Physiological Thermoregulation in the Albacore Thunnus alalunga

Abstract

Shipboard experiments demonstrate that the warm-bodied albacore tuna, Thunnus alalunga, uses physiological mechanisms to control heat loss and gain and is able to defend its body temperature ( ) over a range of ambient temperatures ( ). Thermocouple-implanted fish that were restrained yet capable of unhindered tail activity maintained stable red and white muscle temperatures ( and ) for from 15 to 160 min while exposed to ranging from 11.5 to 18.0 C. Below 11.5 C , and could not be maintained and began to cool. Above 18.0 C , both and increased with in the pattern of a constant temperature excess ( ), The mean triggering thermoregulation was 13.6 C and the mean at the initiation of thermoregulation was 19.7 C. Red and white muscle temperatures varied spatially and temporally. In one fish so tested brain temperature was not regulated while was. All test albacore moved their tails at frequencies and amplitudes that would be sufficient to power a free-swimming fish at velocities needed for hydrostatic equilibrium. At low caudal fin activity was shifted to a higher frequency (+20%) and lower specific amplitude (from 13.6% to 8.6%l [fish fork length]). This mode of swimming favors thermoregulation because a higher beat frequency would produce more heat. It may also be less efficient and thus further contribute to heat production by requiring additional muscle fiber activity to power swimming. The albacore probably controls its heat balance by modulating retial exchange efficiency. This can occur in response to rapid increases or decreases in and is also affected by a fish's recent thermal history, activity and . High activity or swimming in warmer surface waters can increase the albacore's . Shunting of cool blood around retia and to the body core via the dorsal aorta may be an important mechanism for preventing excessive heating or achieving rapid cooling upon return to a cooler . In its natural environment, the albacore doubtlessly makes short forays into water with that are both above and below its range of physiological thermoregulation. Thermal imbalances resulting from this activity are probably compensated for by changes in heat exchange efficiency and the behavioral selection of a that favors rapid restoration of thermal equilibrium.