Model-based evaluation of the genetic impacts of farm-escaped Atlantic salmon on wild populations

Abstract

Genetic interactions (i.e. hybridization) between wild and escaped Atlantic salmon Salmo salar from aquaculture operations have been widely documented, yet the ability to incorporate predictions of risk into aquaculture siting advice has been limited. Here we demonstrate a model-based approach to assessing these potential genetic interactions using a salmon aquaculture expansion scenario in southern Newfoundland as an example. We use an eco-genetic individual-based Atlantic salmon model (IBSEM) parameterized for southern Newfoundland populations, with regional environmental data and field-based estimates of survival, to explore how the proportion of escapees relative to the size of wild populations could potentially influence genetic and demographic changes in wild populations. Our simulations suggest that both demographic decline and genetic change are predicted when the percentage of escapees in a river relative to wild population size is equal to or exceeds 10% annually. The occurrence of escapees in southern Newfoundland rivers under a proposed expansion scenario was predicted using river and site locations and models of dispersal for early and late escapees. Model predictions of escapee dispersal suggest that under the proposed expansion scenario, the number of escapees is expected to increase by 49% and the highest escapee concentrations will shift westward, consistent with the location of proposed expansion (20 rivers total >10% escapees, max 24%). Our results identify susceptible rivers and potential impacts predicted under the proposed aquaculture expansion scenario and illustrate how model-based predictions of both escapee dispersal and genetic impacts can be used to inform both aquaculture management decisions and wild salmon conservation.

Precocial male maturation contributes to the introgression of farmed Atlantic salmon into wild populations

“…In the following years, the proportion of F1 hybrids appears to have decreased drastically, consistent with a large influx of escapees in 2013. However, the low but continued presence of F1 hybrids in the latter years of monitoring also points to continuing smaller escape events, which is consistent with counting fence data that have detected mature escapees entering southern Newfoundland rivers (Bradbury et al 2020).…”

“…Therefore, genetic introgression into wild populations has been identified as a major threat to wild Atlantic salmon populations (Glover et al 2017). Interbreeding can lead to a loss of local adaptation (Taylor 1991, Einum & Fleming 1997, Bourret et al 2011, outbreeding depression (McGinnity et al 2003, Gilk et al 2004, Tymchuk et al 2007, shifts in phenology (Skaala et al 2019, Bolstad et al 2021, Besnier et al 2022, changes in life history traits (Bolstad et al 2021, Besnier et al 2022, population decline (Sylvester et al 2019, Bradbury et al 2020, and in extreme cases, could result in the extinction of wild populations (Levin et al 1996). This is supported by experimental and observational studies which report reduced survival and fitness for domestic offspring relative to wild Atlantic salmon in natural environments (Fleming et al 2000, McGinnity et al 2003, Skaala et al 2012, Sylvester et al 2019.…”

Atlantic salmon survival at sea: temporal changes that lack regional synchrony

“…Nonetheless, all hatchery fish were excluded from our smolt and adult salmon estimates. It is also instructive to observe that the most considerable declining shift in at-sea survival was found for Conne River, the population with the closest proximity to salmon aquaculture operations, whose negative consequences to wild salmon have been well documented (McGinnity et al, 2003;Sylvester et al, 2019;Bradbury et al, 2020).…”

Introgression from farmed escapees affects the full life cycle of wild Atlantic salmon

“…This would also help identifying at which point the level of introgression in the population becomes critical. Modeling studies have investigated this question ( 7 , 8 ), but more work is needed for making firm conclusions.…”

“…Interbreeding between farmed escapees and wild conspecifics thereby induces a genetic load and contributes to maladaptation of the wild population. Theoretical models predict that such interbreeding decreases population fitness and viability (4)(5)(6)(7)(8)(9), but our knowledge of the ecological impact of farmed fish escaping and breeding in the wild is limited.…”