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,”
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
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.