Assessing the effectiveness of hydropeaking mitigation measures through habitat suitability modelling and fish monitoring data

Hydropeaking, rapid discharge fluctuations driven by hydropower operations, poses significant threats to aquatic ecosystems, particularly fish populations. This study assesses the effectiveness of various hydropeaking mitigation strategies on fish habitats in the Bavarian Lech River, Germany, with a focus on larvae of European grayling (Thymallus thymallus) . The ecohydraulic model CASiMiR was employed to simulate four scenarios, ranging from a baseline, with no mitigation, to a combined operational and morphological interventions. Modelling results show that reducing downramping rates through modified turbine operation allows to decrease stranding risk for European grayling larvae by nearly sevenfold. Morphological measures result in further reduction of stranding risk while the scenario combining both operational and morphological strategies achieves the most substantial reduction of stranding risk. Similar trends are observed in the fish monitoring data with the last scenario resulting in the highest fish densities compared to baseline scenario. Decline in fish abundance due to natural flood events reveals the recruitment variability, underscoring the importance of considering both artificial and natural hydrological disturbances. Overall, the findings highlight that operational measures play a key role in reducing stranding risk, while morphological measures are essential in maintaining fish abundance and habitat persistence, both important for the good ecological status according to the European Framework Directive. The study results demonstrate the effectiveness of the combined mitigation approach in reconciling hydropower generation with riverine biodiversity conservation, which is supported by mid-term fish monitoring data.