Subfossil remains indicate that the grey seal Halichoerus grypus used to be common in the Wadden Sea area until the 6th century AD. It is postulated that increased human settlement in the area and intensified hunting reduced this population, and the species became virtually extinct at the end of the 16th century. Because of the growing grey seal population on the Farne Islands in the 20th century, more (young) animals migrated to the Dutch Wadden Sea. In 1980 they founded a colony between the islands of Vlieland and Terschelling. Immigration, and to a lesser extent a number of annually introduced animals, has caused the colony to increase. In 1993, a maximum of 220 animals was observed, and since 1985 pups have been born in the colony. The pronounced seasonal pattern in the numbers of hauled-out animals is similar to that found for the grey seals at the Farne Islands, albeit out of phase by about two months, probably driven by a difference in the timing of the breeding season. Morphodynamic changes of the tidal flat used by the grey seal colony influences the growth of the colony. This could be avoided by management measures promoting establishment of grey seal colonies in other more suitable areas. However, an increasing grey seal stock could come into conflict with other components of the Wadden Sea ecosystem.
