Abstract
A fish‐based index applicable at the pan‐European scale should encompass the relative importance of regional and local factors influencing the distribution of riverine fish. An effective way of using information available from fish assemblages to establish such a multimetric index is through the use of the reference condition approach that involves testing a fish assemblage exposed to a potential stressor against a reference condition. In this study, a variety of metrics based on count and abundance data and reflecting different aspects of the fish assemblage structure and function were selected from the literature for their potential to indicate degradation. Logistic and multiple linear regression procedures were applied, using an initial data set of 1000 reference sites fairly evenly distributed across European rivers (11 countries) and defined by some easily measured regional and local characteristics, to elaborate the simplest possible response model that adequately explains the observed patterns of each metric for a given site. A final list of 10 metrics was retained and the European Fish Index was tested using new independent datasets. The advantage of the resulting index to serve as a practical technical reference for cost‐effective biological assessments of lotic systems in Europe is related to its ability to encompass basins and river size, and its sensitivity to physical and chemical alteration of the river ecosystem.