Metabolic rate depression (MRD) has long been proposed as the key metabolic strategy of hypoxic survival, but surprisingly, the effects of changes in hypoxic O₂ tensions (Pw_O₂ ) on MRD are largely unexplored. We simultaneously measured the O₂ consumption rate (Ṁ_O₂ ) and metabolic heat of goldfish using calorespirometry to test the hypothesis that MRD is employed at hypoxic Pw_O₂ values and initiated just below P_crit, the Pw_O₂below which Ṁ_O₂ is forced to progressively decline as the fish oxyconforms to decreasing Pw_O₂ Specifically, we used closed-chamber and flow-through calorespirometry together with terminal sampling experiments to examine the effects of Pw_O₂ and time on Ṁ_O₂ , metabolic heat and anaerobic metabolism (lactate and ethanol production). The closed-chamber and flow-through experiments yielded slightly different results. Under closed-chamber conditions with a continually decreasing Pw_O₂ , goldfish showed a P_crit of 3.0±0.3 kPa and metabolic heat production was only depressed at Pw_O₂ between 0 and 0.67 kPa. Under flow-through conditions with Pw_O₂ held at a variety of oxygen tensions for 1 and 4 h, goldfish also initiated MRD between 0 and 0.67 kPa but maintained Ṁ_O₂ to 0.67 kPa, indicating that P_crit is at or below this Pw_O₂ Anaerobic metabolism was strongly activated at Pw_O₂≤1.3 kPa, but only used within the first hour at 1.3 and 0.67 kPa, as anaerobic end-products did not accumulate between 1 and 4 h exposure. Taken together, it appears that goldfish reserve MRD for near-anoxia, supporting routine metabolic rate at sub-P_critPw_O₂ values with the help of anaerobic glycolysis in the closed-chamber experiments, and aerobically after an initial (<1 h) activation of anaerobic metabolism in the flow-through experiments, even at 0.67 kPa Pw_O₂.

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