Spatially based methods to assess the ecological status of European fish assemblage types

A spatially based, river type-specific approach was used to develop an ecological assessment method for European rivers based on existing sampling data. The methodology comprised two main steps: (1) description of a river and fish assemblage typology based on minimally or slightly impacted sites and (2) analyses of impacted conditions for each type. Hierarchical cluster analysis of fish species assemblages identified 15 homogeneous groups in 11 European ecoregions. Discriminant analyses, based on abiotic characteristics, were used to predict fish types at impacted sites. The latter encompassed both regional (geographic position in Europe) and local factors (longitudinal zonation) influencing the distribution of riverine fishes. To assess ecological status, the responses of more than 400 metrics (species composition, abundance and age-length structure) to human pressures were tested for each river type separately. A maximum of 10 metrics per river type was selected using discriminant analysis. The density of intolerant species and feeding guilds had the highest capacity to predict the intensity of perturbation.