Migration effort and wild population size influence the prevalence of hybridization between escaped farmed and wild Atlantic salmon

Hybridization of escaped farmed Atlantic salmon Salmo salar with wild populations occurs throughout their native range and can threaten wild population stability and persistence. The extent of hybridization is often population-specific and can drive changes in phenotype and genotype. Current understanding of the forces that contribute to the spatial distribution of hybridization is insufficient despite its potential to inform conservation and management efforts. Using a panel of 95 single nucleotide polymorphisms previously validated for identifying parr of farmed, wild, or hybrid descent, we present a novel exploration of inter- and intra-river distribution of hybrids from 33 locations across 9 rivers in southern Newfoundland, Canada. The proportion of hybrids varied significantly across (p < 0.001) and within rivers (p < 0.05 in 4 rivers). Binomial mixed models and logistic regression showed increased proportions of hybrid and feral offspring within smaller rivers (p < 0.0001). Within-river distribution of hybrid parr was strongly associated with the migration effort required to reach spawning sites; the hybrid proportion decreased significantly (p < 0.05) with increased elevation, geographic distance and the presence of obstructions. These observations support previous hypotheses that the distribution of escaped farmed Atlantic salmon can be restricted by migratory challenges, which result in the reduction of hybrid individuals in upstream spawning sites relative to downstream locations. Our research demonstrates that levels of hybridization vary spatially and are associated with landscape features. We suggest that consideration of spatial variation in levels of hybridization will be essential for the evaluation of impacts that escaped farmed salmon impose on wild Atlantic salmon populations.