Balancing Aerobic and Anaerobic Metabolism, and Its Effect on Recovery Time in Freely‐Diving Steller Sea Lions ( <i>Eumetopias jubatus</i> )

ABSTRACT As breath‐hold divers, marine mammals must coordinate their use of both aerobic and anaerobic metabolism to maximize their time at depth. One indication of the relative reliance between these two energy‐producing systems is the aerobic dive limit (ADL), defined as the dive duration where post‐dive blood lactate concentrations surpass resting levels. Both the timing of the ADL and ensuing rate of lactate accumulation can affect subsequent diving behavior. Despite its importance as a physiological benchmark in diving physiology, the ADL is rarely measured directly. This study investigated the relative reliance on aerobic and anaerobic metabolism in freely diving Steller sea lions ( Eumetopias jubatus ) by measuring post‐dive lactate concentrations and oxygen and carbon dioxide recovery times in relation to dive duration. The rapid onset of elevated post‐dive lactate concentrations suggests that Steller sea lions rely on anaerobic metabolism even for dives with relatively short durations. Further, relative decreases in oxygen recovery times with increasing dive durations are consistent with a downregulation of aerobic metabolism during longer periods of submergence. These findings offer new insights into the metabolic strategies employed by Steller sea lions and could inform the development of physiologically‐based optimal foraging models.