A model-based fish bioassessment index for Eastern Mediterranean rivers: Application in a biogeographically diverse area.

•A novel policy-relevant fish-based bioassessment index is developed for the first time in Eastern Mediterranean rivers.•Predictive models define site-specific reference conditions.•Four trait-based fish metrics discriminate between impaired and reference sites.•The new model-based index assessed rivers in six freshwater ecoregions in Greece.