Changes in the demersal fish assemblages of British coastal waters during the 20th century

Catches of demersal fish from research vessel surveys in three areas around the British Isles from 1901 to 1907 were compared with survey catches in t he same areas from 1989 to 1997. Mesh size and other characteristics of the trawls used suggested that some of these data were comparable, and could b e used to describe changes in demersal fish assemblages over the last 80 ye ars. In Start Bay (NW English Channel) and the Irish Sea, species diversity was the same in both periods, although the most abundant species in each p eriod were not the same. In English coastal regions of the southern North S ea, fish populations became more diverse, as plaice Pleuronectes platessa a nd whiting Merlangius merlangus became less abundant, and the relative abun dance of several non-target species such as dragonet Callionymus spp., bib Trisapterus luscus, and bull-rout Myoxocephalus scorpius increased. The pro portion in the catch of small fish species (maximum body length <30 cm), wh ich would be least vulnerable to capture by commercial trawls, increased be tween the two survey periods in Start Bay and the southern North Sea. None of these small species was commercially exploited. The proportions of large r teleosts (maximum body length >30 cm) in catches decreased in all regions during the time period, except in the Irish Sea where plaice replaced grey gurnard Eutrigla gurnardus as a dominant species. There was a decline in a bundance of large sharks, skates and rays, including the common skate, Raja batis, white skate R a[ba and the angel shark Squatina squatina. During hi storic surveys, 60% of the elasmobranch fauna consisted of thornback ray Ra ja clavata, whereas in contemporary surveys the lesser spotted dogfish Scyl iorhinus canicula was the most abundant elasmobranch. Changes in length-fre quency distribution of fish in both target and non-target categories, and o ther observed changes, were thought to be a response to commercial exploita tion, and corresponded to similar observations recorded elsewhere. (C) 2000 British Crown Copyright.