Fishing effects in northeast Atlantic shelf seas : patterns in fishing effort, diversity and community structure. V. Changes in structure of the North Sea groundfish species assemblage between 1925 and 1996

Abstract We examine long-term changes in the structure and composition of the groundfish species assemblage in four regions of the northwestern North Sea during the period 1925–96. Species diversity in the whole groundfish assemblage has declined in the three areas where fishing pressure has been greatest. In the area where fishing pressure was least, no trend in species diversity was detected. Only in the most intensively fished area was there a negative trend in species diversity in the non-target species assemblage. Marked spatial variation in species diversity was observed. For the whole groundfish assemblage, diversity was greatest in the inshore and southern regions and least in the offshore northern area. For the non-target species assemblage, the spatial diversity gradient was reversed. Multi-variate analyses indicated long-term changes and between-area differences in the species composition of both the whole groundfish assemblage and the non-target species subset. However, these changes consisted mostly of subtle variations in the relative and absolute abundance of a few key species, rather than major species replacement events. Only one species showed any marked increase in abundance: a dominant species which became more abundant. Examination of species aggregated length-frequency distributions indicated a shift towards an assemblage dominated by smaller fish in the whole assemblage, but not in the non-targeted assemblage.

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