Development of Behavioral and Physiological Thermoregulatory Mechanisms with Body Size in Juvenile Bigeye Tuna Thunnus Obesus

Changes in the thermoregulatory mechanisms of juvenile bigeye tuna (Thunnus obesus) as body size increases were investigated by comparing associative (fish associated with floating objects) and characteristic (non-associative) behaviors using archival tag data from 15 fish released in Japanese waters [49-72 cm fork length (FL) at release, 3-503 days]. The thermal excess (body temperature minus ambient temperature) during characteristic behavior was about 0.7-2.3 degrees C higher than that during associative behavior in equally sized fish, and both increased with body size. Heat budget models indicated that the increases in the thermal excess of both behaviors could be attributed to decreases in the whole-body heat transfer coefficient (lambda) while descending. Thermoregulatory mechanisms appear to develop with body size by controlling lambda during descents and ascents, especially in characteristic behavior (associative: 50 cm FL, 2.6-fold; 95 cm FL, 6.4-fold; characteristic: 55 cm FL, 5.5-fold; 95 cm FL, 15.8-fold). The lambda increases while ascending to allow rapid body temperature recovery by absorbing the ambient heat in warmer, shallower water and is restricted for thermoconservation while descending to deeper, cooler water. The heat production results and calculated body temperatures for both behaviors suggest that associative behavior is the inactive state, so physiological thermoregulatory energy may be less necessary than during characteristic behavior. As body bulk lambda decreased with body size, it appears that physiological thermoregulation, by controlling arterial blood flow and/or vasoconstriction, develops to adapt to deeper, offshore waters for better exploitation of prey, in turn supporting the increasing energy demands of larger fish.