Genetic Monitoring and An Experimental Test of Genetic Rescue in Fragmented Populations of Brook Trout

Patch delineation of headwater streams provides a biologically meaningful and practical unit to manage brook trout (Salvelinus fontinalis) populations. At a large regional spatial scale, genetic monitoring of patches can be used to identify those that are most at risk of extirpation. We have initiated a genetic monitoring effort for brook trout in their eastern range. Preliminary findings include a strong relationship between the effective number of breeders (N_b) and patch size. We have also initiated an experimental test of a patch-level management action aimed at preventing population extirpation for the most at-risk patches. Translocation of new individuals into small isolated populations, termed “genetic rescue”, may increase population persistence by alleviating inbreeding depression and increasing individual fitness. Alternatively, offspring of translocated individuals may suffer from reduced fitness or outbreeding depression. In the absence of outbreeding depression, genetic rescue may provide an effective means to increase the probability of persistence of small isolated brook trout populations. We experimentally translocated brook trout from a single source to four geographically proximate fragmented populations in Virginia to perform a replicated multi-generation test of genetic rescue.  We will use genetic markers to reconstruct reproductive success of transplant and resident individuals. We will estimate individual survival as a function of cross type to perform tests for the alleviation of inbreeding depression and the presence of outbreeding depression. Preliminary results from one experimental population showed that 61.6% of the 2012 cohort had at least one transplant parent. Individuals from families with at least one transplant parent were significantly larger than individuals from families with two resident parents (mean difference = 5.841 mm, P =0.012).