Metabolic costs of foraging and the management of O2 and CO2 stores in Steller sea lions

The metabolic costs of foraging and the management of O(2) and CO(2) stores during breath-hold diving was investigated in three female Steller sea lions (Eumetopias jubatus) trained to dive between 10 and 50 m (N=1142 dives). Each trial consisted of two to eight dives separated by surface intervals that were determined by the sea lion (spontaneous trials) or by the researcher (conditioned trials). During conditioned trials, surface intervals were long enough for O(2) to return to pre-dive levels between each dive. The metabolic cost of each dive event (dive + surface interval; DMR) was measured using flow-through respirometry. The respiratory exchange ratio (V) over dot(O2)/(V) over dot (CO2)) was significantly lower during spontaneous trials compared with conditioned trials. DMR was significantly higher during spontaneous trials and decreased exponentially with dive duration. A similar decrease in DMR was not as evident during conditioned trials. DMR could not be accurately estimated from the surface interval (SI) following individual dives that had short SIs (< 50 s), but could be estimated on a dive by dive basis for longer SIs (> 50 s). DMR decreased by 15%, but did not differ significantly from surface metabolic rates (MRS) when dive duration increased from 1 to 7 min. Overall, these data suggest that DMR is almost the same as MRS, and that Steller sea lions incur an O(2) debt during spontaneous diving that is not repaid until the end of the dive bout. This has important consequences in differentiating between the actual and 'apparent' metabolic rate during diving, and may explain some of the differences in metabolic rates reported in pinniped species.