Influence of Swimming Speed on Metabolic Rates of Juvenile Pacific Bluefin Tuna and Yellowfin Tuna

Bluefin tuna are endothermic and have higher temperatures, heart rates, and cardiac outputs than tropical tuna. We hypothesized that the increased cardiovascular capacity to deliver oxygen in bluefin may be associated with the evolution of higher metabolic rates. This study measured the oxygen consumption of juvenile Pacific bluefin Thunnus orientalis and yellowfin tuna Thunnus albacares swimming in a swim‐tunnel respirometer at 20°C. Oxygen consumption (Ṁo2) of bluefin (7.1–9.4 kg) ranged from $$235\pm 38$$ mg kg−1 h−1 at 0.85 body length (BL) s−1 to $$498\pm 55$$ mg kg−1 h−1 at 1.80 BL s−1. Minimal metabolic rates of swimming bluefin were $$222\pm 24$$ mg O2 kg−1 h−1 at speeds of 0.75 to 1.0 BL s−1. Ṁo2 of T. albacares (3.7–7.4 kg) ranged from $$164\pm 18$$ mg kg−1 h−1 at 0.65 BL s−1 to $$405\pm 105$$ mg kg−1 h−1 at 1.8 BL s−1. Bluefin tuna had higher metabolic rates than yellowfin tuna at all swimming speeds tested. At a given speed, bluefin had higher metabolic rates and swam with higher tailbeat frequencies and shorter stride lengths than yellowfin. The higher Ṁo2 recorded in Pacific bluefin tuna is consistent with the elevated cardiac performance and enhanced capacity for excitation‐contraction coupling in cardiac myocytes of these fish. These physiological traits may underlie thermal‐niche expansion of bluefin tuna relative to tropical tuna species.