River barriers as a driver of elevated mortality rates in an iteroparous anadromous fish

Anadromous fishes are vulnerable to anthropogenic stressors, especially river fragmentation, with the conservation of iteroparous populations requiring an understanding of survival during their freshwater migrations. Here, spatial, temporal, and biological drivers of mortality were determined during the spawning migrations of iteroparous anadromous twaite shad Alosa fallax in the highly fragmented River Severn basin, western Britain. Acoustic telemetry tracked 393 individuals over seven successive spawning migrations. Shad mortality rates in areas immediately downstream of migration barriers were more than double compared to unimpeded river sections, but the extent of excess mortality varied between individual barriers. In-river mortality differed between years for returning shad, but there were no significant temporal patterns of mortality within migrations. Newly tagged shad and returning individuals had similar survival rates, and increasing size was the only significant biological predictor of mortality, with sex and spawning history not significant. These results suggest that where migration barriers increase mortality rates, the population-stabilizing and resilience benefits of iteroparity may be reduced, so river fragmentation mitigations should focus on both improving passage and reducing hazard exposure.