Trends in oil pollution in the southernmost (Belgian) part of the North Sea were analysed using a dataset of 37 years (1962-99) of annual national beached bird surveys conducted in February each year. The most abundant seabird groups represented in the beached birds were auks (31%), gulls (28%), scoters (17%) and Kittiwake (9%). Oil rates of most bird species/taxa indicate a decline in oil pollution, though only Larus -gulls, Common Guillemot and Razorbill show significant reductions. The slope in the linear decreasing trend is steeper in inshore and midshore species, than in pelagic species. A power analysis of the results demonstrated that statistically significant trends in annual indices would be expected within 17 years for Razorbill, 29 years for Larus-gulls and 31 years for Common Guillemot. For other species/taxa, at least 50 years of surveying would be required. Long-term oil pollution monitoring in Belgium should be continued with a major focus on a set of abundant bird taxa, sensitive to oil-pollution and occurring in various marine habitats. Most appropriate for this purpose are grebes (inshore), Larus-gulls, Common Guillemot and Razorbill (midshore) and Kittiwake and Fulmar (offshore).

Oil-pollution monitoring at sea through beach bird surveying would undoubtedly benefit from a further standardisation of methods, enhancing the efficiency of data collection. In order to come up with useful recommendations, we evaluated various approaches of beached bird collection at the Belgian coast during seven winters (1993–1999). Data received in a passive way by one major rehabilitation centre were compared to the results of targeted beach surveys carried out at different scales by trained ornithologists: `weekly' surveys – with a mean interval of 9 days – restricted to a fixed 16.7 km beach stretch, `monthly' surveys over the entire coastline (62.1 km) and an annual `international' survey in Belgium over the same distance at the end of February. Data collected through Belgian rehabilitation centres concern injured, living birds collected in a non-systematical way. Oil rates derived from these centres appear to be strongly biased to oiled auks and inshore bird species, and are hence of little use in assessing the extent of oil pollution at sea. The major asset of rehabilitation centres in terms of data collection seems to be their continuous warning function for events of mass mortality. Weekly surveys on a representative and large enough section rendered reliable data on oil rates, estimates of total number of bird victims, representation of various taxonomic groups and species-richness and were most sensitive in detecting events quickly (wrecks, oil-slicks, severe winter mortality, etc.). Monthly surveys gave comparable results, although they overlooked some important beaching events and demonstrated slightly higher oil rates, probably due to the higher chance to miss short-lasting wrecks of auks. Since the monthly surveys in Belgium were carried out by a network of volunteers and were spread over a larger beach section, they should be considered as best performing. Single `international beached bird surveys' in February gave reliable data on total victim number (once the mean ratio between numbers in various months is known) and oil rate (provided a sufficiently large sample can be collected), but failed in tracking events. It is a particularly attractive approach because of its long tradition, resulting in invaluable long-term databases, and the uniformity in which these surveys are organised on a large scale. The minimal distance for a monthly survey amounts to 25–30 km (40–50% of Belgian coastline) up to 40 km (65%) in order to attain sound figures for oil rate and species-richness, respectively. These distances are primarily determined by the number of bird corpses that may be collected and are hence a function of beaching intensity and corpse detection rate.

In 1992, the Institute of Nature Conservation (INC) started ship-based surveys on the Belgian Continental Shelf (BCS) to study the spatial and temporal distribution of seabirds. Among other things, the study showed that the Belgian marine waters hold internationally important numbers (> 1 % of the biogeographical populations) of several coastal and marine bird species. Also the area proved to be of major importance for several species of migratory birds. Out of the 121 bird species encountered at sea during the period 1992-98, 23 were selected as being true marine species that occur in relatively high densities within the Belgian waters. O f these 23 species six were so-called 'focal species', being seabirds which are included in the highest priority lists of international conservation instruments (EC-Birds Directive, Bern Convention or Bonn Convention) and which attain at least 1 % of the flyway population in Belgian marine waters in a particular season. Based on the distribution patterns, conservation value, and sensitivity for disturbance or oil pollution of the six focal seabirds several areas of high ornithological importance as well as areas sensitive for disturbance or pollution could be distinguished at the BCS. The avian hotspots at the Westkustbanken and Vlaamse Banken turned out to be the most sensitive areas for disturbance and oil pollution throughout the year, while the sensitivity of other areas varies with the seasons.

In October 1994, 39 macrobenthic samples, divided between two areas, were taken on the western Belgian Coastal Banks. The two areas could not be clearly divided, concerning their macrobenthic communities. Multivariate techniques revealed five coherent communities, linked with typical sedimentological factors: (1) the Barnea candida community in a very compact muddy sediment (median 14 μm); (2) a second community characterized by the presence of spat of Mytilus edulis, with a medium sandy sediment (median 456 μm); (3) the Lanice conchilega community inhabiting a fine sandy sediment (median 211 μm); (4) the Nephtys cirrosa–Echinocardium cordatum community in a coarser fine sandy sediment (median 242 μm); and (5), closely related to the latter, the N. cirrosa community also occurring in a fine sandy sediment (median 224 μm). Only the L. conchilega community belongs to the diverse transition zone. The other four communities seem to be part of the heterogeneous, species-poor coastal zone associations. No open sea communities have been detected in the area.On the western Coastal Banks only the L. conchilega community, because of the high numbers of Abra alba, Tellina fabula, and Spisula subtruncata, can be interesting as feeding grounds for the common scoter (Melanitta nigra (Aves: Mergini)). Comparison of the spatial distribution of the wintering common scoters and the L. conchilega community revealed no direct similarity. The factors possibly causing this dissimilarity have been discussed.

Pathological investigations were carried out on 67 guillemots ( Uria aalge ) washed up on the Belgian coast between November 1993 and March 1994. Emaciation and acute haemorrhagic gastroenteropathy were observed in more than 70 per cent of the birds. There was no statistical relationship between the level of oil contamination and the severity of the lesions. Differences in bodyweight were accounted for by age, sex, emaciation, and acute haemorrhagic gastroenteropathy. The birds had a severe weight deficit but the concentrations of pollutants were below acutely toxic levels.