Behavioral Thermoregulation Influences Cumulative Welfare in Atlantic Salmon ( Salmo salar )

Cumulative welfare is defined as the balance of all experiences shaping an animal’s quality of life over its lifespan. For cultured fish, optimizing cumulative welfare requires identifying the factors that modulate biological processes linked to negative and positive experiences. This study investigated whether behavioral thermoregulation represents one such factor in Atlantic salmon ( Salmo salar ) during the freshwater phase. The fish were reared for 5 months in thermally enriched (allowing thermoregulation) or constant‐temperature (non‐thermoregulatory) environments, and their thermal behaviors were documented. Welfare was assessed through mortality, growth, and parameters linked to oxidative damage in brain and muscle tissues. These parameters included free radicals, superoxide dismutase 1 ( sod1 ), glutathione peroxidase 1a ( gpx1a ), 8‐hydroxy‐2 ^′ ‐deoxyguanosine (8‐OHdG), telomere‐to‐single‐copy gene ( T / S ) ratio, and 5‐methylcytosine (5‐mC). The ability to behaviorally thermoregulate was associated with lower mortality, greater growth, and elevated antioxidant enzyme expression compared to the non‐thermoregulatory group. Thermoregulatory fish also showed lower 8‐OHdG levels in both tissues, a stable brain T / S ratio over time, and 5‐mC changes indicative of brain hypomethylation and muscle hypermethylation. Despite these complex time, tissue, and group‐dependent patterns, the principal component analysis (PCA)‐derived index revealed consistently higher welfare scores for the thermoregulatory group. Among all indicators evaluated, PCA‐based coefficient values identified 8‐OHdG and mortality as the variables with the largest contribution to the cumulative welfare index. These findings provide experimental evidence of a link between behavioral thermoregulation and the modulation of biological processes related to an optimized cumulative welfare state. This study contributes to the integration of retrospective and prospective dimensions into animal welfare management in aquaculture.