Probing the Depths on a Single Breath
How much energy does a diving sea lion use?

Sea lions must periodically haul out on shore to rest, reproduce, and raise their young – giving scientists a prime opportunity to learn more about them. But studying a sea lion on land is like studying a nesting bird: it only reveals one part of the picture.

Once a sea lion disappears beneath the ocean’s surface in search of food, its body becomes a finely tuned diving machine designed for depth and distance. The longer the dive, the more likely the sea lion will find food. But diving uses energy, and a foraging trip is a careful balance between spending energy – the energetic cost of foraging – and taking it in.

Previous studies suggest that a sea lion can extend the duration of a dive by automatically decreasing its metabolism and by modifying its behavior – taking fewer flipper strokes, for example – to consume less energy at depth. To determine the metabolic ‘costs’ of diving to various depths and durations, a team of scientists recently studied a trio of trained female Steller sea lions diving in open water.

The study, authored by Drs. Gordon Hastie, David Rosen and Andrew Trites (each of the University of British Columbia) was recently published in the Journal of Experimental Marine Biology and Ecology.

What’s in a breath?

The study made use of a floating respiratory dome designed to measure the amount of oxygen consumed before and after a dive. In each open-water trial, a trained sea lion dove repeatedly from under the floating dome to a submerged target light, which was suspended at various depths. When the underwater light was turned off, the sea lion resurfaced under the respiratory dome. Oxygen consumption was then calculated from the pre- and post-dive measurements.

“Diving behavior significantly influenced the rate at which our sea lions consumed oxygen while diving,” the authors write. “In particular, two relatively simple behavioral measures — maximum depth of dives and the swimming distance during dives — appear to reliably predict diving metabolism.”

The sea lions used more energy (i.e., consumed oxygen at a higher rate)as
swimming distance increased, but used less oxygen on deeper dives.The authors suggest that sea lions become less buoyant as they dive deeper (and as water pressure increases), requiring fewer flipper strokes to stay submerged and thereby conserving oxygen. Dive depth and swimming distance also had a much greater effect on their metabolism than environmental factors such as water temperature, which fluctuated widely over the study but did not appear to affect the results.

Predicting population trends

After gathering the data, the team constructed a model to predict the oxygen consumption of sea lions diving in the wild. Using this model, the scientists estimated that an adult Steller sea lion requires between 18 and 8 kg of food per day for dives between 10 and 300 metres, respectively. This model also provides a useful tool to determine how fluctuations in prey availability — such as a shift from capelin to cod — might affect the foraging strategy of wild sea lions and the overall health of their populations.

The relationship between dive
depth and rate of oxygen consumption (relative to average energy expenditure
f[x]=0). Sea lions used less energy when diving to deeper depths.

Although wild sea lions can dive to depths of 100m, the trained sea lions in the study have so far only been studied from 1 to 30m (100 feet) — leaving many questions unanswered about the effects of extreme depth on diving metabolism. Nevertheless, the study does provide the first insights into how Steller sea lions balance their energetic costs and benefitswhen diving to depth.

It also opens the door for future research: much has yet to be learned about the inner workings of a sea lion as it slips beneath the waves in search of food.

23 October 2006

Publication:
The influence of depth on a breath-hold diver: predicting the diving metabolism of Steller sea lions (Eumetopias jubatus). Hastie, G.D, D.A.S. Rosen, A.W. Trites. 2006. Journal of Experimental Marine Biology and Ecology 336:163-170. abstract Diving animals must endeavor to increase their dive depths and prolong the time they spend exploiting resources at depth. Results from captive and wild studies suggest that many diving animals extend their foraging bouts by decreasing their metabolisms while submerged. We measured metabolic rates of Steller sea lions (Eumetopias jubatus) trained to dive to depth in the open ocean to investigate the relationships between diving behaviour and the energetic costs of diving. We also constructed a general linear model to predict the oxygen consumption of sea lions diving in the wild. The resultant model suggests that mean swimming distance and depth of dives significantly influence the oxygen consumption of diving Steller sea lions. The predictive power of the model was tested using a cross-validation approach, whereby models reconstructed using data from pairs of sea lions were found to accurately predict the oxygen consumption of the third diving animal. Predict! ed oxygen consumption during dives to depth ranged from 3.37 L min-1 at 10 meters, to 1.40 L min-1 at 300 meters over a standardized swimming distance of 600 meters. This equated to an estimated metabolic rate of 97.54 and 40.52 MJ day-1, and an estimated daily feeding requirement of 18.92 and 7.96 kg day-1 for dives between 10 and 300 meters, respectively. The model thereby provides information on the potential energetic consequences that alterations in foraging strategies due to changes in prey availability could have on wild populations of sea lions.