STUDIES OF TROPICAL TUNA SWIMMING PERFORMANCE IN A LARGE WATER TUNNEL.2. THERMOREGULATION

The body temperatures (T-b) of nine yellowfin tuna (Thunnus albacares) were monitored while fish swam in a large water tunnel at controlled velocities (U) and ambient temperatures (T-a). Monitoring T-b during s tep changes in T-a constant U permitted estimation of the thermal rate coefficient (k), an index of heat transfer. In the yellowfin, k is de pendent on both T-a and the direction of the thermal gradient (i.e. wh ether T-a is greater or less than T-b) Modulation of k in response to T-a was further demonstrated during tests in which U was varied; the e levation of T-b in response to equal increases in U was 3-4 times less at 30 degrees C than at 25 and 20 degrees C. These experiments demons trate that the yellowfin tuna can modulate heat transfer. This ability could prevent overheating during intense activity, retard heat loss d uring a descent into cool water and permit increased heat gain upon re turning to warm surface waters (i.e. when T-b<T-a).