No significant change in domestication-admixture during the marine phase of an Atlantic salmon population.

Domesticated organisms have been pivotal in food-production and human evolution. Yet, interbreeding between domesticated and wild conspecifics may have significant negative impacts on native populations. Atlantic salmon, subjected to more than 50 years of domestication and extensive escapes from aquaculture, represents a flagship example of this type of interaction. However, currently no data documents the influence of introgression on survival of a naturally-recruiting wild population during the marine phase of the life-cycle. Following analysis of 885 out-migrating smolts and 7275 adults returning to a Norwegian river in the period 2016-2023, we find no statistical evidence that marine survival varies with domestication-admixture, as admixture levels are not significantly different between out-migrating smolts and returning adults. As this wild population is highly admixed (25-30%) following extensive intrusion of escapees during 1989-2012, we conclude that in the current evolutionary state of this population, i.e. several generations after introgression started, there is no statistical evidence of strong selection against admixed fish during the marine phase. This is likely to be driven by natural selection purging mal-adapted individuals during the freshwater stage where fitness differences are well-documented, and potentially, also in the marine stage during the initial generation(s) following introgression.