Oxygen removal from water versus arterial oxygen delivery: calibrating the Fick equation in Pacific salmon

While it is well known that O₂ is directly removed from the water by skin and gill tissues of fish, the mismatch between O₂ removal from water (O₂ uptake; \(\dot{V}{\text{O}}_{ 2}\) ) and the O₂ delivered to tissues by the primary circulation (O₂ consumption; \(\dot{V}{\text{aO}}_{ 2}\) ) has never been measured directly. Using data from four recent studies that simultaneously measured \(\dot{V}{\text{O}}_{ 2}\) and \(\dot{V}{\text{aO}}_{ 2}\) in 2–5 kg Pacific salmon, our analysis revealed that sockeye salmon can remove an additional 12–48 % more O₂ from the water than the primary circulation delivers to the systemic tissues. This percentage did not change significantly during swimming activity, a result that contradicts an earlier prediction that the difference should decrease when \(\dot{V}{\text{O}}_{ 2}\) increases during exercise. In resting Chinook salmon, a similar percentage difference in simultaneously measured \(\dot{V}{\text{O}}_{ 2}\) and \(\dot{V}{\text{O}}_{ 2}\) was observed, yet the difference tended to disappear during acute heat stress to a near lethal temperature. These results emphasize that caution should be exercised when using the Fick equation to estimate cardiac output because the overestimate of cardiac output that results from using the Fick equation in Pacific salmon is not small, may not be fixed and may exist in other teleosts.