Routine metabolic rate of southern bluefin tuna (Thunnus maccoyii)

Routine metabolic rate (RMR) was measured in fasting southern bluefin tuna, Thunnus maccoyii, the largest tuna species studied so far (body mass = 19.6 kg (± 1.9 SE)). Mean mass-specific RMR was 460 mg kg⁻ 1 h^{− 1} (± 34.9) at a mean water temperature of 19 °C. When evaluated southern bluefin tuna standard metabolic rate (SMR) is added to published values of other tuna species, there is a strong allometeric relationship with body mass (423 M^0.86, R² = 0.97). This demonstrates that tuna interspecific SMR scale with respect to body mass similar to that of other active teleosts, but is approximately 4-fold higher. However, RMR (not SMR) is most appropriate in ram-ventilating species that are physiologically unable to achieve complete rest. Respiration was measured in a large (250,000 l) flexible polypropylene respirometer (mesocosm respirometer) that was deployed within a marine-farm sea cage for 29 days. Fasted fish were maintained within the respirometer up to 42 h while dissolved oxygen dropped by 0.056 (± 0.004) mg l^{− 1} h^{− 1}. Fish showed no obvious signs of stress. They swam at 1.1 (± 0.1) fork lengths per second and several fed within the respirometer immediately after measurements.