In order to understand the effects of changing prey availability on dive behavior and energetic costs, behavior and metabolism must be measured simultaneously. Video dive analysis and open-flow respirometry was used to assess changes in dive behavior and energetics of two captive trained California sea lions (Zalophus californianus) in response to high and low prey densities and increased cost of swimming. Differences in dive duration, foraging duration, dive efficiency and foraging efficiency were examined between high and low prey encounter rates, and between standard and cost-increased dives. Changes in foraging metabolic rate (FVO₂) and foraging carbon dioxide elimination (FVCO₂) were also assessed across all four conditions. Both animals had longer dive durations, greater dive efficiency and lower FVO₂ and FVCO₂ levels at high prey encounter rates. Foraging metabolic rate and FVCO₂ showed a slight increasing trend in cost-increased bouts for both animals, and surface interval durations were shorter. The results suggest that at high prey encounter rates animals minimize their metabolic rate to extend dive duration. These results also suggest that although FVCO₂ decreases under high prey encounter rates, increasing the cost of transport keeps VCO₂ levels elevated and increases the animal’s oxygen debt_. The authors hypothesize that as the level of work increases, the ability of the body to sequester CO₂ into bicarbonate diminishes due to an inability of the proton buffering process within the blood and muscles to keep pace with increasing CO₂ production. Therefore, CO₂ may be a more sensitive physiological marker than O₂ when examining the metabolic cost of foraging.

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