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Originally published March 20, 2013 at 7:59 PM | Page modified March 20, 2013 at 8:08 PM

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Trout found that gorges, starves itself to survive

Two UW graduate students discovered that Dolly Varden in Alaska feast on sockeye-salmon eggs for one month, then go without the rest of the year. The key is being able to grow a big stomach when they need it.

Seattle Times science reporter

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Scientists say evolution is partly to blame for the obesity epidemic sweeping the globe today. In a world where the next meal was unpredictable, our ancestors were hard-wired to gorge when the opportunity presented itself.

But researchers at the University of Washington have discovered a type of trout that puts ancient humans to shame when it comes to coping with a feast-or-famine environment.

Living in a remote watershed on the Gulf of Alaska where food is abundant for only five weeks of the year, the fish survive by expanding and shrinking their digestive tracts in a way that modern humans trying to lose weight might envy.

The results demonstrate the remarkable resilience of some fish populations, and could help wildlife managers as they attempt to save bull trout, a closely related species that is threatened in the Pacific Northwest, said Jonathan Armstrong, co-author of a study published Wednesday in the Journal of Animal Ecology.

Working in Alaska’s Chignik watershed, Armstrong and fellow fisheries doctoral student Morgan Bond focused on a type of trout called Dolly Varden. With bright pink spots and an orange belly, the fish reportedly got their name from a character in a Charles Dickens novel who favored brightly colored dresses.

The Chignik watershed is famous for its sockeye-salmon run. Up to a half-million of the crimson fish return like clockwork in late July to spawn, turning the river red. For Dolly Varden, the sockeye’s arrival is like ringing the dinner bell. The trout gobble down salmon eggs unearthed in the spawning frenzy, eating up to half a pound a day.

But by late August, the feast is over. And the rivers and lakes in the Chignik area are so cold and barren that there are few insects or other prey for the Dolly Varden to feed on during the rest of the year, Bond said. He and Armstrong wondered how the fish could survive under such extreme conditions.

The pair surveyed the river in spring, before the sockeye returned, and found it populated with very skinny Dolly Varden. When they cut open some of the fish, they found their digestive systems were tiny, too.

“They are probably on the edge of dying by the time those salmon show up,” Bond said.

Fish captured after the salmon-egg buffet weighed up to 50 percent more, and some parts of their digestive tracts were four times bigger.

And it wasn’t just that their stomachs were stretching, like those of most Americans on Thanksgiving Day. The fish were actually bulking up their entire digestive systems, from liver to intestines, in order to capitalize on the short-lived banquet.

But maintaining a large digestive system is very costly.

A normal-sized gut can consume up to 30 percent of a fish’s energy, more even than the brain or muscles.

So the Dolly Varden essentially jettison their digestive machinery when they no longer need it and live off their fat reserves for nearly 11 months. Presumably, the fish also go into a kind of stasis to conserve energy during the winter, when temperatures in the river hover just above freezing, Armstrong said.

“We don’t know for sure, but my guess is they probably find a deep pool and just hunker down.”

It’s not unusual for animals to be able to survive long periods without food.

Some birds don’t eat during migrations that cover thousands of miles. Pythons and crocodiles can live several months on one big meal, and hibernating bears subsist on their fat stores.

Trout in the high mountain lakes of the Pacific Northwest probably don’t find much to eat during the winter months either, said Chris Donley, inland fish-program manager for the Washington Department of Fish and Wildlife.

But the UW study is the first to document corresponding changes in the digestive tracts of wild fish, and one of the few to show how animals modify their bodies in response to fluctuating food supplies.

By adapting to live almost entirely on sockeye eggs, the Chignik Dolly Varden are also able to avoid the perilous ocean migration that most other populations undertake in search of food, Bond pointed out.

The UW study underscores how important salmon eggs are as a source of food for Dolly Varden and bull trout — which is something fisheries managers are already considering as they work to restore the threatened species, Donley said. “There is a critical link between maintaining healthy salmon runs and bull trout.”

Sandi Doughton at: 206-464-2491 or sdoughton@seattletimes.com

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