Missing a meal can make many of us absolutely ravenous, but the Dolly Varden trout (Salvelinus malma) can live nearly a year without eating.
Surviving such a long time between meals takes a lot of guts—literally. New research shows that a trout can change the size of its intestinal tract based on the availability of salmon eggs, its favorite food. (Also see “Bulging Mutant Trout Created: More Muscle, More Meat.”)The Dolly Varden trout during spawning season. Photograph by Morgan Bond, University of Washington
For the first four or five years of life, Dolly Varden trout travel to the northern Pacific to feed during the spring and summer, then return to freshwater streams and lakes during winter. After that, the fish stay in fresh water, never returning to the sea for the rest of their 12-year life span. (Watch a video of trout spawning.)
The trout get one opportunity to feed: When salmon arrive in these waters to spawn. The researchers couldn’t identify any other food source for the trout. (Read more about Pacific salmon.)
When the researchers calculated how much energy the fish likely obtained from the annual surplus of salmon eggs and compared this with the estimated number of calories the fish would need to survive for an entire year, they found a thousand-calorie deficit.
“We were stumped. How could these large fish—the most [fertile] in the whole [Chignik Lake] population—get enough energy to survive and reproduce in these barren waters?” Armstrong said.
But the team had a breakthrough when they thought about similar predators that sit and wait for their prey, like pythons. These animals can go for long periods of time without eating. (Related: “Python Hearts Double in Size—Now We Know Why.”)
What’s more, migrating birds can expand their guts as they gorge to put on weight and then shrink them back down to make their long journeys easier.
With this in mind, the researchers measured the gut sizes of Dolly Varden trout in Chignik Lake before and after the annual salmon run—and found that the trouts’ guts were 2.6 times larger immediately after the salmon run.
“We were blown away that animals could rapidly and reversibly change their organ size—not just stretch the tissue out, but actually build bigger organs,” said Armstrong. Their study appeared March 19 in the Journal of Animal Ecology.
Feast and Famine
The extra intestinal tissue allows the trout to take advantage of the food surplus, eating a lot and obtaining as much energy as possible. When the salmon eggs run out, however, the fish economize. Smaller guts might not allow the fish to eat as much, but they also require much less energy to maintain. (See pictures of the world’s largest trout in the wild.)
“It’s like trying to choose a car—a sports car can go the fastest, but it also burns up the most gas. Digestive flexibility is a valuable tactic for life in a world of feast and famine—it allows animals to eat more when food is plentiful and to burn less energy when food is scarce,” Armstrong noted.
“What was neat is that this was really just a serendipitous side project. We were both actively pursuing our other projects and snuck in this field sampling on the side,” he added.
“Morgan and some other field researchers basically just went fishing while they were taking a break from other sampling. They brought the fish back to the lab, and when Morgan and when I needed a break from our dissertation projects, we’d squeeze in some time for lab work,” Armstrong said.
Trout Not Harmed by Salmon Fishing
Besides helping us understand the extreme flexibility of the fish digestive system, Armstrong and Bond’s research also provides insight into the current management of salmon fisheries. (Also see “Alaska’s Clash Over Salmon and Gold Goes National.”)
Since up to half of all salmon in an individual population are harvested for commercial fisheries, scientists have wondered whether removing the salmon harms their predators.
At least Dolly Varden trout populations appear unharmed by current salmon harvesting levels—perhaps due to their extraordinary ability to wait a year for their next meal.
So the next time you find yourself counting the seconds until lunch, take solace in the fact that you won’t have to wait nearly as long as this trout.