Abstract
Atlantic salmon (Salmo salar) is an iconic species of great socio-economic value, which is also known for its philopatric behaviour called ‘homing’. After spending one to several years in freshwater, juvenilesmigrate to marine areas for growth, remaining there for one to several years before returning as adults to their natal river to reproduce. However, some individuals can disperse to nearby or more distant rivers, leading to a metapopulation functioning. Theory predicts that dispersing individuals may have a lower fitness than local individuals, but this prediction has been rarely investigated in Atlantic salmon. The objectives of this study were to: i) identify the dispersers in a focal population (Nivelle River, France), and ii) compare their reproductive success with local individuals. We combined pedigree and populationgenetic approaches using 4331 individuals genotyped with 14 microsatellites and/or 164 SNPs over a sixteen years period. Population genetic assignment tests coupled with pedigree reconstruction revealed a high dispersal rate of 19.5% into the Nivelle population. Dispersal was male-biased and higher in single-sea-winter salmon than multi-sea-winter fish. On average, reproductive success was 15% lower for dispersing individuals than local ones. However, after accounting for sex and sea age, dispersers from some populations had a higher reproductive success than local individuals. Overall, we show an origin-dependent reproductive success of dispersing individuals, which illustrates the trade-off between gene flow and local adaptation.