Swimming performance in early life stages of three threatened Iberian Leuciscidae

Rivers are dynamic systems where flow is constantly changing, making early fish life stages with lower swimming abilities potentially vulnerable to rapid changes in water velocity. In this study, we evaluated the response of critical young life stages of three Iberian leuciscids – Achondrostoma occidentale, Iberochondrostoma lusitanicum, and Iberochondrostoma almacai – to increasing water velocities, by determining their critical swimming speed (Ucrit) and its relation with morphometric traits potentially associated with better swimmers. Results suggest a positive relation between both fish length and body depth with Ucrit and species-specific differences in swimming performance. Moreover, most fish could not withstand the maximum velocity tested (20 cm s−1), which can be surpassed in the rivers they occur. This study highlights the importance of considering individual traits for future management of fish stocks in rivers.