Introduced marine species of the North Sea coasts

About 80 non-indigenous species are assumed to have been introduced into th e North Sea by transoceanic shipping and aquaculture. The number is certain ly underestimated as most small organisms received insufficient attention a t the species level. Also, the seafaring tradition of the North Sea countri es is much longer than our biological surveys are. Most exotic invertebrate s originate from the western Atlantic and were introduced by shipping, whil e most algae stem from the Pacific and came with the introduced oysters. A peak of newcomers was observed in the 1970s. Most of the arrivals became es tablished in brackish environments, at harbor sites and in the vicinity of oyster farms, fouling on hard substrates or living as epibionts. A few live in sediments, are holoplanktonic or are parasites. At the open coast, appr oximately 6% of the macrobenthic species are exotics, while in estuaries th eir share is up to 20%. Most exotics have been encountered in the southern North Sea first, and many did not spread further north. About 25% of the es tablished non-natives are widespread and attain locally high abundances. As a consequence, some inshore habitats are entirely dominated by exotics. Th e overall effect on the ecosystem seems to be more additive than one of dis placement. This suggests that the coastal biota of the North Sea are quite capable of accommodating newcomers. However, this is no guarantee that the next introduced species may not cause severe ecological change or economic harm. There is a need to minimize the risk of unintentional introductions b y ballast water treatment and by adhering to quarantine procedures in aquac ulture. Current research on exotics in the North Sea is regarded as inadequ ate for proper evaluation and management requirements.