Abstract

The relationship between heart rate (fh) and oxygen consumption (VO₂) has been determined for several aquatic vertebrates while fasting and resting, but the simplified relationship established from fasted animals may differ or collapse completely in more complex physiological situations, such as when animals are foraging at depth or digesting prey. Our study determined the ability of heart rate to predict oxygen consumption in captive Steller sea lions, Eumetopias jubatus, while digesting at rest and while diving up to 40 m in the open ocean. Results demonstrated that environment, type of diving activity, and digestive state all altered the fh: VO₂ relationship. Different linear equations are needed to predict VO₂ from fh for sea lions fasted on land or in water, but a single equation can predict VO₂ on land regardless of digestive state. Digestion of a 4 kg or 6 kg meal changes the fh: VO₂ relationship in water from the resting state. While fh could be used to predict VO₂ in diving sea lions, the relationship differed between single dive cycles versus dive bout cycles. However, the equation that predicts VO₂ for single dive cycles did not differ from that for sea lions resting on the surface. Neither dive duration, dive depth, nor food consumed significantly affected the fh: VO₂ relationships. Heart rate accurately predicted VO₂ in diving Steller sea lions to within ± 1.1% of direct respirometry measurements, but only over complete dive cycles or dive bouts where animals recovered fully from the O₂ debt incurred underwater. Based on these results, separate equations that distinguish among environmental, digestive, and diving states can be employed to accurately predict energy expenditure from fh for Steller sea lions in the field.

Acknowledgements

The authors wish to thank the veterinary, technical, and training staff at the Vancouver Aquarium and University of British Columbia Open Water Research Station for their expertise and assistance in this study.