Abstract BACKGROUND Slaughter is a critical phase in aquaculture that can severely compromise both animal welfare and product quality. Stress responses triggered during this stage may accelerate post‐mortem biochemical degradation and promote oxidative damage in fish fillets. Essential oils, known for their antioxidant and anti‐inflammatory properties, have been proposed as dietary supplements to help mitigate stress and preserve flesh quality. This study investigated the effects of dietary essential oil supplementation and different slaughter methods, air asphyxia and percussion, on stress biomarkers, oxidative processes, and fillet quality in rainbow trout ( Oncorhynchus mykiss ), both immediately after slaughter and during frozen storage. RESULTS Air asphyxiation significantly accelerated ATP degradation, increased lipid and protein oxidation products and caused texture loss in fillets assessed immediately post‐slaughter. These effects were markedly less pronounced in percussion‐slaughtered fish. Lipid mediators such as 12‐HpEPE+15‐HpEPE and PGD3 + PGE3 were elevated only in asphyxiated fish, providing potential biomarkers for slaughter‐induced stress. Proteomic analysis identified several glycolytic enzymes as highly responsive to air asphyxia. Dietary supplementation with essential oils (0.02%) did not mitigate the immediate physiological stress responses to slaughter and was associated with increased muscle protein oxidation at death. However, during 45 days of frozen storage, fillets from fish fed the supplemented diet exhibited delayed progression of oxidative damage, with improved texture and colour retention, especially under high‐stress conditions such as air asphyxia. CONCLUSION Although dietary essential oils did not reduce slaughter‐induced stress, they proved to be an effective strategy for delaying fish fillet degradation and preserving texture and colour during frozen storage. © 2026 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
Méndez et al. (2026) studied this question.