1. Management of running waters and assessment of water quality trends require the use of biological methods. Among potential indicators, fish assemblages are of particular interest because of their ability to integrate environmental variability at different spatial scales.
2. The French Water Agencies and the Ministry of the Environment initiated a research programme to develop a fish-based index that would be applicable nation-wide. Such an index should encompass the relative importance of geographic, ecoregional and local factors influencing the distribution of riverine fishes.
3. An effective way of using the information available from fish assemblages to establish such an index is through the use of the ‘reference condition approach’ which involves testing a fish assemblage exposed to a potential stress against a reference condition that is unexposed to such a stress.
4. Logistic regression procedures were applied, using a fish data set of 650 reference sites fairly evenly distributed across French rivers and defined by some easily measured regional and local characteristics, to elaborate the simplest possible response model that adequately explains the observed patterns of occurrence for each species of a fish assemblage at a given site of any given river. This allows us to predict a ‘theoretical’ assemblage at a site.
5. The models were validated using a second independent data set of 88 reference sites. Using a third data set of 88 disturbed sites, the observed assemblages were then compared against the reference condition as defined by the ‘theoretical’ assemblages. The amount of deviation between the expected and observed assemblages within these sites is used as a measure of degradation.
6. This approach could be used as a framework for adapting and calibrating a multimetric index, thereby serving as a practical technical reference for conducting cost-effective biological assessments of lotic systems.