Variable Introgression from Supplemental Stocking in Southern Ontario Populations of Lake Trout

Abstract

An unintended consequence of fish stocking is genetic homogenization from interbreeding between indigenous populations and genetically distinct hatchery strains. Lake trout populations in southern Ontario have been extensively supplemented with hatchery strains originating from Great Lakes sources, but evaluation of introgressive admixture has been challenging without data or samples that precede historical stocking events. We used complementary genetic markers (mitochondrial PCR-RFLP and 12 microsatellite DNA loci) to resolve native and introgressed (hatchery descendent) genetic profiles for lake trout from four unstocked lakes and eight stocked lakes using samples from an introduced population (i.e., established by introduction of stocked fish) and source hatchery strains for comparison. We predicted that some inland populations would retain a composite native genetic profile similar to profiles of populations in unstocked lakes and that introduced and introgressed populations would resemble hatchery sources. Allele frequency-based methods and Bayesian individual assignment techniques gave largely congruent results for inferred population ancestries. Four of the eight supplemented populations included in this analysis exhibited genetic profiles consistent with native ancestry, indicating limited introgressive admixture. The remaining supplemented populations, however, showed evidence of introgression and homogenization with genetically distinct stocked fish. Recorded stocking history alone was not indicative of admixture in these populations, which suggests that other genetic, ecological, or anthropogenic factors facilitated reproduction between native and stocked fish.