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

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.

Genetic diversity, admixture, and hatchery influence in Brook Trout (Salvelinus fontinalis) throughout western New York State

“…Only 1.96%–3.10% of the Brook Trout individuals examined showed signs of hatchery introgression. This result is consistent with other studies on wild, stream‐dwelling Brook Trout populations, which have found hatchery introgression levels to be minimal (Annett, Gerlach, King, & Whiteley, ; Kelson et al, ; White et al, ). Variables such as stocking intensity (Marie et al, ), availability of high‐quality habitat (Marie et al, ), and the amount of time since stocking occurred (Létourneau et al, ) have been shown to be correlated with the degree of hatchery introgression in Brook Trout.…”

“…However, the degree to which hatchery introgression occurs can vary substantially. For Brook Trout, studies assessing stocked lakes have found moderate-to-high levels of hatchery introgression (Harbicht, Alshamlih, Wilson, & Fraser, 2014;Lamaze et al, 2012;Létourneau et al, 2017;Marie et al, 2010), whereas introgression in small stream systems is often limited (Bruce & Wright, 2018;Kelson, Kapuscinski, Timmins, & Ardren, 2015;White, Miller, Dowell, Bartron, & Wagner, 2018). This trend could be due to greater dispersal ability for Brook Trout stocked into streams, as fish can more easily move into neighboring tributaries, rather than remain confined to a single lake.…”

“…The software program STRUCTURE is commonly used to detect population subdivisions within a data set, but has been shown to produce erroneous population assignments when unbalanced sample sizes occur and when K values are very large (Kalinowski, 2011;Wang, 2017). A simulation study by Kalinowski (2011) Trout populations, which have found hatchery introgression levels to be minimal (Annett, Gerlach, King, & Whiteley, 2012;Kelson et al, 2015;White et al, 2018). Variables such as stocking intensity (Marie et al, 2010), availability of high-quality habitat , and the amount of time since stocking occurred (Létourneau et al, 2017) have been shown to be correlated with the degree of hatchery introgression in Brook Trout.…”

Genetic Assessment Reveals Population Fragmentation and Inbreeding in Populations of Brook Trout in the Laurel Hill of Pennsylvania

“…; White et al. ). This low level of hatchery introgression has been attributed to variables such as stocking intensity (Marie et al.…”

Influence of historical and human factors on genetic structure and diversity patterns in peripheral populations: implications for the conservation of Moroccan trout

“…The high proportion of shared mitochondrial haplotypes and nuclear microsatellite alleles between Sidi Hamza and Sidi Rachid with other trout populations is also difficult to explain unless one considers these two populations as the main sources of a trout stock that is repopulating the other introgressed populations. Research efforts are also hampered by stocking practices as they make it even more difficult to accurately infer the evolutionary history of salmonids (Valiquette, Perrier, Thibault & Bernatchez, 2014; but see White, Miller, Dowell, Bartron & Wagner, 2018).…”