Walvisachtigen in de zuidelijke Noordzee: twee survey methoden vergeleken
Sula , Volume 8 - Issue 3 p. 207- 225
Surveys, dedicated to cetaceans in the North and Baltic seas have been few. The existing data all refer to only a part of the area, and often to the periphery of the sea. Most sightings have been made by observers that were not primarily interested in marine mammals, such as mariners, boaters, fishermen, seawatchers and marine ornithologists. With the exception of the studies of seabird distribution and seabird migration (.seawatching) in which cetaceans were recorded, the effort is generally not known. One of the largest existing databases on the distribution of cetaceans in the North Sea is the ’European Seabirds At Sea' (ESAS) database, in which offshore seabird surveys, conducted by nearly all North Sea states, are compiled. Effort, in terms of kilometers sailed is known. This database contains the work of over 100 observers and contains thousands of cetacean sightings. In summer 1994 the North Sea at large was synoptically surveyed for cetaceans, using survey techniques specifically developed for this group. In this paper, the results of the three Dutch ships involved in this survey are compared to the data in ESAS for the southern North Sea. Only observations in seastates 0-4 B have been used. Numbers of cetaceans seen were relatively low during the seabird counts, indicating that during seabird counts a high proportion of the cetaceans will be missed by the observers. Still the distribution patterns of the different species of cetacean matched well between the two datasets. A major discrepancy is the lack of sightings of Harbour Porpoise in the eastern North Sea in the ESAS data. Other than that, both datasets show that the northwestern part of the southern North Sea holds most cetaceans, and that Harbour Porpoise, Whitebeaked Dolphin and Minke Whale are the most abundant species in this area. The ESAS database therefore seems a usefull tool for predicting the relative occurrence of cetaceans. ESAS spans the entire year and may thus be used to study cetacean distribution in other seasons.
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