Riverscape features and isolation-by-distance shape spatial genetic structure of Brook Trout in a Colorado headwater stream network

Abstract

Understanding the influences of riverscape characteristics on gene flow in stream networks is crucial for managing population connectivity in freshwater species. In this study, we examined the fine-scale genetic structure of non-native Brook Trout (Salvelinus fontinalis) in a headwater stream network proposed for future reintroduction of native trout. Using 12 microsatellite loci, we genotyped 757 individual Brook Trout from 22 sampling sites throughout the dendritic stream network and modelled the effects of physical riverscape features on gene flow. Genetic clustering analysis identified four distinct tributary groups, indicating fine-scale population structure, while pairwise genetic differentiation estimates (mean FST = 0.04; mean Jost's D = 0.06) revealed some genetic connectivity across the network. Riverscape genetics models identified vertical barriers and steep stream gradients as key factors impeding gene flow, whereas higher order mainstem streams were more conducive to trout movement. Gene flow was stronger in the downstream direction, and models with interaction terms revealed that asymmetries between upstream and downstream gene flow were more pronounced in stream reaches with barriers and steep gradients. Mantel tests confirmed that both waterway distance between sites and riverscape resistance significantly influence genetic connectivity across the network. Overall, this study demonstrates that spatial genetic patterns in stream networks are shaped by a combination of isolation-by-distance, riverscape resistance, and asymmetric stream flow. Our findings suggest that this reintroduction area provides sufficient genetic connectivity to support a metapopulation of native trout.