Inbred source populations result in genetic rescue of imperiled trout populations

Genetic rescue—assisted translocation to reduce inbreeding and increase population viability—is a promising conservation strategy for mitigating the effects of habitat loss and fragmentation. Most empirical studies of genetic rescue conducted in the wild focus on single populations, where environmental variability and concurrent management actions confound results. Moreover, nearly all genetic rescue studies in wild populations rely on outbred sources, but inbred donor populations are often the only available option for imperiled species. We conducted a rare experimental test of genetic rescue using wild, isolated westslope cutthroat trout populations with severely elevated levels of inbreeding. A small number of fish were translocated from inbred sources into four isolated recipient populations spanning a gradient of genetic variation and inbreeding. To explicitly evaluate effects of genetic rescue, we developed an integrated population model (IPM) that incorporates Mendelian inheritance and ancestry-specific vital rates. Three recipient populations with the highest degrees of inbreeding showed increased genetic variation, aggregate fitness, and abundance. There was a scaling effect of genetic rescue, with the strongest responses in the most inbred populations. Importantly, we provide strong evidence that inbred donor populations can facilitate genetic rescue, providing critical empirical support for genetic rescue as a viable conservation strategy for many threatened taxa in which ideal (outbred) donor populations no longer exist.