Catching fish in the North Pacific Ocean is no picnic. For a sea lion, it is a fine balance between expending sufficient energy to catch prey, but not so much energy that it spends more than it gains. Sea lions have a particularly challenging time searching for prey between grabbing quick breaths of air at the surface.
A trained Steller sea lion leaping out of water while wearing
a harness carrying scientific instruments.
The energy a sea lion burns while obtaining food (also known as its metabolic cost of foraging) is a key part of its natural history. Knowing how much energy an animal burns while swimming and hunting for food allows scientists to predict the amount of prey the animal requires for survival and reproduction, as well as how environmental and physiological changes will affect its ability to forage.
Consortium researchers Andreas Fahlman, David Rosen, Andrew Trites (all of the UBC Marine Mammal Research Unit), Caroline Svärd (UBC and Linkopings University), and David Jones (UBC Department of Zoology) joined forces to determine how much energy sea lions use to dive, and how sea lions manage their oxygen stores to optimize the time they can spend underwater. The results of their study were published in The Journal of Experimental Biology.
Animation: The sea lions are trained to dive and capture fish at different depths then resurface and breathe inside a plexiglass dome. The scientists measure the concentration of oxygen and carbondioxide inside the dome.
The
short movie clip above requires a FLASH plugin to view it.
Price of Diving
Steller sea lions that encounter a school of fish will typically make repeated dives as they attempt to capture prey and stay with the school. Andreas Fahlman and his colleagues created a simulated foraging site where trained sea lions could dive to depths as deep as 50 m to capture prey pumped down through a pipe. The fish was delivered at different rates creating simulated prey patches of different densities at different depths. The sea lions could feed for as long as they wanted, but were trained to resurface to breathe inside a dome that floated on the surface. In this way, the amount of oxygen that the sea lions consumed and the amount of carbon dioxide they produced could be measured to estimate how much energy was spent on each dive.
Consortium researchers found that longer series of continuous dives are less costly than shorter ones. They also found that the first dive in a series had the lowest metabolic cost compared with the last dive, which had the highest metabolic cost.
One possible explanation for the unexpected finding that all dives were not energetically equal was that the sea lions did not completely replenish their oxygen supplies when they surfaced for air, and were consequently diving with an oxygen debt until the series of dives was completed. To test this theory, the researchers carried out a second set of experiments by allowing the trained sea lions to forage freely, diving and surfacing at will, while at other times they had the animals stay at the surface in the dome until their breathing returned to normal.
Running on Half-empty
As in the first study, the sea lions used less oxygen (and apparently less energy) during their first dive, and more oxygen (and more energy) during the last dive. However, the sea lions used equal amounts of energy during dives when the sea lions were required to stay in the chamber to stabilize their oxygen levels before diving again. The researchers therefore concluded that sea lions were diving on “half-empty tanks” to maximize their foraging time underwater.
On the research barge with 1) one of the study animals, 2) a research technician monitoring the scientific instruments and underwater cameras, and 3) a researcher connecting the air tube from the oxygen and carbon dioxide analyzers to the dome where the sea lions breathe between dives.
In addition to providing new insights into what it costs sea lions to capture prey, the researchers also disproved a long-held theory that the time a sea lion spends on the surface following a foraging bout is dependent on the time needed to replenish their oxygen stores. Instead, the researchers concluded that the surface duration is primarily determined by the need to remove carbon dioxide. Such new insights help to understand what limits the ability of sea lions to capture prey, as well as the energetic consequences associated with feeding on fish stocks that are depleted or at deeper depths.
Fahlman, A., Svärd, C., Rosen, D.A.S., Jones, D.R. and Trites, A.W. 2008.
Journal of Experimental Biology 211:3573-3580.
abstract
The metabolic costs of foraging and the management of O2 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 2 to 8 dives separated by surface intervals (SI) that were determined by the sea lion (spontaneous trials) or by the researcher (conditioned trials). During conditioned trials, SI was long enough for O2 to return to pre-dive levels between each dive. The metabolic cost of each dive event (DMR = dive + surface interval) was measured using flow-through respirometry. The respiratory exchange ratio (VCO2 ·VCO2-1) 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 SI following individual dives that had short surface intervals (SI < 50 sec), but could be estimated on a dive by dive basis for longer SIs (SI > 50 sec). 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 O2 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 metabolic differences reported between pinniped species.
Steller Sea Lions: Marine Mammal Research Consortium
