Brook Trout Populations in Headwater Stream Networks: Reproductive Biology, Riverscape Genetics and Climate Change Impact on Abundance

I used neutral genetic markers to study reproduction and landscape influence on spatial population structure, and examined climate change impact on size-specific abundance for brook trout Salvelinus fontinalis populations in two headwater stream channel networks in Connecticut. Sibship reconstruction revealed the prevalence of polygamy in males and females, a balanced sex ratio, a balanced reproductive success among individuals, and limited dispersal. The two study streams were characterized with low effective population sizes (91 and 210). Both-sex polygamy, a balanced reproductive success, and a balanced sex ratio may help maintain genetic diversity of brook trout populations with small breeder sizes persisting in headwater streams.^ Spatial population structure was observed and it was attributable to riverscape habitat variables. A weak pattern of isolation-by-distance was observed consistently despite the fine scale of investigation and spatially continuous distribution. Genetic differentiation between individuals was positively correlated with total number of confluences in both study sites, indicating the importance of dendritic habitat patterns on spatial genetic structure. In one study site, stream gradient, temperature and habitat volume were significant variables for explaining individual genetic differentiation, although their effect was confounded by correlation among riverscape variables and the presence of first-order tributaries that were identified as genetically distinguishable from each other. ^ Hierarchical regression modeling was used to understand size-specific trout abundance as a function of habitat variables. Increased stream temperature reduced abundance of nearly all size classes. Young-of-the-year trout were positively associated with the amount of riffle habitat in upstream reaches and tributaries, but the amount of pool area and maximum stream depth were important for larger trout. Under climate change scenarios, in which temperature elevation was assumed based on paired stream-air temperature records and stream flow reduction was assumed to affect pool and riffle habitats differently, brook trout of all size classes were projected to decrease in abundance overall. However, some stream segments, particularly tributaries, were projected to remain thermally suitable and harbor populations dominated by small-sized trout even under the most severe climate change scenario. As a result, the stream channel networks were projected to experience habitat loss and fragmentation as climate change progresses.