DEMERSAL FISH POPULATIONS IN THE COASTAL WATERS OF THE UK AND CONTINENTAL NW EUROPE FROM BEAM TRAWL SURVEY DATA COLLECTED FROM 1990 TO 1995

Samples of the demersal fish fauna have been collected by beam trawl f rom the coastal waters of northwest Europe (49-57 degrees N, 8 degrees W-9 degrees E) by the UK, Netherlands, Germany and Belgium, since 199 0, during the third quarter of the year. Changes in community structur e within small spatial scales were subtle as species compositions form ed part of a continuum over the entire continental shelf. Populations of low diversity were particularly evident in the German Eight and on the North Sea continental coast, where dab Limanda limanda were abunda nt. In the Channel and to the west of the UK the demersal assemblages were more species-rich than in the North Sea and, although dab was sti ll an important member of the underlying fish assemblage, the abundanc e of other species, especially poor cod Trisopterus minutus, solenette Buglossidium luteum, plaice Pleuronectes platessa, and the lesser wee ver, Echiichthys vipera, allowed a range of different groups to be ide ntified. Despite the greater species diversity in this westerly region only eight out of a total nineteen flatfish species were found in abu ndance. The dominance of different species in different size classes w as a key feature of the community structure. Flatfish were the largest group by weight in the smaller-length classes (<30 cm), and in wester n areas the elasmobranchs dominated the larger-size classes. Observed patterns in community structure were partly explained by the zoogeogra phy of the region and the presence of the British Isles at the boundar y between two faunal types. The additional influence on demersal popul ations of depth and substrate type, which may regulate the abundance o f flatfish at key stages in their life history, was also discussed. In addition to these natural processes, recent increases in fishing effo rt are thought to have affected the structure of the demersal assembla ge, and an examination of aggregated length-frequency distributions fr om these surveys tends to support this conclusion. Without further inf ormation on the distribution of fishing effort, it is not possible to separate the influence of natural faunal changes between regions from that of artificial changes caused by fishing activity. (C) 1998 Elsevi er Science B.V. All rights reserved.