The energetic consequence of specific dynamic action in southern bluefin tuna Thunnus maccoyii

The effect of feeding on the rate of oxygen consumption (Ṁ_O2) of four groups of three southern bluefin tuna Thunnus maccoyii (SBT) was examined in a large static respirometer at water temperatures of 18.2-20.3°C. Six feeding events of rations between 2.1-8.5% body mass (%M_b) of Australian sardines (Sardinops neopilchardus) were recorded (two of the groups were fed twice). Before feeding, fish swam between 0.71 and 1.4 body lengths s^-1 (BL s^-1) and the routine metabolic rate (RMR) was 366±32.5 mg kg^-1 h^-1 (mean ± s.e.m.). For all trials, Ṁ_O2 was elevated post feeding, presumably as a result of specific dynamic action (SDA). Swimming velocity was also elevated post feeding for periods similar to that of Ṁ_O2 (between 20-45 h, longest for the largest rations). Post feeding swimming velocity increased to between 0.87-2.6 BL s^-1 and was also dependent on ration consumed. It is suggested that the purpose of increased post-feeding swimming velocity was to increase ventilation volume as a response to the enhanced metabolic demand associated with SDA. Peak post-prandial Ṁ_O2 increased linearly with ration size to a maximum of 1290 mg kg^-1 h^-1, corresponding to 2.8 times the RMR. When converted to its energy equivalent, total magnitude of SDA was linearly correlated with ration size to a maximum of 192 kJ kg^-1 h^-1, and as a proportion of gross energy ingested (SDA coefficient), it averaged 35±2.2%. These results demonstrate that, although the factorial increase of SDA in SBT is similar to that of other fish species, the absolute energetic cost of SDA is much higher. These results support the contention that tuna are energy speculators, gambling high rates of energy expenditure for potentially higher rates of energy returns. The ration that southern bluefin tuna require to equal the combined metabolic costs of SDA and RMR is estimated in this study to be 3.5%M_b of Australian sardines per day.