Impact of fishing on size composition and diversity of demersal fish communities

By analysing data sets from different world regions we add evidence to documented changes in demersal fish community structure that may be related to fishing. Changes are analysed by community properties that might be expected to capture relevant overall changes - size spectra slopes and intercepts, Shannon-Wiener diversity, and dominance. Cross-system differences in the shape of the integrated community size spectra appear to be related to ecosystem productivity. The slope of size spectra appears to respond in a consistent way to changes in exploitation levels. In most areas studied. but particularly in high-latitude regions, we observe a decreasing trend in the slope, reflecting changes in size composition toward a relative decline in larger fish. The results from tropical regions are less conclusive, partly owing to the difficulty in obtaining consistent data series, but probably also because the generally higher growth rates of the constituent species make the slope less sensitive to changes in fishing. No evidence was found of any decline in species richness, while changes in diversity (richness and evenness) were caused either by changes in patterns of dominance or by changes in the number of species identified resulting from improved survey protocols. (C) 2000 International Council for the Exploration of the Sea.