Abstract
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 1,742 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 5.6% of wild‐caught fish assigned to introgressed. Models examining environmental drivers of introgression explained <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.
Highlights
Supplementation of wild populations with captive-raised individuals is an increasingly important management strategy for species of social, commercial, and recreational values (Araki, Cooper, & Blouin, 2007; Naish et al, 2007; Stowell, Kennedy, Beals, Metcalf, & Martin, 2015)
TA B L E 1 Site name abbreviations, sample sizes, measures of population genetic diversity (NA—average number of alleles per locus, HE-Nei’s unbiased estimate of heterozygosity), and average stocking densities for 30 wild populations and five hatchery strains of brook trout used to determine the degree of introgression in the Loyalsock Creek watershed
After sampling 30 wild brook trout populations, many of which at or near recent stocking locations, we found low incidences of introgression
Summary
Supplementation of wild populations with captive-raised individuals is an increasingly important management strategy for species of social, commercial, and recreational values (Araki, Cooper, & Blouin, 2007; Naish et al, 2007; Stowell, Kennedy, Beals, Metcalf, & Martin, 2015). Introgression can rapidly (i.e., in as little as one generation; Muhlfeld et al, 2009) modify wild population genetic diversity (Bowman et al, 2017; Ryman & Laikre, 1991), disrupt locally adapted gene complexes (Hallerman, 2003; Naish et al, 2007), homogenize genetic structure (Hindar, Ryman, & Utter, 1991; Marie, Bernatchez, & Garant, 2010), introduce maladaptive phenotypes into a population (Bolstad et al, 2017), disrupt expression of biologically relevant genes (Lamaze, Garant, & Bernatchez, 2013), and increase disease susceptibility (Currens et al, 1997) These genetic consequences of introgression, combined with reduced survival, reproduction, and competitive ability of introgressed offspring, can compromise population resiliency and future adaptive potential by reducing population sizes and eroding among-population genetic variability (Tufto, 2017). Movement of stocked individuals into nearby wild populations could result in unintended and unforeseen interactions between hatchery and wild trout that extend beyond the spatial scale of direct stocking efforts
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