Phenotypic variation in migration is widespread, yet the extent to which it reflects underlying genetic differentiation versus environmental modulation remains unresolved in partially migratory species. Using a replicated year‑long mesocosm experiment, we quantified migration reaction norms in brown trout from migratory, resident, and cross‑bred strains tracked with PIT telemetry under high‑ and low‑density conditions. Low density enhanced growth, condition, and migratory propensity, but density did not interact with genetic background. Instead, strains exhibited distinct reaction norms, demonstrating clear genetic divergence and supporting an environmental threshold model in which genetically determined thresholds shape sensitivity to environmental cues. Our results reveal how genetic background constrains plasticity in migration strategies and highlight the importance of conserving genetic diversity to maintain adaptive potential under environmental change.