Spatial genetic structure of suspected remnant and naturalized populations of muskellunge and evidence for introgression between stocked and native strains

Achievement of management goals to maintain, enhance, or re-establish fish species of management importance in the Great Lakes often relies on hatchery supplementation. Issues may arise when individuals of hatchery origin are super-imposed upon natural stocks, particularly when resident species are naturally in low abundance such as with most Muskellunge (Esox masquinongy) populations. We used 12 microsatellite loci to survey 450 individuals from 13 populations to quantify the contributions of stocked individuals to the current Muskellunge stock structure in Michigan and document evidence of inter-strain hybridization. Genetic differentiation among populations based on variance in allele frequency was moderately high (mean Fst = 0.18), and was largely attributed to stocking history. The major genetic discordance was found among populations inhabiting waters with native Great Lakes and native and introduced Northern Muskellunge strains. We identified five genetic lineages, corresponding to native stocks (one Great Lake and two Northern strains) and two Northern Muskellunge strains obtained from other states and stocked across Michigan. Analyses revealed that the majority of populations sampled were composed of multiple hatchery strains of Northern Muskellunge, including waters connected to the Great Lakes and in waters with remnant native stocks. Admixtures of stocked strains and evidence of inter-strain hybridization were widespread. Collectively, data reveal that hatchery programs have the potential to restructure native fish populations on a statewide basis. Greater attention to current genetic stocks of both donor and recipient populations is advised to ensure that future supplementation efforts do not further erode the integrity of native stocks.