Flow and Phenotype Determine Climbing Success in Juvenile European Eel ( <scp> <i>Anguilla anguilla</i> </scp> ): A Test of Two Ramp Designs

ABSTRACT The European eel ( Anguilla anguilla ) faces significant challenges during its migratory lifecycle due to anthropogenic habitat fragmentation. Fish passage solutions, such as eel ramps, aim to mitigate such barriers, but their effectiveness varies and may impose selective pressures on eel phenotypes. This study evaluates the impact of ramp design and water flow on the climbing success of juvenile eels, with particular focus on effects of individual eel exploratory phenotypes. Choice between two ramp designs—laterally flat and V‐shaped—was evaluated under low (3 L min −1 ) and high (9 L min −1 ) flow conditions. The proportion of eels climbing the flat and V‐shaped ramps was similar at low flows (36% and 32%, respectively), while a V‐shaped ramp led to a higher proportion of climbs than a flat ramp at high flows (30% and 2%, respectively). Additionally, individuals with lower activity scores had a higher probability of climbing. These findings suggest that ramp design influences eel passage efficiency and highlight the potential for unintended selective pressures against high‐activity phenotypes. Optimizing eel passage design is crucial to allow upstream and downstream migration and maintaining population diversity. Further studies are needed to assess if upstream migration over multiple eel ramps can affect the migration or phenotypic selection to ensure that passage design does not inadvertently disadvantage climbing success for certain phenotypes within the already threatened eel population.