Mystery Trout Can Quadruple Its Stomach Size

By Pierre Dumont , Mar 21, 2013 01:00 PM EDT

Researchers discovered that a certain type of trout, called the Dolly Varden, feasts just once a year and, in doing so, quadruples the size of its stomach.

The particular type of fish lives in a watershed on the Gulf of Alaska, where food is available in large quantities only five weeks out of the year. The Dolly Varden survive by gorging during that time frame: a method facilitated by the expansion and contraction of their digestive tracts. The trout feast on the eggs of sockeye salmon, which are diminishing in population across North America.

"They basically are fasting themselves right down to the brink of death," study researcher and doctoral student at the University of Washington Morgan Bond told LiveScience. "They're really skinny, there's almost no fat left in their body, and then days later, the salmon show up."

Dolly Varden trout reach a length of about two feet. During salmon season, which takes place in the late summer or early fall, the trout can consume a third to a half-pound of eggs per day. Bond and his fellow researchers were researching the trout in Alaska's Alec River in the Chignik Lake watershed. While the fish should have traveled to salt water to obtain food, it was discovered that the fish were remaining in fresh water.

The fact that the fish can quadruple the size of its digestive tract makes for a distinct advantage. This is amplified by the fact that, aside from that, the fish are also adding new gut tissue. Other animals have been discovered to grow and shrink their digestive tracks, but this is the first time researchers have found a wild fish to do so.

"The fact that these fish can change the size of their organs to change how much energy they need just to live is a really novel thing," Bond said. "Nobody's ever showed that in wild fish before."

The study also showed that the Dolly Varden live in lakes until around the age of 3, after which they travel back and forth between fresh water and the ocean for a few years. At about age 5, they remain in the fresh water lakes and rivers. While regulated fishing allows for a large number of salmon eggs in the Chignik Lake watershed, salmon populations in the lower 48 states are much less. There are efforts at conservation, however, which involve taking salmon from rivers and moving them to hatcheries to lay their eggs.

"It's sort of an interesting dynamic where we'd like to see the fishery regulated not only to provide enough fish for returning the salmon population, but also to provide enough of these (egg) subsidies for the Dolly Varden and birds and bears and other organisms that rely on that subsidy," Bond said.

Results of the study are published in the Journal of Animal Ecology.

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