Widespread hybridization in sympatric populations of allis shad Alosa alosa (L.1758) and twaite shad Alosa fallax (Lacépède 1803): A transnational perspective for conservation and management.

The dynamics of hybridization between co-occurring species in environments that have been modified by anthropogenic activities are important for generating new understandings of how species respond to disturbance and to support conservation management. Accordingly, the patterns of hybridization between anadromous allis shad Alosa alosa (L.1758) and twaite shad Alosa fallax (Lacépède 1803) were investigated, where these species historically co-occurred across rivers in Europe. Genetic and meristic data from 1102 individuals sampled at 24 coastal and riverine sites revealed hybridization occurred across their overlapping Atlantic range, with hybrids detected in all regions, including both first-generation and later-generation crosses, with prevalence and hybrid class distribution varying geographically. Four main hybridization patterns were identified, shaped by local species composition, spawning behaviour and habitat connectivity. Gill raker counts reliably distinguished parental species and F1 hybrids, but not later-generation hybrids. Fork length, gill raker number and body condition differed between species and sexes, and were influenced by regional and habitat context. Intermediate traits in hybrids suggest potential ecological distinctiveness. Straying by A. alosa into catchments dominated by A. fallax likely promotes repeated hybridization and introgression. It is suggested that hybridization in this species complex is not solely a by-product of anthropogenic disturbance but may also reflect adaptive processes enhancing population resilience and colonization capacity. Conservation and management frameworks should thus account for hybrids as a dynamic and functionally relevant component of Alosa spp. populations, and consider phenotypic thresholds (e.g. size) for improved identification and protection in the field.