Reproductive Dynamics and Phenotypic Differentiation of Chub (Squalius cephalus) Across a Hydropower Barrier

River fragmentation can disrupt fish movements and modify local environmental conditions, potentially affecting reproduction and phenotypic development. We investigated reproductive dynamics, scale and asteriscus shape, and otolith bilateral asymmetry in chub (Squalius cephalus) from river sections separated by a hydropower barrier without a fish passage. Reproductive dynamics were assessed using the gonadosomatic index (GSI), scale and asteriscus shape were analyzed using geometric morphometrics, and bilateral asymmetry was evaluated from paired asterisci based on length, width, and mass. GSI differed significantly with barrier position, sampling date, and their interaction, suggesting different temporal patterns of gonadal development upstream and downstream of the dam. Pairwise geometric morphometric analyses indicated site-related differences in both scale and asteriscus shape, although the strength and pattern of differentiation were inconsistent across all pairwise comparisons and analytical approaches. In contrast, asteriscus fluctuating asymmetry (FA) showed no consistent upstream–downstream pattern; length and mass asymmetry did not differ significantly among sites, while the overall difference in width asymmetry was not supported by adjusted pairwise comparisons. Overall, the results suggest site-related differences in reproductive dynamics and in scale and asteriscus shape in S. cephalus, whereas asteriscus FA showed no consistent pattern in relation to barrier position.