Hybridisation and genetic population structure of Alosa population in United Kingdom.

Human-mediated habitat fragmentation has been proposed as the main factor driving hybridization between the sympatric migratory European shads, Alosa alosa and Alosa fallax, and the observed, substantial, decline in A. alosa. In river systems across the British Isles, shad are negatively affected by navigation weirs constructed over the past few hundred years that impede their migration. Consequently, A. alosa is now considered rare, and A. fallax threatened, in the British Isles. Understanding the impact of human disturbances on hybridization holds an important role for prioritizing both settings conservation guidelines and river restoration projects. In this study, five British populations were genotyped using twenty-four microsatellite loci, in order to measure the extent of hybridisation between A. alosa and A. fallax. Our results revealed a high level of hybridisation in British shad populations, except for one that spawns in Poole Bay where only pure A. fallax individuals were found. The extent of hybridisation varied across the rivers from 31.8% to 53.3% in the River Severn and Solway Firth, respectively. Pure A. alosa individuals were identified only in the River Tamar and Solway Firth. Overall, the results reveal that whilst genetic introgression between European Alosa spp. remains an issue for their genetic integrity, both species remain present in British waters.