Size structure of benthic freshwater fish communities in relation to environmental gradients

The benthic fish communities of 161 Swedish lakes, sampled with multi-mesh gillnets, were examined by direct gradient analyses with ten environmental factors. A geographic gradient, correlated with climatic factors and produc tivity, was most important for ordination of species optima. By contrast, t he distribution of biomass in different size-classes and trophic groups, wa s influenced more directly by local factors such as water quality (PH) and lake morphometry (area and maximum depth). The communities are not only str uctured at the species level, but also by size-related allocation of resour ces, within and between coexisting species. This national survey confirmed patterns observed in previous local field studies, as well as experiments d esigned for testing mechanisms acting on size-structured populations. Howev er, it highlighted the need for more extensive studies on if and how large- scale environmental variation affects the cues for size-related ontogenetic niche shifts in facultative piscivores, because species such as perch Perc a fluviatilis, brown trout Salmo trutta, and Arctic charr Salvelinlus alpin us often dominate the benthic fish community. (C) 2000 The Fisheries Societ y of the British Isles.