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We report genetic differences for resistance to the pathogen Listonella anguillarum within and among one cultured and two wild Canadian populations of Atlantic salmon, Salmo salar, using a common-garden experimental protocol. Following exposure to the causative agent for vibriosis, parr originating from the endangered Stewiacke River population experienced significantly higher mortality than cultured parr, four generations removed from the Saint John River population, and wild parr from Tusket River. Pathogen resistance differed between sexes; males consistently experienced higher survival than females. There was no evidence that maturity influenced pathogen resistance in male parr. The population and sex differences in pathogen resistance documented here have implications for risk assessments of the demographic consequences of interbreeding between wild and farmed Atlantic salmon.

Introgression between wild and farmed Atlantic salmon Salmo salar has been hypothesized to influence the persistence of wild populations, particularly those at heightened risk of extinction. Based on pure and hybrid crosses involving one farmed and two wild populations (Tusket River and Stewiacke River, an endangered population) in a common-garden environment, we examined the consequences of introgression to survival, growth, and disease resistance in the first year of life. Introgression with farmed salmon had either no effect (Stewiacke) or a negative effect (Tusket) on survival. Importantly, the among-family variance in survival (a negative correlate of population persistence) was higher for the hybrids than it was among the pure crosses. There was also evidence of genetic differences in growth rate and disease susceptibility. Introgression with their farmed counterparts is unlikely to have a positive effect on the fitness of wild Atlantic salmon. The degree to which genetic interactions between farmed and wild salmon threaten the latter's persistence almost certainly depends on the degree to which individuals are adapted to their local environment, on the genetic differentiation between farmed and wild individuals, and on the relative proportions of farmed and wild salmon during spawning.