Data from: Limited hatchery introgression into wild brook trout (Salvelinus fontinalis) populations despite reoccurring stocking

Due to increased anthropogenic pressures on many fish populations,
supplementing wild populations with captive-raised individuals has become
an increasingly common management practice. Stocking programs can be
controversial due to uncertainty about the long-term fitness effects of
genetic introgression on wild populations. In particular, introgression
between hatchery and wild individuals can cause declines in wild
population fitness, resiliency, and adaptive potential, and contribute to
local population extirpation. However, low survival and fitness of
captive-raised individuals can minimize the long-term genetic consequences
of stocking in wild populations, and to date the prevalence of
introgression in actively stocked ecosystems has not been rigorously
evaluated. We quantified the extent of introgression in 30 populations of
wild brook trout (Salvelinus fontinalis) in a Pennsylvania watershed, and
examined the correlation between introgression and 11 environmental
covariates. Genetic assignment tests were used to determine the origin
(wild vs. captive-raised) for 1742 wild-caught and 300 hatchery brook
trout. To avoid assignment biases, individuals were assigned to two
simulated populations that represented the average allele frequencies in
wild and hatchery groups. Fish with intermediate probabilities of wild
ancestry were classified as introgressed, with threshold values determined
through simulation. Even with reoccurring stocking at most sites, over 93%
of wild-caught individuals probabilistically assigned to wild origin, and
only 6% of wild-caught fish assigned to introgressed. Models examining
environmental drivers of introgression explained less than 3% of the
among-population variability, and all estimated effects were highly
uncertain. This was not surprising given overall low introgression
observed in this study. Our results suggest that introgression of
hatchery-derived genotypes can occur at low rates, even in actively
stocked ecosystems and across a range of habitats. However, a cautious
approach to stocking may still be warranted, as the potential effects of
stocking on wild population fitness and the mechanisms limiting
introgression are not known.