To the Depths on a Breath

When a sea lion dives below the ocean’s surface in search of food, its foraging success largely depends on how long it can hold its breath. A complex metabolic reaction called a dive response helps the animal to use a single breath as efficiently as possible, to maximize its dive time.

By gathering information on the rate at which diving sea lions consume the oxygen in a single breath, scientists seek to predict the energy needs of individual animals and entire populations. This information also helps to explain the physiological constraints that drive the foraging decisions a sea lion makes while diving – such as when to end the dive and how a shift in prey type may affect those decisions.

To better understand diving metabolism, a group of scientists from the Marine Mammal Research Unit at the University of British Columbia recently conducted a study with trained Steller sea lions in an open ocean environment. The study, authored by Drs. Gordon Hastie, David Rosen, and Andrew W. Trites, was recently published in the journal Marine Mammal Science.

Metabolic Depression
Using sea lions trained to dive to specific depths and remain there for short periods, the scientists measured oxygen consumption at the surface and at varying depths. The sea lions cooperated with all data collection and were never restrained or confined during any of the trials.

The sea lions showed a marked decrease in oxygen consumption while diving – as much as 45% lower than the corresponding surface rates – suggesting that a diving sea lion can automatically depress its metabolism to efficiently use a single breath. The study provides the first estimates of diving oxygen consumption for Steller sea lions, and is the first study to demonstrate metabolic depression in an otariid (an “eared” seal or sea lion) while freely diving in the open ocean.

The results were then combined with existing information on oxygen storage capacity among sea lions to estimate the aerobic dive limit (ADL), or the maximum length of time a Steller sea lion may remain submerged on one breath. The ADL is a theoretical calculation; seals and sea lions have been observed to approach or exceed the expected ADL during a dive.

“Recent work suggests that there is a concise link between foraging ecology and the tendency to approach physiological limitations during dives,” the authors write. “Given that Steller sea lions generally feed at or near the seabed, we might expect that they regularly approach or exceed their aerobic limitations in the wild.”

Foraging Strategies
The authors note that the physiological mechanisms for efficiently using oxygen – which vary between species of pinniped – likely arise from the fundamental link between foraging strategy and aerobic limitations while diving. In other words, the techniques used by each pinniped species to capture its key prey will determine its dive limits and the physiological mechanisms that enable it to approach those limits while foraging.

The results of this study will enable scientists to better interpret diving behavior in wild Steller sea lions. However, the authors note that further work is required to assess how active foraging influences diving metabolism (the sea lions in the current study were stationary at depth), and how the energetic costs of diving by wild animals vary under a range of realistic diving scenarios. Each foraging dive is an adventure unto itself, and it will take time to unravel the complexities of hunting beneath the waves.

June 11, 2007

Publication:
Reductions in oxygen consumption during dives and estimated submergence limitations of Steller sea lions (Eumetopias jubatus). Hastie, G.D., D.A.S. Rosen and A.W. Trites. 2007. Marine Mammal Science 23:272-286. abstract Accurate estimates of diving metabolic rate are central to assessing the energy needs of marine mammals. To circumvent some of the limitations inherent with conducting energy studies in both the wild and captivity, we measured diving oxygen consumption of two trained Steller sea lions (Eumetopias jubatus) in the open ocean. The animals dived to predetermined depths (5–30 m) for controlled periods of time (50–200 s). Rates of oxygen consumption were measured using open-circuit respirometry before and after each dive. Mean resting rates of oxygen consumption prior to the dives were 1.34 (±0.18) and 1.95 (±0.19) liter/min for individual sea lions. Mean rates of oxygen consumption during the dives were 0.71 (±0.24) and 1.10 (±0.39) liter/min, respectively. Overall, rates of oxygen consumption during dives were significantly lower (45% and 41%) than the corresponding rates measured before dives. These results provide the first estimates of diving oxygen consumption rate for Steller sea lions and show that this species can exhibit a marked decrease in oxygen consumption relative to surface rates while submerged. This has important consequences in the evaluation of physiological limitations associated with diving such as dive duration and subsequent interpretations of diving behavior in the wild.