Figuring out what a northern fur seal has eaten recently
can be a messy business, says fisheries biologist Jeremy
Sterling of the National Oceanic and Atmospheric
Administration’s National Marine Mammal Laboratory in Seattle,
Wash. Determining where they have been eating is a bit more

The Alaska Ecosystems Program, which includes Sterling
and more than a dozen other researchers, has been using
satellites to track northern fur seals and Steller sea lions
in the Gulf of Alaska and the Bering Sea for nearly a decade.
With a transmitter attached to an animal’s back sending
signals to the Argos instrument on the National Oceanographic
Atmospheric Administration’s polar-orbiting satellite, the
scientists can record the marine mammal’s path as it swims
hundreds of kilometers, or miles, from land foraging for food.

Now the scientists are combining their information about
where the animals go to find food with data from another
satellite, Topex/Poseidon, to understand why the animals
choose to go where they do. NASA’s Jet Propulsion Laboratory,
Pasadena, Calif., manages the Topex/Poseidon program for
NASA’s Earth Science Enterprise, Washington, D.C.

With fewer than 40,000 individuals, Steller sea lions are
an endangered species. Northern fur seals now number less than
1,000,000 in U.S. waters. The Alaska Ecosystems Program
monitors both species and conducts research on the animals’
numbers, health, mortality and basic life histories–where
they live, how they reproduce, how well they survive. What the
scientists learn is important in deciding how best to protect the animals
and manage the fisheries on which they and many other species,
including humans, also depend.

“We’d like to see the populations increasing and looking
well,” says Sterling, “but both are declining. Our job is to
try to figure out why. Is there a climatic change? Is there a
disease? Are the animals not able to reproduce? Are they not
getting enough food? Has there been some change in the food

“We use lots of different methods to find out what’s
going on,” says Sterling. “Members of our research group go
out and count animals, note seasonal and annual changes, weigh
pups. Satellites are another tool.”

While satellite tracking provides information about where
the animals go for food, the researchers wanted to know more
about the environment where and when the seals and sea lions
were foraging. “We want to figure out why animals go to
certain areas, what affects their decision-making,”says

They turned to Topex/Poseidon altimeter measurements of
the Bering Sea provided by Dr. Robert Leben, research
associate professor, Colorado Center for Astrodynamics
Research, University of Colorado. Topex/Poseidon measures sea
surface topography, the hills and valleys of the ocean’s
surface, revealing the location of currents and eddies. The
altimeter data also indicate ocean temperature and other physical conditions.

The researchers combined some of their tracking data from
1999 and 2000 with Topex/Poseidon measurements made during the
same periods. “We can definitely see some patterns, ” says
Sterling. “It appears that the animals travel on the edges
where the eddies and gyres occur. Eddies concentrate food
where animals can feed.”

Topex/Poseidon has been making continuous measurements of
sea surface height since 1992. Jason 1, scheduled for launch
on December 7, will carry on Topex/Poseidon’s mission of
monitoring the globe’s oceans even further into this century.
Like Topex/Poseidon, Jason 1 is a joint project between NASA
and France’s Centre National d’Etudes Spatiales.

“We’ve been tagging animals since the early 1990s,” says
Sterling. “We now have the potential to go back and see what
patterns we can find that can help us understand the animals’
behavior. We can also get new maps of the Bering Sea every
three days to use with our current studies.”

“Our goal is to learn everything we can to help us manage
these species and the fisheries,” he says.

More information about the Alaska Ecosystems Program is
available at: .

JPL is managed for NASA by the California Institute of
Technology in Pasadena.