Man-Made Barriers and Augmentation Drive Spatial and Temporal Trends of Genetic Diversity and Effective Population...

Man-made barriers impede fish movement up and downstream, between the channel and floodplain, and from marginal habitats to those that promote survival and reproductive output. Barriers also increase genetic isolation resulting in loss of genetic variation. Species with drifting eggs and larvae, like the endangered Rio Grande silvery minnow (Hybognathus amarus), are especially susceptible to disruptions to demographic and genetic connectivity. Using archived Rio Grande silvery minnow DNA from 25 temporal collections spanning 38 years, and targeted amplicon sequencing, we show that unidirectional stream flow and longitudinal position of populations affect patterns of N_e and genetic diversity. N_e is reduced in the upstream-most reach (Angostura) and there is a strong correlation between upstream and range-wide N_e suggesting impacts to the entire population. In the absence of population augmentation, allelic diversity was reduced upstream but stocking with captive-reared fish restored diversity. Long-term genetic analysis indicates: (1) that there is no longer sufficient spawning and rearing to maintain natural diversity in the Angostura reach; (2) upstream movement of adults is insufficient to recover diversity; and/or (3) there is higher variance in reproductive success in resident fish resulting in small values of N_e; all consequences of a highly engineered and disconnected river corridor. Similar impacts are predicted for species with pelagic life histories in altered rivers, underscoring the need for fish passage structures that facilitate connectivity and habitat restoration aimed toward propagule retention in upstream reaches. Long-term genetic data also show periodic recruitment bottlenecks compound losses to genetic diversity imposed by river regulation that cannot be restored by hatchery stocking alone.