BLANK, J.M.*; FARWELL, C.J.; MORRISSETTE, J.M.; SCHALLERT, R.J.; BLOCK, B.A.; Stanford University; Monterey Bay Aquarium; Stanford University; Monterey Bay Aquarium; Stanford University: Effects of temperature on metabolic rates of Pacific bluefin tuna, Thunnus orientalis

Tunas are remarkable among teleosts for systemic endothermy, high metabolic rates and exceptional cardiac performance. Among tunas, the 3 bluefin species attain the largest body sizes, inhabit the broadest thermal niches, and maintain the highest body temperatures. We measured oxygen consumption of 7 to 10 kg Pacific bluefin tuna as the animals swam in an 870 L swim tunnel respirometer for up to 6 days at temperatures of 6 to 25 C. Routine metabolic rates of bluefin at 20 C (222 � 24 mgO2/kg/hr) were higher than those of yellowfin tuna in this study (162 � 19 mgO2/kg/hr). VO2 was minimized at higher speeds in bluefin than in yellowfin. At a given speed, bluefin swam with higher tailbeat frequencies and shorter stride lengths than yellowfin tuna. This difference may be related to interspecific differences in internal temperature. VO2 of bluefin swimming at 1 BL/s was minimized at temperatures of 15 to 20 C and increased at ambient temperatures of 8 to 10 C and at 25 C. Yellowfin tuna showed a similar pattern, but were unable to tolerate temperatures below 10 C. Archival tag data indicate that visceral thermal excess remained constant as ambient temperature in the swim tunnel changed. The cold-induced increase in VO2 may be related to endothermy, muscle mechanics, or behavior. Interspecific differences in metabolic rate and thermal tolerance correspond to differences in cardiac performance and biochemistry. Funded by NSF and Monterey Bay Aquarium.