What Walrus Eat
INTRO: Scientists learn a lot about animals, birds and fish by peering into their stomachs to see what they eat. A new study on how walrus digest their food has scientists rethinking this marine mammal's impact on the ocean food chain. Doug Schneider has more in this week's Arctic Science Journeys Radio.
STORY: Biologists have long known that walrus comb the bottom of the sea, rooting out clams, snails, worms and other creatures from the seabed. But when they've had occasion to examine the inside of a walrus stomachsuch as after they've been harvested by Native hunterswhat they usually find are just the leftover, hard-to-digest pieces of clams. That's led scientists to presume that walrus ate far more clams than anything else.
Gay Sheffield is a biologist with the Alaska Department of Fish and Game.
SHEFFIELD: "With all these invertebrates, you have no bones. And so you're trying to go back and look at the diet, at a variety of prey, none of which have hard parts. So basically you'd just find large clam feet and siphons."
The task of learning what and how much walrus eat fell to Sheffield, who until recently was a graduate student at the University of Alaska Fairbanks. Sheffield constructed an artificial walrus stomach in her lab, using a container that mimicked the acidity and temperature of a walrus digestive tract. Her results were published in the April issue of the Journal of the Society for Marine Mammology.
SHEFFIELD: "What I did was take a variety of invertebrate preyechiurid worms, sipunculid worms, clams and snails with their shells removedand I wanted to see which of these invertebrate prey would digest the quickest. I wanted to see how they came apart given the temperature and the acidity of a walrus stomach. And we found that they came apart differently. Some disappeared rather quickly, like sipunculid worms and the viscera and mantle of clams. The clam foot and siphon would stay behind."
Since small invertebrates passed through the digestive system quickly, and other prey, like clams, remained, it's not surprising that scientists tended to overestimate clams in the walrus diet. Dr. Brendan Kelly is a marine mammal scientist at the University of Alaska Fairbanks.
KELLY: "While she hasn't completely changed the depiction of what's in the diet, it certainly shows us that we've tended to overestimate some of the things that digest more slowly, like some of the clams. You end up with kind of a skewed picture of what's in the diet. So she was able to come up with some guidelines for how soon after feeding you would need to sample in order to really have a complete picture of what that individual walrus ate."
Sheffield's findings have caused the scientific community to go over data as far back as 1952, to recalculate the types and amount of prey walrus consume. They still eat lots of clams, but they eat lots of other things as well.
SHEFFIELD: "Turns out that walrus are eating a lot more things, a lot more frequently, than clams. They eat everything from tiny amphipods, jellyfish, clams, all the way up to birds and seals."
The U.S. Fish and Wildlife Service estimates 300,000 walrus inhabit the Bering Sea. Knowing more about what walrus eat will help researchers better gauge the health of both walrus and the prey they depend on.
OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
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Thanks to the following individuals for help preparing this script:
Gay Sheffield, Marine Mammal Biologist
Dr. Brendan Kelly, Assistant Professor
Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks.
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