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Although human fragmentation of freshwater habitats is ubiquitous, the genetic consequences of isolation and a roadmap to address them are poorly documented for most fishes. This is unfortunate, because translocation for genetic rescue could help mitigate problems. We used genetic data (32 SNPs) from 203 populations of westslope cutthroat trout (Oncorhynchus clarkii lewisi) to (1) document the effect of fragmentation on genetic variation and population structure, (2) identify candidate populations for genetic rescue, and (3) quantify the potential benefits of strategic translocation efforts. Human-isolated populations had substantially lower genetic variation and elevated genetic differentiation, indicating that many populations are strongly influenced by random genetic drift. Based on simple criteria, 23 populations were candidates for genetic rescue, which represented a majority (51%) of suitable populations in one major region (Missouri drainage). Population genetic theory suggests that translocation of a small number of individuals (∼5 adults) from nearby populations could dramatically increase heterozygosity by up to 58% (average across populations). This effort provides a clear template for future conservation of westslope cutthroat trout, while simultaneously highlighting the potential need for similar efforts in many freshwater species.

The number of effective breeders (Nb) has been touted as a means to monitor freshwater fishes, but the realized application of Nbhas been limited. Using genetic monitoring data for two Arctic grayling (Thymallus arcticus) populations of conservation concern, we describe temporal trends in genetic variation and Nb, determine how sampling and variance in reproductive success influence estimates of Nb, and quantify the relationship among Nb, effective population size (Ne), and adult abundance (Nc). Temporal trends in allelic richness (AR) and Nbtracked known or suspected population trajectories. Nbincreased in one population where there has been extensive conservation action, and both Nband ARtracked a decline in the other population where harsh winter conditions have resulted in overwinter mortality events. After accounting for population demography, Neestimates for each population were 190.7 and 308.8. Overall, this study demonstrates that temporal genetic data effectively resolve demographic and evolutionary status and trend in Arctic grayling, provides insight into the demographic factors that influence genetic variation, and emphasizes the value of temporal genetic data for conservation and management.