Rapid increase of the grey seal (<Emphasis Type="Italic">Halichoerus grypus</Emphasis>) breeding stock at Helgoland |
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Authors: | K Abt J Engler |
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Institution: | (1) Wildlife Consulting, Samwerstr. 32, 24118 Kiel, Germany;(2) Faculty of Geography/Geosciences, Department of Biogeography, University of Trier, 54286 Trier, Germany |
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Abstract: | Over the past 25 years, the grey seal (Halichoerus grypus) population in the North Sea increased on average at 5% per year, while founding a number of new colonies. Based on published
data, this note compares recent trends in pup production at nine breeding sites, with focus on the German island of Helgoland.
Since 1997, numbers born at Helgoland increased at 35% per year (95% CI: 29–42), reaching an all-time high of 53 in 2007.
Yet, this was less than 1% of all grey seal offspring in the North Sea area. Some other small stocks had similarly high growth
rates of 20–50%, which must be explained by immigration from the larger colonies, as sustained autochthonous increase cannot
exceed 13% per year in the species. Further, the Helgoland breeding stock increased significantly faster than the neighbouring
colonies at Amrum (13%) and in the Dutch Wadden Sea (20%), indicating higher offspring survival and/or higher relative influx
of migrants. This is attributed to the fact that at Helgoland newborn seals are safe from floodwater. In the other two sites,
in contrast, pups are born on sandbanks and are thus often hit by winterly storm tides, which poses a risk to their survival.
Because of the favourable breeding conditions at Helgoland, this new colony is expected to soon become an important stronghold
of the grey seal in the Dutch-German North Sea area. |
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Keywords: | Halichoerus grypus North Sea Population dynamics Pup production Colonization |
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