Assessment of the ecological status of regulated lakes: stressor-specific metrics from littoral fish assemblages

Following the European Water Framework Directive (WFD), the ecological status of lakes should be classified based mainly on biota, including fish fauna. In Finnish regulated lakes, littoral fish assemblages may be especially vulnerable to winter drawdown in water level. In this study, we sampled littoral fish assemblages by electrofishing in 14 regulated and 9 reference lakes, and developed three stressor-specific metrics based on total fish density, proportion of disturbance-sensitive species, and occurrence of juveniles. Ecological status was classified from bad to high in regulated lakes, whereas standard WFD assessment based on NORDIC gillnet sampling indicated high status and no response to water level regulation. We suggest that stressor-specific metrics should be integrated into the assessment of the ecological status of regulated lakes in the WFD.