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

By analysing data sets from different world regions we add evidence to docu mented changes in demersal fish community structure that may be related to fishing. Changes are analysed by community properties that might be expecte d to capture relevant overall changes - size spectra slopes and intercepts, Shannon-Wiener diversity, and dominance. Cross-system differences in the s hape of the integrated community size spectra appear to be related to ecosy stem productivity. The slope of size spectra appears to respond in a consis tent way to changes in exploitation levels. In most areas studied. but part icularly in high-latitude regions, we observe a decreasing trend in the slo pe, reflecting changes in size composition toward a relative decline in lar ger fish. The results from tropical regions are less conclusive, partly owi ng 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 an y decline in species richness, while changes in diversity (richness and eve nness) were caused either by changes in patterns of dominance or by changes in the number of species identified resulting from improved survey protoco ls. (C) 2000 International Council for the Exploration of the Sea.