The importance of the North Sea for winter dispersal of harbour seals Phoca vitulina from the Wadden Sea Original Research Article

Harbour seals Phoca vitulina were routinely recorded during ship-based seabird counts in the eastern North Sea in winter. This area borders on the Wadden Sea, a major breeding ground of the species in Europe. Line transect theory was used to estimate total numbers of seals at sea. A total estimate of 2200 seals was obtained (c.v. 35%; 95% confidence INTERVAL = 855–3785). Stratification of the data according to geographical subregion did not improve precision of this estimate. Our estimate is uncorrected for animals missed on the tracklines and is thus a minimum. Considering this, at least 20% of the estimated Wadden Sea summer population is to be found in the adjacent North Sea in winter. This implies that seal conservation policy needs to consider this wintering range in the North Sea, in addition to the existing management plan that protects the breeding and moulting areas in the Wadden Sea.