Long‐term effects of moderate and severe hypoxia on growth, intermediary metabolism, monoamine neurotransmitters, and fatty acids in juveniles of coho salmon, <i>Oncorhynchus kisutch</i>

Abstract Hypoxia concerns have increased in recent decades, reducing fish fitness and becoming a critical challenge in salmon aquaculture. We investigated the effect of long‐term hypoxia on the physiological response of coho salmon ( Oncorhynchus kisutch ). Twenty‐five juveniles were randomly distributed across five experimental conditions: 100 (control or normoxia), 60, 50, 35, and 25% oxygen saturation (OS) for 30 days. Mortality occurred only at 25% OS, while significant growth reductions were observed at 35 and 25% OS, and FCR was highest under these conditions. Serum glucose and lactate levels declined under increasing hypoxia, whereas triglycerides (TAGs) significantly increased at 35 and 25% OS. Protein, albumin, and NH 4 + levels were not affected by hypoxia. In the liver, glycogen, amino acids, and lactate decreased, while glucose and TAG levels rose with increasing hypoxia. All liver enzymes (except HOAD) were significantly affected by hypoxia, with the enzyme activity of GPt, GPa, G6PDH, HK, LDH, PK, and G3PDH decreasing with hypoxia, whereas FBP, GDH, and GPT activities increased, particularly at 35 and 25% OS. In the white muscle, glucose levels increased, while glycogen and water content decreased with hypoxia. Lactate and TAG levels were significantly high only at 60% OS. Muscle enzyme activities (except HK) significantly declined under severe hypoxia. Fatty acid profile was altered, with elevated levels of C21:0, C16:1, C18:2n6, and C18:3n6 under severe hypoxia, while C17:0 and C22:6n3 tended to decrease with greater severity of hypoxia. These changes were most evident under the 25% OS condition. Multivariate analysis revealed distinct metabolic responses between moderate and severe hypoxia in liver and muscle tissues. In the brain, serotonin and dopamine levels followed a quadratic trend, decreasing the levels of both neurotransmitters with hypoxia but without significant difference between 25% OS and normoxia, whereas norepinephrine and TAG levels increased with hypoxia severity. In conclusion, 30 days of hypoxia significantly affected the physiology of O. kisutch , generating metabolic depression and stress. These effects followed a mostly linear pattern, with the stronger effects observed at severe hypoxia of 35 and 25% OS, whereas moderate hypoxia of 60 and 50% OS had limited effects, suggesting a physiological and productive resilience of O. kisutch in the range of moderate hypoxia. Future research should investigate how combined stressors with hypoxia may exacerbate the physiological and productive responses of O. kisutch .