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Dolly Varden trout store up fat for an early retirement

Retired Dolly Varden trout eat enough eggs in the summer to last them through the winter.

By Brooks Hays
Dolly Varden trout store up fat for an early retirement
Adolescent Dolly Varden trout swim up Alaska's Newhalen River. Photo by UW/Morgan Bond

SEATTLE, Oct. 28 (UPI) -- Trout retire too. But unlike humans, who tend to work their lives in cities to retire to the country or the coast, Alaskan sea trout save up fat to trade their ocean adventures for the full-time tranquility of freshwater.

According to new research out the University of Washington, Dolly Varden trout are the first species to exhibit such behavior.

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"As far as we know, no one has ever seen a population of large-bodied fish come back to freshwater and just park there for the rest of their lives," Morgan Bond, a postdoctoral researcher at the National Oceanic and Atmospheric Administration, said in a press release.

Bond is the lead author of a new paper on the discovery. She conducted the research while she was a Washington doctoral student.

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Like salmon, steelhead and some cutthroat and bull trout, Dolly Varden live most of their days in coastal tributaries but trek to the ocean each summer to feed and grow.

But the migration pattern, called anadromy, comes with risks. The ocean is full of food, but also predators.

"Small fish gain enough to make this risk worthwhile because they can pack on a good deal of growth in a summer at sea," said senior author Tom Quinn, a professor of aquatic and fishery sciences at Washington.

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In analyzing the bones of Dolly Varden trout, Bond and her research partners realized the species ditch their migration pattern as soon as they can -- that is, as soon as they are big and fat enough.

The ear bone of a trout, the otolith, has rings like a tree trunk. Each ring marks a new year, and within the ring is trapped information about the fish's environment. The evidence shows that for the first few years of a their lives, Dolly Varden journey back and forth between the stream and sea. But at a certain point, when they surpass a foot in length, they retire to freshwater life.

There, scientists say, the trout subsist on salmon eggs, laid in abundance each year during the summer spawning run. A unique ability to expand and contract their digestive organs enables retired Dolly Varden to gorge on eggs each summer and survive the lean months in between.

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"Fish that are already big will only grow a bit more, and there are still predators that can eat them," explained Bond. "So, in this case the big fish retire from anadromy at a certain age and stay in the river, waiting for the ocean to come to them -- in the form of the eggs released by the salmon."

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But just as retirees may be forced back to work by an economic depression, an diminished salmon spawn run may sometimes force bigger fish back to the sea.

Researchers are now trying to figure out whether other anadromous fish ever retire, given the right ecological conditions.

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