Riverscape genetics of nonnative Brook Trout to inform native cutthroat trout conservation

ABSTRACT Objective Understanding how riverscape features influence gene flow is critical for managing population connectivity in freshwater species. We examined how landscape and stream characteristics shape the spatial genetic structure of nonnative Brook Trout Salvelinus fontinalis in a headwater stream network proposed for reintroduction of federally threatened Greenback Cutthroat Trout Oncorhynchus virginalis stomias. Brook Trout were studied to evaluate the suitability of this habitat for supporting a native trout metapopulation. Methods We genotyped 757 Brook Trout from 22 sites across a 60-km stream network using 12 microsatellite loci. Spatial genetic structure was assessed using clustering analysis (program STRUCTURE) and pairwise differentiation metrics (FST and Jost’s D). A spatial network modeling approach was used to quantify the effects of riverscape features (e.g., stream gradient, stream order, waterfalls, and flow direction) on trout gene flow. Results Genetic clustering identified four distinct tributary groups, while estimates of pairwise genetic differentiation indicated some genetic connectivity across the network (mean FST = 0.04; mean Jost’s D = 0.06). Trout gene flow was impeded by waterfalls, steep stream gradients, and increased hydrologic distance. Higher stream order and downstream flow direction were associated with stronger gene flow, and stream segments containing waterfalls and steeper gradients showed greater asymmetries between upstream and downstream gene flow. Conclusions Brook Trout populations in this stream network are spatially structured, but gene flow persists and is mediated by physical riverscape features and hydrologic distance. The observed patterns of genetic connectivity suggest that this habitat can support connectivity among populations of reintroduced Greenback Cutthroat Trout. In future native trout reintroduction efforts, prioritizing habitats with gradual stream gradients and fewer waterfalls would promote population connectivity.