HEART-RATE AND PLASMA LACTATE RESPONSES DURING SUBMERGED SWIMMING ANDTRAINED DIVING IN CALIFORNIA SEA LIONS, ZALOPHUS-CALIFORNIANUS

California sea lions, Zalophus californianus, were trained to elicit m aximum 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 wer e dominated by a bradycardia of 20-50 bpm, and demonstrated that otari id diving heart rates were at or below resting heart rate. Venous bloo d 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 s ubmerged for periods longer than 3 min lead us to conclude that an aer obic limit had been reached. Due to the similarity of heart rate respo nses and swimming velocities recorded during submerged swimming and tr ained diving, this 2.3-min limit should approximate the aerobic dive l imit in these 40-kg sea lions. Total body O-2 stores, based on measure ments of blood and muscle O-2 stores in these animals, and prior lung O-2 Store analyses, were 37-43 ml O-2 kg(-1). The aerobic dive limit, calculated with these O-2 stores and prior measurements of at-sea meta bolic rates of sea lions, is 1.8-2 min, similar to that measured by th e change in post-submersion lactate concentration.