Microhabitat used in a 0(+) rheophilous cyprinid assemblage: Quantitative assessment of community structure and fish density

DC electrofishing frames (2 m(2)) were used to study the evolution of fish density and habitat use by the O+ rheophilous cyprinid community of the River Ourthe (barbel zone) in summer-autumn 1994. 16155 0(+) fish (10-75 mm) belonging to 15 species were captured at 78 sampling sites. The most frequently encountered species were Leuciscus cephalus, L. leuciscus and Condrostoma nasus (respectively 43.6, 20.7 and 16.7% of summer captures). Correspondence analysis of the site-by-species x size matrix showed a progressive shift along the time axis and subdivision of the community into running water fish (Barbus barbus, L. leuciscus) and more phytophilous species preferring smooth substratum (L. cephalus and C. nasus). Densities as high as 1500 25-mm fish m(-2) were recorded in late June, corresponding to a biomass of approximately 250 g m(-2). A stepwise multiple-regression model accounted for 60.0% of the variation of fish density, based on velocity, water temperature and sampling date, the latter variable integrating the dispersion-mortality concept into the model. Implications of the methodology and results on a river management scale are briefly discussed.