadden Sea and along the North Sea coast show distinctly different oil rates in the respective areas (Figures 23-24), whereas pelagic seabirds found within the Wadden Sea had the same oil rates as those found along the North Sea coast (Figures 25-26). Representatives of the latter group would only enter the Wadden Sea when already weakened or otherwise in serious trouble (and, most likely, already oiled). The results of beached bird surveys over the past 30 years show consistent declines in oil rates in all areas and seasons and in virtually all species. As expected following the power-analysis in the previous study (Camphuysen 1995) nearly all trends were highly significant: In winter along the North Sea coast (1976/77-1996/97), declines in oil rales were found in all species!groups of birds except in scoters. All trends were significant, except in divers (because of rather small sample: Table 6, Figure 28-33). In winter in the Wadden Sea area significant declines in oil rates were found in all tested species or groups of birds. There was a tendency of even more rapid declines in oil rates in comparison with the results of beached bird surveys along the North Sea coast (Table 7, Figures 34-37). In summer along the North Sea coast (1977-96), all tested groups/species of birds showed significant declines in oil rates (Table 8, Figures 38-41). In summer in the Wadden Sea area, significant declines in oil rates were found, but it should be noted that rather little material was available (Table 8, Figure 42). Changes in oil rates of different groups/species of birds in winter and summer along the North Sea coast and in the Wadden Sea area are summarized in Table 10. The logit index along the North Sea coast in winter declined over the last 20 years in all species except scoters by at least 1.2 units. For all species found along the North Sea coast, the logit index changed from 1.2 to – 0.5 (76 -» 38% oiled), which could be expressed as an overall decline of about 50% in the percentage of oiled birds since winter 1976/77. Logit-transformed oil rales of coastal species found along the North Sea coast declined by 2.3-3.7 units, of pelagic seabirds by 1.2-2.6 units, and of land birds by 3.3-4.0 units. Over a longer period (1968/69-1996/97), the overall oil rate in this area has declined by 57% (Figure 43). Within the Wadden Sea in winter, the overall logit index declined from 0.3 to -2.2 (i.e. 57 -* 10% oiled), which would represent a 82% decline since 1976/77, or an 88% decline in the proportion of oiled birds since the late 1960s (Figure 43). Within the Wadden Sea, declines in oil rates were steeper than along the North Sea coast in winter and summer and in all tested groups/species of birds. Based on the material in this report, there is no evidence for a sudden improvement when MARPOL Annex I came into effect in 1983. A decline in oil rates commenced somewhere in the early 1970s, and the gradual implementation of MARPOL may only have assisted the continuation of that trend. Despite MARPOL, substantial amounts of oil are still illegally discharged into the North Sea and massive seabird mortality as a result of shipping accidents and chronic pollution may still be expected to occur at times. The results of aerial surveillances for oil slicks are currently not available in a way that allows direct comparisons with beached bird survey results. These surveys, however, still show many reported oil slicks in the Dutch sector of the North Sea. A preliminary comparison of results show a positive, nonsignificant relationship between the number of slicks per hour of observation and the oil rate in beached birds in winter (data 1983-94). Beached bird surveys are now believed to provide information on temporal changes or spatial differences in the occurrence of oil pollution in the marine environment and, hence, provide a valuable monitoring tool. Beached bird surveys are organised in over 20 European countries, using roughly the same methods. A critical analysis of the results would provide a much belter insight in the spatial and temporal fluctuations in the level of oil pollution in this pan of the world. It is suggested that attempts to set up a ’European Beached Bird Survey' are further stimulated, leading to a monitoring programme of marine oil pollution of the European seas. The now available data should be analysed along the lines described in this report, to examine any possible trends in the material in time all over Europe. The results of beached bird surveys in The Netherlands suggest that there is a clear improvement in the level of oil pollution in the southern North Sea and, particularly so, in the Wadden Sea. This suggestion is supported by the overall impression that Dutch beaches are nowadays cleaner in terms of oil loadings than 20-30 years ago. Measures to reduce (illegal) discharges of oil at sea appear to have been successful. It is concluded, however, that oil rales in The Netherlands are still high in comparison with 'clean ‘ areas such as around the Shetland Islands (Table 11).