De verspreiding van zeevogels in de Noordzee: naar een beter begrip van patronen en verbanden

The spatial distribution of seabirds at sea in the North Sea area has been studied systematically since the late 1970s. In the middle of the 1990s, , the combined effort of ten institutions in seven countries finally led to the publication of eight allasses on seabird distribution. Some 80 seabirds and coastal birds frequently occur within the North Sea, over 60 of which are common and 40 are breeding birds (table I). Both diversity and total numbers of seabirds were found to peak in autumn (c. 8-9 million individuals). Distribution patterns and total numbers of seabirds in the North Sea were used for some gross generalisations, from which total fish consumption of North Sea seabirds was assessed. The annual consumption of c. 600 000 tonnes of fish, including c. 200 000 tonnes of sanded, estimated for the 18 most abundant seabirds in the North Sea is compared to the c. 6 600 000 tonnes consumed by predatory fish (table 4; Anon. 1994). On the basis of the same data, six areas of high international importance were identified on the basis of their conservation value (figure 8, table 5; Skov et al. 1995). Now that spatial distribution patterns and total numbers of seabirds at sea are much better known, the lack of information on aspects underlying these patterns is even more obvious than before. Feeding strategies of seabirds at sea, particularly outside the breeding season, _ . , „ . ore very poorly known and specific information on seabird diets is lacking in many cases. The paper reviews the work that has been conducted since the late 1970s and offers suggestions for further, co-operative studies in the North Sea which should lead to a better understanding of distribution patterns and relationships between feeding seabirds and other top-predators at sea. Important steps in a better knowledge of seabirds at sea should be (1) the integration of information collected in colonies (recruitment, total numbers), from the coast (seawatching: migration patterns, trends in wintering numbers), on the beach (beached bird surveys: mortality patterns, wrecks) and at sea (distribution patterns, total numbers), (2) more detailed studies of diets and prey selection at sea outside the breeding season, and (3) ship-based surveys during which seabird numbers, seabird feeding behaviour, associations between seabirds and marine mammals, fish abundance (acoustic surveys, catches) and oceanographic parameters (e.g. depth, salinity, temperature, stratification, turbidity, fronts, upwelling) are collected simultaneously.