RECOLONIZATION OF THE DUTCH WADDEN SEA BY THE GREY SEAL HALICHOERUS-GRYPUS

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 po stulated that increased human settlement in the area and intensified h unting reduced this population, and the species became virtually extin ct at the end of the 16th century. Because of the growing grey seal po pulation on the Farne Islands in the 20th century, more (young) animal s migrated to the Dutch Wadden Sea. In 1980 they founded a colony betw een the islands of Vlieland and Terschelling. Immigration, and to a le sser extent a number of annually introduced animals, has caused the co lony 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 breed ing season. Morphodynamic changes of the tidal flat used by the grey s eal colony influences the growth of the colony. This could be avoided by management measures promoting establishment of grey seal colonies i n other more suitable areas. However, an increasing grey seal stock co uld come into conflict with other components of the Wadden Sea ecosyst em.