Etablering og udvikling af ynglebestanden af Bramgås på Saltholm, 1992-2010

摘要

Formerly, the breeding grounds of the Russian Barnacle Goose population were confined to the Barents Sea in the Arctic. In 1971, however, breeding Barnacle Geese were reported from Gotland in the temperate Baltic Sea. Since then the Baltic population has increased rapidly, and breeding colonies are now established on islands and in coastal areas of Sweden, Finland, Estonia, Denmark, Germany and the Netherlands.Saltholm is a 17 km2 flat island in 0resund, between Copenhagen and Malmo (Fig. 1). Around it is a 1.5 km wide zone of shallow water. The habitat of the island is brackish grassland, used for livestock grazing without use of fertilizers or pesticides. Saltholm and surrounding shallows represent one of the most important breeding, moulting and staging sites for waterbirds in eastern Denmark. It is protected as a Natura 2000-area where hunting and public access is highly restricted.The Barnacle Goose was first found breeding on Saltholm in 1992, when a single pair nested on the islets Svaneklapperne south-east of the main island. Since then, Barnacle Geese have increased rapidly, so that in 2008 the number amounted to 1317 pairs, and in 2010 probably more than 2300 pairs, now dispersed over much of the main island.During 1992-1999, the mean annual growth rate of the population was 64 %, falling to 40 % during 2002-2008 (Fig. 2). The rapid growth initially suggests that the population was supplemented by immigrants from Baltic or Russian colonies, birds from which pass the island on the spring migration. However, semi-domesticated birds from the city of Malmo may also have been involved, since some of Salt-holm's geese move to Malmo after the post-nuptial moult.In addition to the extensive grazing, an important factor for the breeding birds is the absence of mammalian predators. If that condition can be maintained, the number of Barnacle Geese will probably continue to increase for several years.