The Benefits of Buoyancy?

Life in the North Pacific Ocean exacts a demanding physical toll on its denizens, as seen in female Steller sea lions whose body composition changes with the seasons. Their percentage body fat is greatest in the spring, before the summer pupping season, and leanest in autumn, when weather worsens and foraging becomes more difficult. These changes in body composition influence the buoyancy of a swimming sea lion and hence, the ability to forage effectively at depth.

Do these changes in buoyancy and their effect on underwater swimming affect the amount of energy a foraging sea lion expends at depth? Does a fatter and more buoyant sea lion make shorter foraging dives? To answer these and other questions about buoyancy and diving energetics, Consortium researchers conducted a series of simulated foraging dives using three trained Steller sea lions in an open water environment.

The study, authored by Andreas Fahlman, Gordon Hastie, David Rosen, Andrew Trites (each of the University of British Columbia) and Y. Naito (of the Japanese National Institute of Polar Research) was recently published in the journal Aquatic Biology.

Complicated relationship
“We assessed how changes in body composition might alter the foraging efficiency of Steller sea lions by artificially adjusting the buoyancy of trained individuals,” the authors write. “PVC tubes were attached to harnesses worn by Steller sea lions who had been trained to feed at fixed depths of 10 to 30m, which is within the dive range most commonly observed in adult female Steller sea lions in the wild.”

Equipped with harnesses of varying buoyancy, the sea lions were instructed to feed at a predetermined depth and to resurface inside a metabolic dome, which measured the amount of oxygen consumed during the dive, a measure of the metabolic cost. The PVC tubing was altered to simulate the effects on buoyancy of naturally occurring changes in body composition – typically a 12-26% change in subcutaneous fat – reported in wild adult females. Diving characteristics such as transit times and time-at-depth were also measured.

“The relationship between buoyancy, diving behavior and energetics is complicated,” the authors note. “An animal that is positively buoyant at the surface will have to swim actively to depth, but will be aided by the buoyant lift during ascent.”

Useful strategy
The authors were surprised to find that changes in body composition did not affect the diving metabolic rate of Steller sea lions for dives between 10 and 30m. As a possible explanation, the authors hypothesize that Steller sea lions may adjust their diving lung volume to compensate for changes in buoyancy to avoid additional energy expenditures at depth. This strategy would be particularly useful during shallow dives.

Alternatively, the additional cost incurred by descending with additional buoyancy may equal the energetic saving reaped during ascent, and vice versa. Either of these scenarios might explain why metabolic costs did not change with buoyancy.

These findings leave the door open for future studies of this hypothesis, which will help to shed more light on the factors that potentially limit the ability of Steller sea lions to successfully forage.

October 14, 2008

Publication:
Buoyancy does not affect diving metabolism during shallow dives in Steller sea lions Eumetopias jubatus. Fahlman, A., G.D. Hastie, D.A.S. Rosen, Y. Naito and A.W. Trites. 2008. Aquatic Biology 3:147-154. abstract hanges in buoyancy due to seasonal or abnormal changes in body composition are thought to significantly affect the energy budget of marine mammals through changes in diving costs. We assessed how changes in body composition might alter the foraging efficiency of Steller sea lions Eumetopias jubatus by artificially adjusting the buoyancy of trained individuals. PVC tubes were attached to harnesses worn by Steller sea lions that had been trained to feed at fixed depths (10 to 30 m) and to resurface inside a metabolic dome. Buoyancy was altered to simulate the naturally occurring differences in body composition reported in adult females (~12 to 26% subcutaneous fat). Diving characteristics (transit times and time at depth) and aerobic energy expenditure (gas exchange) were measured. We found that foraging cost decreased with the duration of the dive and increased with dive depth. However, changes in body composition did not affect the diving metabolic rate of Steller sea lions for dives between 10 and 30 m. We propose that Steller sea lions may adjust their diving lung volume to compensate for changes in buoyancy to avoid additional metabolic costs.