Binge-Eating Trout Can Quadruple Size Of Its Stomach

When salmon spawn in the waters of Alaska, it's as if all the year's opportunities for binge eating occur in the same instance for the Dolly Varden, a kind of trout that researchers have learned can quadruple its stomach size to make room for eggs from spawning salmon each August.

The Dolly Varden vacuum up the eggs and nibble at the carcasses of spawned-out salmon in a veritable last supper, for once the buffet of salmon eggs ends, there is but little for the Dolly Varden to eat for the rest of the year.

To adapt to challenge of eating a proper meal only once a year, the Dolly Varden's stomach expands to four times its normal size when it is binging through its annual feast, then the fishes' stomach shrinks back down to normal for the rest of the year and the fish lives off intestinal reserves.

While other animals like have been known to grow and shrink their digestive track in response to gorging, this is the first time researchers have documented a wild fish doing so, according to researchers.

The Dolly Varden live in an area where there is little to eat most of the year; the fishes' survival depends on there being lots of returning salmon each season.

By shrinking their guts the fish can cut their energy costs and survive until the next round of spawning salmon arrives.

"They live close to the edge though, getting skinnier and skinnier each month until salmon return in the late summer," said Morgan Bond, a doctoral student at the University of Washington who co-authored a study on the fish.

An adult Dolly Varden can be two-feet-long and eat up to half a pound of salmon eggs per day.

"For a long time Dolly Varden were vilified as being bad for salmon, in part because they eat the eggs," Bond said. "But they don't dig up eggs, other salmon do. Dolly Varden are eating eggs that aren't viable."

The survival of the Dolly Varden depends on healthy river systems full of spawning salmon each season.

"Wild salmon runs have been dramatically reduced across much of the lower 48 states and often are replaced with hatchery fish," said Jonathan Armstrong, co-author of the study. "When salmon are spawned in hatcheries, bull trout - which are threatened in the Pacific Northwest - as well as juvenile coho salmon and other species of concern to conservationists no longer have the opportunity to feed on salmon eggs, which are an incredible food source.

"Our society invests heavily in restoring stream ecosystems. Our study emphasizes the importance of conserving food webs. You can pay millions of dollars to add wood to a stream and take other steps to restore habitat but if there's nothing for fish to eat, you might not see positive results," Armstrong said.

The full study is published in Journal of Animal Ecology