Escapes of farmed Atlantic salmon from net pens can lead to interactions and hybridization with wild salmon populations. An understanding of the outcomes and consequences of this, however, remain limited, particularly in North American waters. A single, large escape event in Newfoundland, resulted in wide-spread interbreeding (27.1% hybrid or feral juveniles, 17 of 18 sampled rivers). The presence of domestic alleles from this cohort decreased annually, consistent with selection against domestic genotypes in the wild, though persistent introgression involving backcrosses to wild fish was evident. In Newfoundland, the current farmed stock derives from the St. John River, over 1,000 km distant, and there has been recent site-specific permission to farm European origin (EO) salmon as triploids, which are highly genetically divergent from North American (NA) salmon. To assess the potential interactions and consequences of hybridization among these farm strains and wild Newfoundland (NF) salmon, we created a series of controlled crosses. The offspring from these crosses were then used to quantify trait and performance differences.
We undertook experiments to compare the early-life traits of NA and EO farm strains relative to their wild conspecifics and hybrids, including early development, gene expression profiles as late yolk sac fry, juvenile behaviour in different contexts and competition and growth in semi-natural stream and tank environments. While there were clear differences in early development, most of these were driven by population of origin and maternal effects associated with egg size. The gene expression study indicated greater transcriptome differences of the wild population from the EO than the NA farmed strain. Moreover, the largest differences in global gene expression were between the two farmed strains, with few significantly differentially expressed transcripts between F1 hybrids and domesticated/wild maternal strains. These findings indicated that most of the differences seen were driven by geographic origin, with little evidence of common differences due to domestication. By contrast, the juvenile behaviour study and the competition and growth study indicated clear differences related to effects of domestication, and less so geographic origin. Both NA and EO farm fish were similarly more explorative, bold, aggressive and dominant than wild fish and related hybrids. Furthermore, both farm strains outgrew wild conspecifics in tank environments, but only EO farmed fish outgrew wild fish in semi-natural environments. These results indicate domestication may be an important driver of behavioural, growth and competitive differences, while geographical origin is a key driver of transcriptome differences, with the latter implicating greater concern related to the escaped EO than NA farmed salmon for the productivity and viability of wild NF populations.