MARINE MAMMAL RESEARCH NEWSLETTER   |   September 2015 (Issue 13)

From the Lab

Sea lions give up earlier when prey are scarce

All animals share a common trait when it comes to eating—they all seek as much as they can using the least amount of energy to do so. However, diving mammals like Steller sea lions have another complication—they have to hold their breath while chasing down prey in the dark depths of the ocean.

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Sitka, one of 4 Steller sea lions at the Open Water Research Station.

For her MSc thesis, Liz Goundie sought to determine how depth and density of prey affect the diving behavior and foraging costs of Steller sea lions. “This is really important because the decline of Steller sea lion populations in Alaska may be due to changes in prey availability,” Liz explains. “We need to know whether sea lions can alter their behavior to compensate for changes in prey availability or whether they are having a harder time obtaining enough energy.”

Liz conducted her study at UBC’s Open Water Research Station with four adult, female Steller sea lions trained to dive freely in the open ocean. Her study is the first to make direct measurements of both energy expenditure and intake of animals diving in a realistic setting to capture prey of different densities at different depths.

Liz Goundie at open water research station with Steller sea lion.

Liz Goundie at the Open Water Research Station with one of the girls.

Liz created simulated prey patches of different densities at 10 and 40 m (33 and 131 feet), and monitored dive behavior and the amount of energy the sea lions spent and gained while diving on these patches. “This was a really unique set-up that allowed us to measure the diving behavior and costs of animals that were diving to realistic depths in a natural setting,” Liz reports. “It is something that couldn’t be done anywhere else and really helped to bridge the gap between purely wild studies and traditional captive studies where animals are confined to pools.”

Liz found that sea lions altered their dive behavior in response to both the depth and density of the prey they were eating. In line with theoretical models, the sea lions spent the longest time foraging on deep patches with high prey density, and the least time foraging on shallow, low-density prey patches. As a result of these behavioral differences, the sea lions spent proportionally less time foraging in the low density prey patches. “This means that not only was less food available to the animals at these low-density prey patches, but by giving up earlier, they were also foraging less efficiently.” Liz explains.  This resulted in the sea lions gaining five-times less energy in the low-density prey patches compared to the high-density prey patches, even though the difference in the amount of food available was only three-times less.

This study provides valuable insight into how the Steller sea lions in Alaska may be affected by changes in prey distribution or abundance. If prey becomes less available, and the sea lions become less efficient foragers, they will need to spend more time foraging. Spending more time foraging increases their risk of being eaten by killer whales, and could take time and energy away from other important activities, such as caring for their young. Such impacts of prey availability on individual animals have implications for the health of the entire population. Liz’s study is the most recent conducted with the Steller sea lions at the Open Water Research Station. Studies such as this continue to help better understand and responsibly manage sea lions in the wild.

Liz Goundie recently completed her M.Sc. degree at the University of British Columbia. Her research was recently published in the Journal of Experimental Marine Biology and Ecology.

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