Effects of hypoxia on oxygen consumption, swimming velocity and gut evacuation in southern bluefin tuna (Thunnus maccoyii)

Of the few measurements of the behavioural and physiological responses of tuna to hypoxia, most are restricted to shallow diving tropical species. Furthermore, when wild tuna experience low dissolved oxygen, they are likely to have an increased oxygen demand associated with the metabolic cost of food digestion and assimilation (specific dynamic action). However the response of postprandial tuna to hypoxia has never been examined. This study focuses on the metabolic and behavioural responses of both fasted and postprandial southern bluefin tuna (Thunnus maccoyii) to low dissolved oxygen. Fasted T. maccoyii were exposed to dissolved oxygen levels of 4.44, 3.23, 2.49 and 1.57 mg·l⁻¹ for 20–21 h. In moderate hypoxia (4.44 and 3.23 mg·l⁻¹), swimming speed was enhanced (1.5 and 1.3 times normoxic speed, respectively) presumably to increase ventilation volume. Routine metabolic rate (R_r) was similarly elevated (1.3 and 1.2 times normoxic R_r, respectively), most likely due to increased metabolic demand of faster swimming. At 2.49 mg·l⁻¹, swimming speed increased to over double the normoxic speed, possibly as an escape response. At 1.57 mg·l⁻¹, both swimming speed and R_r were reduced (0.8 and 0.9 times normoxic level, respectively), and tuna failed to survive the entire 20 h exposure period. This reveals that the critical oxygen level of T. maccoyii is between 1.57 and 2.49 mg·l⁻¹, demonstrating that they are remarkably well adapted to low dissolved oxygen. Feeding did not greatly influence their hypoxia tolerance with tuna surviving exposure to dissolved oxygen levels of 2.96 and 1.81 mg·l⁻¹ for 21 h, after ingesting a ration of 6.7% body weight of sardines (Sardinops sagax). In a subsequent experiment to determine the effects of hypoxia on digestion rate, T. maccoyii were fed to satiation and exposed to a dissolved oxygen level of 2.84 mg·l⁻¹ for 6.5–8 h. There was no significant difference in swimming speed, R_r and gastric evacuation rates of tuna in hypoxia compared to those in normoxia. This demonstrates that in moderate to severe hypoxia, T. maccoyii are still capable of aerobically supporting maintenance metabolism, routine swimming and specific dynamic action. It is hypothesized that adaptations which support the large metabolic scope of tuna are also likely to be beneficial for oxygen extraction and delivery in conditions of hypoxia.