Magnitude and ecological implications of thermal inertia in skipjack tuna,Katsuwonus pelamis (Linnaeus)

Synopsis

Heat exchange experiments with sedated and free-swimming skipjack tuna,Katsuwonus pelamis (Linnaeus), yielded the following results: For fish between 0.4 and 3.5 kg in weight (W),

• 1) inertial resistance to cooling and warming were virtually equal over the same span of temperature (18° to 30° C);

• 2) thermal inertia of red muscle, white muscle, and brain (in intact, living animals) was proportional to W^0.45 (i.e., coefficient of temperature change, k, ∞ W^−0.45 for each tissue);

• 3) white muscle, brain, and ventricular blood equilibrated with a changed environmental temperature about 1.1, 3.3, and 20 times as rapidly as red muscle;

• 4) the countercurrent heat exchanger was about 95% efficient as a thermal barrier between gills and red muscle; consequently, only about half (30%–80%, depending on W) the total heat transfer between the red muscle and the environment occurred across the gills;

• 5) under conditions of thermal equilibrium, the red muscle and white muscle of sedated fish were warmer than the environment by amounts independent of environmental temperature but proportional to W^0.58 and W^0.61, respectively; in contrast, the excess temperature of the brain was independent of fish weight but bore a weak, positive relation to environmental temperature; and,

• 6) in two free-swimming fish stimulated to violent activity by chasing, the red muscle warmed at rates up to 0.3° C min⁻¹, ultimately attaining temperatures 1.5° and 3.4° C above pre-chasing equilibrium levels.

Comparison of our results with those of other researchers indicated that skipjack tuna exchange core heat with the environment only about 60% as rapidly as do typical teleosts and even somewhat more slowly than do air-breathing aquatic reptiles. Results 1) and 5) were taken to imply no short-term physiological thermoregulation in skipjack tuna; problematic evidence for physiological thermoregulation in other tunas and in aquatic reptiles is discussed. Calculations based on thermal inertia, excess temperature, and rate of warming indicated that minimum and maximum rates of metabolism in the red muscle of skipjack tuna are about 4 and 25 cal g⁻¹ hr⁻¹, respectively. Similar considerations suggested that large thermal inertia and high rates of metabolism may pose an ecological problem for skipjack tuna as they grow in body mass; excess core temperature may become so large that the muscle of the fish overheats, especially during periods of greatest activity in warm waters; speculative upper temperature limits are offered for skipjack tuna as a function of body size and activity level. Two potential benefits of large thermal inertia are discussed and illustrated with simulation models; these are 1) substantial independence from rapid fluctuations of environmental temperatures as the fish move between the upper mixed layer and the thermocline, and 2) inertial ‘memory’ of thermal history to permit or enhance perception of weak temperature gradients.