Impact of downstream passage through hydropower plants on the physiological and health status of a critically endangered species: The European eel Anguilla anguilla

Hydropower plants (HPPs) are a source of “green” energy but also a threat to migrating fish such as the European eel (Anguilla anguilla) owing to the disruption of river connectivity and the obstruction of downstream migration. The impact of HPP are well-documented in terms of fish survival and damages but there is no available information concerning the condition of surviving and unharmed fish. The aim of this study is to assess the impact of the passage through HPP on the survival, the physiological and health status of adult eels. Two trials were carried with variants of the Kaplan turbine - one of the most common types in Europe. After a deliberate passage through the turbines, we studied direct mortality, external and internal damages, stress and immune biomarkers such as plasma cortisol and glucose levels, alternative complement (ACH50), lysozyme and peroxidase activities, and total immunoglobulin (Ig) content. Our results showed a lower survival and a higher external and internal damages rates in the HPP groups. Glucose levels, ACH50, lysozyme and peroxidase activities and TIgc were also affected by the passage depending on HPP characteristics. Those findings suggest a greater energy expenditure and a disruption in innate immunity due to this passage. HPPs can not only have an impact in terms of direct mortality and injuries but also affect the physiological and health condition of the surviving eels. This impact may explain the delayed mortality observed in telemetric studies and the passage through many HPPs may compromise the ability of adult eels to migrate successfully to the ocean. [Display omitted] •Hydropower plants are considered as a threat to downstream migrating silver eels.•Eels passage through the turbine affected survival, and caused internal and external damages.•This passage affected energy expenditure and innate immunity.•This impact may explain the delayed mortality observed in migrating eels.•The cumulative impact may compromise the ability of eel to escape to the ocean.