Effects of Acute Hypoxia and Reoxygenation on Hematological Parameters, Antioxidant Activity, and Hypoxia-Related Genes Expressions in Rainbow Trout and Steelhead

Global warming and water eutrophication are expanding oceanic anoxic zones and increasingly exposing offshore salmonid aquaculture to hypoxia during summer time. To evaluate the potential hypoxic tolerance difference between landlocked rainbow trout (Oncorhynchus mykiss) and anadromous steelhead (O. mykiss) in physiological responses, hematological parameters, liver antioxidant activities, and gene expression were measured before stress, after 12 and 24 h of hypoxia, and after 24 h of reoxygenation. Serum alanine aminotransferase (ALT), alkaline phosphatase (ALP), and lactate dehydrogenase (LDH) activities initially increased significantly in both fishes, then declined during extended hypoxia and reoxygenation. Peak values occurred at 12 h in steelhead and 24 h in rainbow trout, indicating a superior metabolic adjustment in steelhead. In rainbow trout liver, hypoxia significantly elevated catalase (CAT) activity but reduced superoxide dismutase (SOD) activity and lipid peroxidation (LPO) content. Conversely, steelhead showed increased SOD activity and decreased CAT activity, malondialdehyde (MDA) and LPO contents. Total antioxidative capacity (T-AOC) was significantly higher in steelhead than in rainbow trout under hypoxic stress. The Integrated Biomarker Response Version 2 (IBR_v2) index was higher in rainbow trout, reflecting greater physiological stress. Both fishes exhibited significant upregulation of liver erythropoietin (epo) expression after 12 h of hypoxia, suggesting an adaptive enhancement of erythropoiesis. Overall, rainbow trout is more sensitive to hypoxia than steelhead.