Demersal fish populations in the coastal waters of the UK and continental NW Europe from beam trawl survey data collected from 1990 to 1995

Abstract

Samples of the demersal fish fauna have been collected by beam trawl from the coastal waters of northwest Europe (49–57°N, 8°W–9°E) by the UK, Netherlands, Germany and Belgium, since 1990, during the third quarter of the year. Changes in community structure within small spatial scales were subtle as species compositions formed part of a continuum over the entire continental shelf. Populations of low diversity were particularly evident in the German Bight and on the North Sea continental coast, where dab Limanda limanda were abundant. 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 still an important member of the underlying fish assemblage, the abundance of other species, especially poor cod Trisopterus minutus, solenette Buglossidium luteum, plaice Pleuronectes platessa, and the lesser weever, Echiichthys vipera, allowed a range of different groups to be identified. Despite the greater species diversity in this westerly region only eight out of a total nineteen flatfish species were found in abundance. The dominance of different species in different size classes was a key feature of the community structure. Flatfish were the largest group by weight in the smaller-length classes (<30 cm), and in western areas the elasmobranchs dominated the larger-size classes. Observed patterns in community structure were partly explained by the zoogeography of the region and the presence of the British Isles at the boundary between two faunal types. The additional influence on demersal populations of depth and substrate type, which may regulate the abundance of flatfish at key stages in their life history, was also discussed. In addition to these natural processes, recent increases in fishing effort are thought to have affected the structure of the demersal assemblage, and an examination of aggregated length-frequency distributions from these surveys tends to support this conclusion. Without further information 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.