When "safe" dams kill: analyzing combination of impacts of overflow dams on the migration of silver eels

• Published in: Ecological Engineering
• Main Authors: Trancart, Thomas, Carpentier, Alexandre, Acou, Anthony, Charrier, Fabien, Mazel, Virgile, Danet, Valentin, Feunteun, Éric
• Other Authors: Station marine Dinard, Centre De Recherche et d'Enseignement sur les Systèmes Côtiers (CRESCO), Muséum national d'Histoire naturelle (MNHN)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Muséum national d'Histoire naturelle (MNHN)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), Université de Caen Normandie (UNICAEN), Normandie Université (NU)-Normandie Université (NU)-Muséum national d'Histoire naturelle (MNHN)-Institut de Recherche pour le Développement (IRD)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA), Patrimoine naturel (PatriNat), Muséum national d'Histoire naturelle (MNHN)-Centre National de la Recherche Scientifique (CNRS)-Office français de la biodiversité (OFB), Laboratoire d'Ichtyologie générale et appliquée, Muséum national d'Histoire naturelle (MNHN), Fish Pass
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2020
• Subjects: acoustic telemetry; silver eels; migration; mortality; non-powered dams; turbines drinking water intake; [SDE]Environmental Sciences; [SDE.BE]Environmental Sciences/Biodiversity and Ecology; Anguilla anguilla
• Online Access: https://hal.sorbonne-universite.fr/hal-03007018; https://hal.sorbonne-universite.fr/hal-03007018v1/document; https://hal.sorbonne-universite.fr/hal-03007018v1/file/Trancart%20et%20al.%20-%202020%20-%20When%20%E2%80%9Csafe%E2%80%9D%20dams%20kill%20Analyzing%20combination%20of%20im.pdf; https://doi.org/10.1016/j.ecoleng.2020.105741

International audience 1. The drastic decline in European eel Anguilla anguilla stock is now widely recognized. However, while various causes for this decline have been identified, the relative importance of each cause remains unclear.2. During the catadromous migration of silver eels, the negative impact of dams is frequently highlighted, but mainly for powered dams (with turbines) or where connectivity is completely ruptured. Mechanical impact due to turbine blades is often considered the most important cause of mortality of silver eels during downstream migration. Consequently, non-powered dams equipped with spillways are often considered safe for the passage of migrating silver eels.3. We hypothesized that, to understand the negative impacts of dams, a much wider context must be considered than turbine mortality alone. Using an acoustic telemetry survey of silver eels, we demonstrated the negative effects of non-powered dams on downstream migration.4. Five main impacts on eel populations were highlighted: (i) the attenuation or loss of triggering factors, leading to an absence of or delay in migration; (ii) extra delays and extra distances travelled when crossing the dam; (iii) extra energetic costs of the additional distance travelled as result of exploring the dam and the reservoir to find other escape passages; (iv) the selection of a more risky behavioral phenotype, i.e., bold eels; and (v) direct blocking once migration has started. Mortality was evaluated as a supplementary impact. Some of these effects (attenuation of triggers, extra delays to cross the dam) might be more important than the same effect from powered dams, probably due to the constant high water discharge required for turbines that facilitate the passage of eels.5. As these “safe” dams are very widespread, they must be considered a potential threat to effective eel conservation.