Microsatellites Reveal Population Identity of Individual Pink Salmon to Allow Supportive Breeding of a Population at Risk of Extinction

Efforts to restore depressed populations of Pacific salmon Oncorhynchus spp. are often hampered by the inability to assign population identity to individuals in an admixture. This knowledge is of particular concern in supportive breeding programs, in which misidentification of individuals to population may result in progeny of mixed heritage, which, in turn, results in the erosion of the genetic population structure and of the existing genetic diversity and local adaptations of the target population. We evaluated two classes of genetic markers, allozymes and microsatellites, for estimating population identity of pink salmon Oncorhynchus gorbuscha in a supportive breeding program on the Dungeness River in Washington State. Fall-run pink salmon of the Dungeness River are the target of restoration, but they presumably overlap, in terms of timing, with an earlier summer run. Both marker types revealed similarly low estimates of relative genetic differentiation (θÌ, = 0.02), which suggests that there is little variation in allele frequency among populations. However, microsatellites provided a more accurate estimate of population identity. When applying a log-likelihood ratio criterion of greater than 1.3, 74.8% of individuals were correctly assigned to population using microsatellites (versus 3.1% of individuals using allozymes). The difference in assignment accuracy was best predicted by the statistic δÌ, which estimates cumulative allele frequency differences among populations. Our results suggest that genetic markers with many alleles are preferred when populations exhibit little genetic differentiation (as is the case in pink salmon), because δÌ, is more likely to be large, presumably as a result of genetic drift at each allele. The use of microsatellites to select fall-run pink salmon for supportive breeding confirmed the run-timing overlap and prevented unintentional crosses between the two populations.