Heart rate and plasma lactate responses during submerged swimming and trained diving in California sea lions, Zalophus californianus.

California sea lions, Zalophus californianus, were trained to elicit maximum voluntary breath holds during stationary underwater targeting, submerged swimming, and trained diving. Lowest heart rate during rest periods was 57 bpm. The heart rate profiles in all three protocols were dominated by a bradycardia of 20-50 bpm, and demonstrated that otariid diving heart rates were at or below resting heart rate. Venous blood samples were collected after submerged swimming periods of 1-3 min. Plasma lactate began to increase only after 2.3-min submersions. This rise in lactate and our inability to train sea lions to dive or swim submerged for periods longer than 3 min lead us to conclude that an aerobic limit had been reached. Due to the similarity of heart rate responses and swimming velocities recorded during submerged swimming and trained diving, this 2.3-min limit should approximate the aerobic dive limit in these 40-kg sea lions. Total body O2 stores, based on measurements of blood and muscle O2 stores in these animals, and prior lung O2 store analyses, were 37-43 ml O2 kg-1. The aerobic dive limit, calculated with these O2 stores and prior measurements of at-sea metabolic rates of sea lions, is 1.8-2 min, similar to that measured by the change in post-submersion lactate concentration.