Development of fish-based assessment method for the ecological status of rivers in the Baltic region

A fish-based assessment method for determining the ecological status of rivers in the southern part of the Baltic region was developed, classifying fish assemblages of near natural sites, and testing the response of a wide range of fish metrics to human pressure. However, the potential for developing a statistically robust method was limited because of insufficient data and the impact of fishery exploitation. A combination of expert judgement, historical data and spatially based methods were needed to establish reference conditions and classification criteria for fish metrics. The final list of metrics (6-9 metrics per six different river types) comprises five ecological guilds (feeding, overall tolerance, habitat, migration and spawning strategy) and three sentinel (dominant) species (Salmo trutta L., Cottus gobio L. and Alburnoides bipunctatus [Bloch]). The method tends to underscore the ecological status of specific river types because the selected metrics probably do not account for all possible variability in fish community response to degradation, as some community-specific characteristics were poorly or not represented. Exploitation pressure might have considerable effects on functionality of fish metrics. Results suggest that the method needs modification to account for these limitations.