Selection against dispersers: sex, age and origin-dependent fitness differences in wild Atlantic salmon

Local adaptation is often portrayed as uniform fitness disadvantages of immigrants relative to locals. Yet dispersal costs vary with origin, sex and life-history traits, shaping the balance between gene flow and adaptation. We quantified these heterogeneous costs by comparing the reproductive success (RS) of local and immigrant individuals using a 15-year genetic pedigree of Atlantic salmon (Salmo salar), including over 1100 adults and 3400 juveniles. Immigrants represented 19.5% of adults, showed male-biased dispersal and averaged 15% lower RS than locals. However, outcomes were strongly origin-dependent: immigrants from some populations suffered severe fitness costs while others matched or exceeded locals. Sex and life-history further modulated RS: male immigrants underperformed local males, whereas multi-sea-winter females often equalled or outperformed local females. Immigrant males partially compensated their lower RS by acquiring more mates. These findings highlight that philopatry and local adaptation are reciprocally linked, while showing that the origin of immigrants strongly modulates the fitness costs that shape gene flow–adaptation dynamics.