Sensitivity to hypercapnia and elimination of CO2 following diving in Steller sea lions (Eumetopias jubatus)

The diving ability of marine mammals is a function of how they use and store oxygen and the physiological control of ventilation, which is in turn dependent on the accumulation of CO₂. To assess the influence of CO₂ on physiological control of dive behaviour, we tested how increasing levels of inspired CO₂ (hypercarbia) and decreasing inspired O₂ (hypoxia) affected the diving metabolic rate, submergence times, and dive recovery times (time to replenish O₂ stores and eliminate CO₂) of freely diving Steller sea lions. We also measured changes in breathing frequency of diving and non-diving individuals. Our findings show that hypercarbia increased breathing frequency (as low as 2 % CO₂), but did not affect metabolic rate, or the duration of dives or surface intervals (up to 3 % CO₂). Changes in breathing rates indicated respiratory drive was altered by hypercarbia at rest, but blood CO₂ levels remained below the threshold that would alter normal dive behaviour. It took the sea lions longer to remove accumulated CO₂ than it did for them to replenish their O₂ stores following dives (whether breathing ambient air, hypercarbia, or hypoxia). This difference between O₂ and CO₂ recovery times grew with increasing dive durations, increasing hypercarbia, and was greater for bout dives, suggesting there could be a build-up of CO₂ load with repeated dives. Although we saw no evidence of CO₂ limiting dive behaviour, the longer time required to remove CO₂ may eventually exhibit control over the overall time they can spend in apnoea and overall foraging duration.