Transcriptional Responses to Chronic Thermal Stress in Chum Salmon (Oncorhynchus keta) Smolt

Understanding the chronic thermal acclimation capacity of chum salmon (Oncorhynchus keta) is essential for predicting species resilience and developing mitigation strategies under ocean warming. We investigated the upper limit of chronic thermal acclimation and its underlying molecular mechanisms in chum salmon smolts exposed to four constant temperatures (10, 14, 18, and 22 °C) for 6 weeks. Transcriptional responses of genes related to cellular stress protection, endocrine feedback regulation, antioxidant defense, metabolic regulation (AMPKα and mTOR), and protein degradation were quantified in the liver, skeletal muscle, and brain. Chronic exposure to elevated temperature elicited tissue-specific molecular responses, with the most pronounced effects observed at 22 °C. At this temperature, all tissues showed marked induction of heat shock proteins and ubiquitin, accompanied by suppression of antioxidant defenses, glucocorticoid receptor signaling, and AMPKα–mTOR-mediated metabolic regulation, particularly in the liver and muscle. These responses were consistent with previously reported impairments in growth performance, lipid reserves, and hematological indices from the same growth trial. In contrast, smolts maintained at 18 °C exhibited molecular signatures indicative of effective physiological compensation without severe cellular stress. Collectively, these results indicate that chum salmon smolts can acclimate to chronic warming up to 18 °C, whereas exposure to 22 °C exceeds their acclimation capacity and induces a tertiary stress response.