GENOMIC EVIDENCE OF RECENT EUROPEAN INTROGRESSION INTO NORTH AMERICAN FARMED AND WILD ATLANTIC SALMON
Escape events from Atlantic Salmon, Salmo salar , net-pen aquaculture occur regularly and evidence from across the North Atlantic has repeatedly demonstrated interbreeding and subsequent introgression between wild Atlantic Salmon and escaped domestic individuals. T he genetic impacts of gene flow from domestic into wild salmon populations depends both on the degree of domestication and the original source of the domesticated population. Atlantic salmon used i n North American aquaculture are ostensibly of North American origin. However, evidence of European introgression into North American aquaculture salmon has accumulated in recent decades, yet t he full extent as well as the potential impacts on wild salmon in the Northwest Atlantic remains uncertain.
Here, we extend previous work comparing North American and European wild salmon (n=5799) using a 220K SNP array to quantify levels of recent European introgression into samples of aquaculture salmon, aquaculture escapees, and wild salmon collected throughout Atlantic Canada. Analysis of North American farmed salmon (n=403) and escapees (n=289) displayed significantly elevated levels of European ancestry by comparison with wild North American individuals (p<0.001). Of North American farmed salmon sampled between 2011 and 2018, ~17% had more than 10% European ancestry and several individuals exceeded 40% European ancestry. Samples of escaped farmed salmon similarly displayed elevated levels of European ancestry, with two individuals classified as 100% European. Analysis of juvenile salmon collected in rivers proximate to aquaculture locations also revealed evidence of elevated European ancestry and larger admixture tract in comparison to individuals collected at distance from aquaculture.
Overall, our results demonstrate that individuals of full and partial European ancestry have been in use over the last decade and
that some of these individuals have escaped and hybridized with wild salmon throughout Atlantic Canada. Ultimately, the presence of trans-Atlantic hybridization in aquaculture salmon in Atlantic Canada elevates the potential impact and uncertainty regarding the risk posed by escapees to wild and often at-risk salmon populations.
