Blank, J.M.*; Davie, P.S.; Morrissette, J.M.; Block, B.A.: EFFECTS OF TEMPERATURE ON CARDIAC PERFORMANCE IN YELLOWFIN TUNA

Tunas are remarkable among teleosts for their endothermy, high metabolic rates and high capacity for exercise. While tunas warm their swimming muscles and viscera, their hearts are thought to remain near ambient temperature. This raises the possibility that cold water may impair cardiac performance in vivo, limiting the scope for metabolism of tuna in the wild. We have investigated the effects of acute temperature change on cardiac function in yellowfin tuna (Thunnus albacares) using an in situ perfused heart preparation. We found that as temperature was reduced, heart rate fell but stroke volume increased. Heart rate exhibited a Q10 of ~2.0 between 15�C and 25�C while stroke volume showed a Q10 of ~-0.5 over the same range. Cardiac output was 15-30% lower at 15�C than at 20�C or 25�C. Maximal output pressure attainable without flow compromise remained nearly constant at 12 kPa over this temperature range. This reduction in cardiac performance with decreasing temperature suggests that cold water may reduce blood flow and hence oxygen delivery in wild yellowfin tuna.