Riverscape genetics of non-native Brook Trout to inform native Cutthroat Trout conservation

Understanding how riverscape features influence gene flow is crucial for
managing population connectivity in freshwater species. We examined the
spatial genetic structure of non-native brook trout (Salvelinus
fontinalis) in a headwater stream network proposed for the reintroduction
of federally threatened greenback cutthroat trout (Oncorhynchus virginalis
stomias). Brook trout were used as a surrogate species to evaluate the
suitability of this habitat for supporting a native trout metapopulation.
Using 12 microsatellite loci, we genotyped 757 individual brook trout from
22 sampling sites and modelled the effects of physical riverscape features
on gene flow. Genetic clustering analysis identified four distinct
tributary groups, while pairwise genetic differentiation (mean FST = 0.04;
mean Jost’s D = 0.06) indicated some genetic connectivity across the
network. Vertical barriers and steep stream gradients impeded gene flow,
whereas high-order mainstem streams facilitated trout movement. Gene flow
was stronger in the downstream direction, and streams with barriers and
steeper gradients showed increased asymmetry between upstream and
downstream movement. Results suggest that this stream network provides
sufficient genetic connectivity to support a metapopulation of native
trout. Managers should prioritize habitats with gradual stream gradients
and fewer barriers to promote genetic connectivity in reintroduced native
trout.