Spatiotemporal variability in reproductive readiness of silver carp H ypophthalmichthys molitrix along a Western invasion front

Invasive species may exhibit varied reproductive capacity at the edge of their distributions. The dynamic habitat conditions of prairie streams along the edge of invasive Silver Carp Hypophthalmichthys molitrix ranges may limit reproduction. The objectives of this study were to assess the spatial and seasonal variation in female Silver Carp reproductive readiness, reproductive phases and reproductive subphases across the western invasion front in the Platte River basin in Nebraska, USA. Batch fecundity, Gonadosomatic Index (GSI) and reproductive phases were assessed for female Silver Carp collected seasonally in 2022 and 2023. Batch fecundity and GSI were lower at the Platte River confluence with the Missouri River across all seasons compared to upstream reaches in the lower Platte River and its tributaries. Spawning capable females were found in all reaches where Silver Carp were present. Evidence of prior spawning (e.g., post-ovulatory follicles) was found in all reaches and across seasons but lacked spatial and temporal trends. Mass atresia of Silver Carp was prevalent in the fall. Reproductive readiness of female Silver Carp was greater at the leading edge in the lower Platte River basin compared to downstream reaches. Silver Carp may exhibit spatial variation in reproductive capacity and spawning in heterogeneous environments, including prairie streams. Reproduction of Silver Carp in prairie streams appears probable given the prevalence of reproductively capable individuals. As such, reproduction may not be isolated to large mainstem rivers, further enhancing the risk of establishment and spread of invasive Silver Carp within prairie rivers and streams.