Trout gut balloons for annual food frenzy

U. WASHINGTON (US) — Once a year, a type of trout that lives in Alaska’s Chignik Lake watershed expands its stomach up to four times the usual size to go on a month-long eating binge.

Organs such as the stomach and intestines in the Dolly Varden trout double to quadruple in size when eggs from spawning sockeye salmon became available each August, acting like vacuums sucking up the eggs and nipping at the flesh of spawned-out salmon carcasses.

Then, once the pulse of eggs and spawning salmon cease, the guts shrink and the fish live for nearly a year off the reserves they’d built up because there is little else to eat.

The gut of the Dolly Varden trout becomes up to four times its normal size each August when eggs from spawning salmon become available and then shrinks back to normal size for the rest of the year when food is no longer available. Above, Brightly colored sockeye salmon surge by as a Dolly Varden waits its chance to binge on salmon eggs. (Credit: J. Armstrong/U. Washington)


Certain snakes, birds about to migrate, and Atlantic cod in the laboratory are known to grow and shrink their digestive track in response to gorging, but this is the first time researchers have documented wild fish doing so, according to a new study published online in the Journal of Animal Ecology.

Dolly Varden, bull trout, and brook trout are among the North American members of the char family, part of the larger trout and salmon lineage. Dolly Varden trout live where insects and other prey are scarce because of the long, cold winters. The fish can’t do without the yearly egg “subsidy.” Survival depends on there being plenty of returning salmon, spawning naturally.

Depleted runs or rivers full of salmon returning to hatcheries just won’t do it, a factor for managers to consider if it turns out other fish have evolved the ability to grow and shrink their guts to take advantage of food pulses.

“Wild salmon runs have been dramatically reduced across much of the lower 48 states and often are replaced with hatchery fish,” says Jonathan Armstrong, a doctoral student in aquatic and fishery sciences at the University of Washington. “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.”

It appears that Alaska is protecting coastal watersheds and conserving the inherent productivity of salmon runs, and there are enough salmon to support commercial fisheries that harvest half, or more, of the returning fish as well as upstream consumers like Dolly Varden.

“The intact ecosystems of the Bristol Bay region produce staggering amounts of salmon. Even after half of the sockeye run is harvested, there are still enough fish left over to produce the iconic image of a stream turned red by aggregations of salmon in their bright spawning colors, salmon are so abundant that we can count them from airplanes,” Armstrong says.

In the Alec River, where the work was conducted, so many sockeye salmon return that as females dig nests in the stream they unearth caches of eggs left by earlier spawners. The eggs roll along the stream beds, get caught in crannies along banks, and concentrate in swirling eddies where Dolly Varden and other animals chow down on them.

A 2-foot-long (60-centimeter) adult Dolly Varden can eat one-third to one-half pound (150 to 230 grams) of eggs a day.

“For a long time Dolly Varden were vilified as being bad for salmon, in part because they eat the eggs,” says doctoral student Morgan Bond says. “But they don’t dig up eggs, other salmon do. Dolly Varden are eating eggs that aren’t viable.”

After the monthlong all-you-can-eat buffet, it’s important that the Dolly Varden digestive track shrinks because big guts demand a lot of energy from an animal. A normal-size gut, for example, uses about 30 percent of the animal’s energy when it is at rest. By weight, the fish’s digestive track takes more energy to maintain than its muscles or brain. By shrinking their gut, Dolly Varden 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,” Bond says.

Conserving energy is crucial when the fish is essentially fasting most of the year. Many fish—including young Dolly Varden—head to the sea seeking food all summer long before returning to their home rivers.

But once Dolly Varden reach 16 or 17 inches (40 to 43 centimeters) in length they begin staying put, waiting for the feast to come to them. It’s an evolutionary adaptation that could be protecting them from the predators and other hazards encountered by fish going to sea.

“These are pretty large-bodied fish living in a place that is relatively nutrient poor but the egg subsidy allows the fish to remain in fresh water year after year. They don’t have to go to sea,” Bond says.

The work was funded by the National Science Foundation, Gordon and Betty Moore Foundation, Alaska salmon processors, and the University of Washington School of Aquatic and Fishery Sciences.

Source: University of Washington