Calorespirometry reveals that goldfish prioritize aerobic metabolism over metabolic rate depression in all but near-anoxic environments.

Metabolic rate depression (MRD) has long been proposed as the key metabolic strategy of hypoxic survival, but surprisingly, the effects of changes in hypoxic O tensions (w ) on MRD are largely unexplored. We simultaneously measured the O consumption rate ( ) and metabolic heat of goldfish using calorespirometry to test the hypothesis that MRD is employed at hypoxic w  values and initiated just below , the wbelow which  is forced to progressively decline as the fish oxyconforms to decreasing w Specifically, we used closed-chamber and flow-through calorespirometry together with terminal sampling experiments to examine the effects of w  and time on , 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 w , goldfish showed a of 3.0±0.3 kPa and metabolic heat production was only depressed at w  between 0 and 0.67 kPa. Under flow-through conditions with w  held at a variety of oxygen tensions for 1 and 4 h, goldfish also initiated MRD between 0 and 0.67 kPa but maintained  to 0.67 kPa, indicating that is at or below this w Anaerobic metabolism was strongly activated at w≤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-w  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 w.