Résumé
The Atlantic bluefin tuna (ABFT) is an extremely valuable pelagic marine fish, with interestingphysiological adaptations to a migratory lifestyle of ceaseless swimming, notably partial endothermy and obligatory ram ventilation. Very little is known, however, about ABFT physiological energetics. In 2023, ABFT were bred for the first time in captivity, offering unprecedented access to live age 0+ juveniles. We used swimming respirometry to evaluate standard and routine metabolic rates (SMR and RMR) and aerobic metabolic scope (AS) in juveniles (mass ~550g, forklength ~30cm) at 19 °C. We performed respirometry over a range of swimming speeds but tuna lost equilibrium and became agitated below 1.4 bodylengths per second (BL s$^-1$), refused to swim faster than 2.2 BL s$^-1$, and had their highest metabolic rates when agitated. Tuna SMR was about 1.5 times those of other Mediterranean fishes at similar sizes and temperatures whereas their AS was two to threefold higher. Video analysis of tuna in their rearing tank revealed they were actually cruising spontaneously at speeds between 2.5 and 3.5 BL s$^-1$. Extrapolation of respirometry data to 3 BL s$^-1$ indicated an RMR about fourfold higher than direct measures, by tank respirometry, in other Mediterranean species. These preliminary observations would confirm longstanding predictions about the comparative energetics of ABFT but more work is clearly needed. Analysis of videos of tuna swimming in the tunnel may provide insights into their poor performance and permit further estimates of RMR at cruising speeds in the tank.