UBC Reports | Vol. 50 | No. 11 | Dec. 2, 2004

A Less than Steller Diet

By Michelle Cook

Adolescent Steller sea lions may not be consuming enough prey to satisfy their nutritional needs. But, unlike some young humans, it’s not due to bad eating habits, and it could be contributing to their declining numbers in the wild, according to a recent study by two UBC researchers.

The number of Stellers has declined drastically in recent years with their worldwide population reduced by an estimated 85 percent since 1970. Although the exact cause is unknown, a change in their food sources, either due to fishing or environmental reasons, is thought to be a factor in the decline.

The study focused on whether young sea lions could physically adjust to eating lower quality prey when the high-energy fish, such as herring, that normally make up their diet are not available or in limited supply.

“There were questions about whether the quality and abundance of fish were affecting the population of young Steller sea lions in the wild,” says David Rosen, a research associate who co-authored the study for the North Pacific Universities Marine Mammal Research Consortium with zoology professor Andrew Trites.

“As a physiologist, a logical question for me seemed to be that even if they had the behavioural instinct to change their food intake, was that limited by a physical capacity to process the food?” says Rosen.

The year-long study, done with five captive yearling Stellers at the Vancouver Aquarium, was designed to determine the physiological -- not behavioural -- factors that influence the amount of fish that young sea lions can eat. The goal was to understand how changes in the availability and type of fish affect their total food intake. The study was done with young animals because they are thought to be the portion of the Steller population most at risk, and they have high energy requirements.

For the study, the sea lions were allowed to eat as much as they wanted. Researchers tried to remove any behavioural constraints to getting fish so the sea lions didn’t have to work for their food. Using different feeding schedules, the Stellers were offered either high-energy herring or low-energy capelin (a small silvery fish and a relative of the freshwater smelt) daily or every other day.

Researchers quickly found out their study participants were not picky eaters. Rosen says the sea lions only took one to two days to adapt to changes in their food supply.

“We didn’t expect the sea lions to eat much. We thought if they were used to eating seven kilograms of fish, they would eat seven kilograms of fish. But they were able to adapt much more quickly to changes in their food supply,” Rosen says.

The sea lions increased their intake of herring when herring was only available every other day. When low-energy capelin was on the menu every other day, the sea lions consumed more compared to when they ate herring or when they ate capelin every day.

The problem was the sea lions appeared to reach a limit on how much fish they could consume and process.

In order to get a similar energy intake with the lower quality food, they had to gorge themselves, eating up to 80 per cent more. That left them stuffed and lethargic.

Rosen says these results suggest that, in the wild, younger Stellers may be having physical difficulty eating enough quantities of lower energy prey, particularly when they’re not eating on a daily basis. This limitation of food intake may be an important clue to understanding how changes in fish availability and species might have contributed to the decline of Stellers in the north Pacific.

Sea Lion Research Wins Award

It’s not a stretch to say the Steller Sea Lion Open Ocean Research Project is making waves. The world’s first-ever open water study of Stellers earned UBC trainers from the North Pacific Universities Marine Mammal Research Consortium the top prize at the International Marine Animal Trainers Association in Sweden earlier this year.

The study involves training Sitka and Bonilla, two Stellers raised at the Vancouver Aquarium, to dive in the wild and return to the surface in order to measure the energy they expend. The goal is to figure out how much energy Stellers need to swim, forage for food and capture prey.

This is the first time Stellers have been successfully trained in open water. Researchers hope to see consistent results in a wild setting that will help them to better understand the animals’ biology and behaviour, and increase the chances of conserving the dwindling numbers of Sitka and Bonilla’s counterparts in the wild.

For more in project visit www.marinemammal.org.