Based on the North Sea International Bottom Trawl Survey, the number of species recorded after 20 hauls is used as an index of biodiversity at a spatial scale of 10*10nm. The results show a clear pattern: species richness is lowest in the central North Sea and highest in Scottish waters, in the Kattegat and in the Channel area. When the community is split into its northerly and southerly components, the former reaches its highest diversity in waters typically deeper than 100m and the latter in waters less than 50m. The area of high richness of northerly species extends from Scottish waters along the Norwegian trench into the Kattegat. High richness of southerly species is not restricted to the southern North Sea but is observed also along the Scottish coast and in the Kattegat. These patterns are discussed in relation to hydrographical features that may control these differences. Temporal trends indicate that both components are characterized by a gradual increase in species richness over the past 25 years, a process that has affected the whole area while rates of change did hardly differ between the components or areas. A standardized index of abundance also indicates long-term gradual increases for both northerly and southerly species, although in this case the increase in southern species is larger. I argue that overexploitation is a more plausible explanation for the observed phenomena, although climate change may have had add-on effects.
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