Genetic Effects of Supplemental Stocking on Native Brook Trout Populations

Supplemental stocking has commonly been used as a management tool, with little thought to the long-term genetic consequences on wild populations until recently.  In Ontario, brook trout have been widely stocked for over a century, with 1.5 million fish stocked annually into inland lakes. We studied the genetic ancestry and spatial structure of wild brook trout populations in Algonquin Park, and investigated the long-term consequences of stocking on the genetic structure of these populations.  For most of the 20^th century, brook trout populations in Algonquin Park were stocked with two non-native strains: a domesticated strain (Hills Lake) and a ‘wild’ strain (Lake Nipigon). Brook trout from 27 self-sustaining populations (both native and historically stocked) from three of the five major watersheds in Algonquin Park were sampled and genotyped for 14 polymorphic microsatellite loci, along with representative broodstock from the two stocking sources. Bayesian assignment tests identified five primary genetic clusters, three of native origins and two of admixed/hatchery origins that corresponded to each of the two non-native strains. Fine spatial population structure was detected among native populations; local gene pools were present within and among watersheds. Native population were highly differentiated from the ‘wild’ hatchery strain (average F_ST of 0.42) and the domesticated strain (average F_ST of 0.26). Principal Component Analysis (PCA) of the genetic data resulted in three significant axes which explained 60% of the variation and identified native, introduced, and admixed populations. Tests showed concordant results of varying degrees of introgression in wild populations. Genetic introgression from the ‘wild’ strain was minimal, whereas homogenization of genetic structure of Brook Trout populations stocked from the domesticated strain was readily apparent in stocked populations.  Introgression appears to have been male-mediated, based on existing mitochondrial DNA data from these populations. Geographic structuring among the native populations suggests that watersheds are the appropriate management scale for conserving native gene pools and locally-adapted genotypes. Collaborative work is investigating ecological and anthropogenic influences on introgression, as well as the effects of introgression on relative fitness of populations.