Minimal Captive Introgression in Wild Brook Trout (Salvelinus fontinalis) Populations in the Loyalsock Creek Watershed, Pennsylvania

Elaine Langone Center, Terrace Room

Due to increased anthropogenic pressures on many fish populations, stocking wild populations with hatchery individuals has become a common management practice. Stocking has been the subject of much controversy due, in large part, to the potential for captive individuals to breed with wild stocks. By modulating the abundance of locally adapted gene complexes and introducing maladaptive genotypes, genetic introgression can cause declines in wild population fitness, resiliency, and accelerate local population extirpation. However, the rate of introgression in highly stocked river systems has not been rigorously evaluated, and so the relative risk of genetic erosion from stocking is unknown. We quantified the proportion of introgressed individuals in 30 populations of wild brook trout (Salvelinus fontinalis) distributed throughout the Loyalsock Creek watershed in Pennsylvania. Genetic assignment tests were used to determine the origin (wild vs. captive) for 1748 wild-caught and 300 hatchery brook trout. These assignment tests generated the probability of an individual fish belonging to either a simulated wild or simulated hatchery population. Fish with intermediate probabilities of wild descent were classified as introgressed, with cutoff values determined through simulation of first-generation crosses between wild and hatchery individuals. Even though streams in Loyalsock Creek are annually stocked with high densities of adult trout, we found minimal evidence for genetic introgression in the populations studied. Over 93% of all wild-caught individuals assigned to wild origin, and only 5% of wild-caught fish showed evidence of recent introgression. There was variation in introgression across populations; however, average within-site wild probability was 97%. Our results suggest that introgression with hatchery fish can occur at low rates, even in heavily managed ecosystems. However, results from this study should be viewed cautiously. Higher rates of introgression are not uncommon in other species of salmonids, and introgression may be more common under different environmental conditions. Further, we did not evaluate potential declines in wild brook trout fitness from competition with hatchery individuals, and so negative effects of stocking could still occur despite limited introgression.