Changes in the demersal fish assemblage in the south-eastern North Sea following the establishment of a protected area ('plaice box')

Abstract

This paper studies the effect of the reduction in the trawling effort of large beam trawlers (> 300 hp) in the coastal waters of the south-eastern North Sea following the establishment in 1989 of a protected area, the 'plaice box', using data from annual beam trawl surveys carried out since 1985. Two different aspects of the demersal fish assemblage were analysed: (1) the size distribution using multiple analysis of variance; and (2) the species composition using multivariate techniques such as principal component analysis, multidimensional scaling and multiple analysis of variance. It is shown that the overall size structure of the commercially exploited fish species was affected by the change in trawling effort whereas that of the non-target species was not. In particular, the abundance of commercial fish within the marketable size-range of 25-40 cm increased when fishing effort was reduced. Multiple analysis of variance showed that, in contrast to the size structure of the fish assemblage, the species composition was not significantly affected by the change in fishing effort. However principal component analysis does indicate that after the closure of the 'plaice box' a considerable proportion of the variation in the abundance of the large fish (≤25 cm) over the years can be explained by a higher abundance in the 'box' area than in the reference area of most fish species, including the two main commercial species plaice and sole. Other trends that were observed during the study period both within and outside the closed area were: (1) a decrease of the relative abundance of plaice and (2) a general increase of species richness due to the influx of southerly species.