Feature: Seals, Sea Lions and Satellites
12/3/01
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 high-tech.
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 Oceanic and Atmospheric Administration's polar-orbiting satellite, the scientists can record the marine mammal's path as
it swims hundreds of kilometers 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.
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 base?"
"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
Sterling.
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
indicates 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. JPL manages the program for
NASA's Earth Science Enterprise.
"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:
http://nmml.afsc.noaa.gov. More information about Topex/Poseidon and Jason 1 is available at the Jason 1 launch page.
Image courtesy and related link:
All images courtesy of:
National Oceanic and Atmospheric Administration,
National Marine Fisheries Service,
Alaska Fisheries Science Center,
National Marine Mammal Laboratory
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