CARDIOVASCULAR AND RESPIRATORY PHYSIOLOGY OF TUNA - ADAPTATIONS FOR SUPPORT OF EXCEPTIONALLY HIGH METABOLIC RATES

Both physical and physiological modifications to the oxygen transport system promote high metabolic performance of tuna. The large surface area of the gills and thin blood-water barrier means that O-2 utilization is high (30-50%) even when ram ventilation approaches 101 min(-1)kg(-1). The heart is extremely large and generates peak blood pressures in the range of 70-100 mmHg at frequencies of 1-5 Hz. The blood O-2 capacity approaches 16 ml dl(-1) and a large Bohr coefficient (-0.83 to -1.17) ensures adequate loading and unloading of O-2 from the well buffered blood (20.9 slykes). Tuna muscles have aerobic oxidation rates that are 3-5 times higher than in other teleosts and extremely high glycolytic capacity (150 mu mol g(-1) lactate generated) due to enhanced concentration of glycolytic enzymes. Gill resistance in tuna is high and may be more than 50% of total peripheral resistance so that dorsal aortic pressure is similar to that in other active fishes such as salmon or trout. An O-2 delivery/demand model predicts the maximum sustained swimming speed of small yellowfin and skipjack tuna is 5.6 BL s(-1) and 3.5 BL sec(-1), respectively. The surplus O-2 delivery capacity at lower swimming speeds allows tuna to repay large oxygen debts while swimming at 2-2.5 BL s(-1). Maximum oxygen consumption (7-9 x above the standard metabolic rate) at maximum exercise is provided by approximately 2 x increases in each of heart rate, stroke volume, and arterial-venous O-2 content difference.