Water Temperature as the Main Cause of Failure to Meet the Requirements of the European Water Framework Directive in the Fish Fauna Quality Element: Comparison of Two Low Mountain Rivers in Bavaria, Germany

This study addresses a long-term failure to achieve the good ecological status for fish fauna required by the European Water Framework Directive (WFD) in the salmonid–rhithral zone in Bavaria, Germany. To identify the causes, we compared extensive fish population data from the Eger River with data from its twin river, the Röslau. The proportion of autochthonous cold, oligo-stenothermic fish species is significantly lower in the Eger (4.97% vs. 37.31%). In addition, continuous water temperature measurements were taken at five locations from spring to early autumn in 2023. The Eger showed significantly higher water temperatures throughout the measurement period. In midsummer, differences in daily maximum temperatures exceeded 10 °C at the same altitude. The proportion of cold-water fish in the population appeared to be negatively related to the mean of summer water temperature peaks (MWTP) (R = 0.95, p < 0.01). Accordingly, restoration measures in the Eger would need to reduce the MWTP from the current 19.1–20.5 °C to at least 16–16.5 °C, which requires more detailed data on the thermal footprint of individual hydraulic structures. The anthropogenic thermal impact on the Eger was overlooked as the root cause of the problem for many years, which largely explains the ineffectiveness of previous restoration measures. At least within the salmonid–rhithral zone, we consider the ability of the standardized fish-based assessment tool (fiBS) to detect thermal deterioration to be sufficient, provided it is applied with due care.