The Relationship Between Food Intake and Visceral Warming in Southern Bluefin Tuna (Thunnus maccoyii)

Abstract

Using archival tags implanted in the body cavity of juvenile southern bluefin tuna (SBT) being held in fattening cages, we examine the effect of varying ration (of clupeoid bait fish) on the patterns and extent of warming in the viscera. Our experiments were run in summer and winter to examine the effect of ambient water temperature on the relationship between ration and visceral warming. In both seasons, we found strong relationships between the amount of food eaten and the extent of visceral warming. For clupeoid prey and the size of fish we worked with, the relationships make it possible to predict ration and intake from various measures of visceral warming calculated from archival tag data. The time taken for the viscera to reach maximum heat increment seems the easiest and most robust measure for prediction. Our three year old SBT maintained a constant differential of 2–3°C between their basal visceral temperature and ambient water temperature throughout a 6–7°C seasonal change in water temperature. For a given ration, the extent of warming of the viscera during digestion relative to the basal visceral temperature was significantly higher in winter than in summer, indicating either improved heat conservation and/or higher aerobic metabolic rates in winter. Despite the greater warming, viscera were still significantly cooler during digestion in winter than in summer. Even though visceral temperatures were cooler in winter than in summer, rates of digestion were very similar in the two seasons. Lipid content of the food varied between seasons and may account for a slowing of digestion in summer. However, it is unlikely that this accounts for the apparent lack of a thermal effect on digestion rates. We hypothesise that this is facilitated through positive thermal compensation during winter, through the activation of cold adapted isozymes in the digestive system.