Introduced Aquatic Species of the North Sea Coasts and Adjacent Brackish Waters

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Introduced aquatic species have received more attention in north-western Europe following the summaries from the German North Sea coast (Gollasch 1996; Nehring and Leuchs 1999), Britain (and Ireland) (Eno et al. 1997; Minchin and Eno 2002), Norway (Hopkins 2002) and a more general account for the North Sea (Reise et al. 1999). Since then, several inventories have appeared: for the German coast (Nehring 2005), the Dutch coast (Wolff 2005) and the Danish coast (Jensen and Knudsen, 2005). In this account we review, summarise and update all those previous accounts. We have also included NIS (=non-indigenous introduced species) which were known from the North Sea but most probably are extinct in this area today, and species that have been recorded, but for which we have no proof of self-sustaining populations.

For the purpose of this account:

  • The North Sea is defined from a line between Dover and the Belgian border in the south-west to a parallel line from the Shetland Islands to Norway in the north, and also includes the Skagerrak in the east (modified after North Sea Task Force, 1993). The boundary between the North and Baltic Seas, as defined by the Helsinki Commission (, is the parallel of the Skaw in the Skagerrak at 57°44.43′N (Fig. 29.1).

  • We define marine and brackish-water species as those aquatic species which do not complete their entire life cycle in freshwater (modified after ICES 2005). Marine species are those having their main distribution in salinities higher than 18 psu; brackish-water species have their main distribution in salinities between 1 and 18 psu.

  • Introduced species (= non-indigenous, exotic or alien species, NIS) are species transported intentionally or accidentally by a human-mediated vector into habitats outside their native range. Note that secondary introductions may be transported by human-mediated vectors or by natural means (ICES 2005).

  • A vector is any living or non-living carrier that transports living organisms intentionally or unintentionally (ICES 2005).