Guts of Glory

By Bill Hauser

I happened to see a very brief article about Dolly Varden in the Alaska Dispatch News a while back. Then I found a slightly different version entitled Guts of glory: Alaska trout avoid migration by feasting on massive scale Published by the Journal of Animal Biology, March 20, 2013. J. B. Armstrong and M. Bond of the University of Washington; An Alaska Dolly Varden caught during salmon spawning season weighs twice as much as a Dolly Varden of the same length caught during the lean months.

"University of Washington scientists studying Dolly Varden in the Chignik Lake drainage in Southwest Alaska have discovered the fish living year-round in fresh water when they should be making their annual journey to salt water looking for food. Further investigation turned up a trait that surprised the researchers -- a trait previously found in snakes and birds but unknown in wild fish. Some Alaska birds, for example, feed heavily to bulk up before they begin a migration (especially long distance, over ocean migrators) but during the migration, they are able to shrink their gut and internal organs to reduce weight and have more energy available for flying. The Dolly Varden could not remain in fresh water if it weren't for the massive salmon runs in the Chignik drainage", reports Live Science.

This new study finds Dolly Varden feast once a year, expanding their stomach muscles up to four times the usual size to make the space for heavy feeding. During sockeye salmon spawning in the late summer or early fall, the Dolly Varden lurk as female salmon create redds (nests) for their eggs. All this digging turns up unhatched eggs from earlier spawning fish, which Dolly Varden view as a huge source of energy packed, oil rich food. In the monthlong spawning season, the trout can consume a third to a half-pound of eggs each day. Eating a quarter of their body weight daily for a month is no easy task.

"Remarkably, the Dolly Varden digestive tract then shrinks to decrease energy requirements during most of the year and preserve the fish through the long fasting season ahead," says researcher Morgan Bond.

"It's an evolutionary adaptation that could be protecting them from the predators and other hazards encountered by fish going to sea," the co-authors say.

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

Information from the original publication provides even more details... but in my own words...

Muscle and internal organs (including the gut) require energy from food or stored fat to maintain but large digestive organs increase rates of energy gain when food is plentiful but are costly to maintain and increase rates of energy loss when food is scarce. The physiological adaptations to this trade-off differ depending on the scale and predictability of variation in food abundance.

The authors investigated the physiological and behavioural tactics of the Dolly Varden that rear in watersheds with low natural productivity but experience annual food resource pulses from the spawning migrations of Pacific salmon. The eggs of Pacific salmon provide high-energy food for Dolly Varden for a brief part of a year. If they are not anadromous during most of the year, a large stomach and digestive system are a liability - an energy drain.

Dolly Varden sampled six weeks prior to the return of spawning sockeye salmon had shrunken stomachs, digestive organs, intestines, and liver. Throughout the portion of the growing season prior to the resource pulse of spawning salmon, fish had empty stomachs, low indices of energy condition and muscle isotope signatures reflecting the previous resource pulse.

During the resource pulse, Dolly Varden exhibited an abruptly enlarged digestive machinery, gorged on salmon eggs and rapidly stored energy as fat reserves, muscle growth and gonad development. Dolly Varden appeared to achieve nearly their entire annual energy surplus during the approximately five week period when sockeye salmon spawn.

Digestive flexibility provides Dolly Varden the energy efficiency required to survive and reproduce when resource abundance is concentrated into an annual pulse that is predictable, yet highly ephemeral. Although fish are known to incur extremely variable energy budgets, this study is one of the first to document digestive flexibility in wild fish. It emphasizes that fish can rely heavily on rare, high-magnitude foraging opportunities. Human actions that attenuate spikes in food abundance may have stronger than anticipated effects on consumer energy budgets.

Thus, while fish from most populations of Dolly Varden are known or presumed to be anadromous throughout their life, the fish from this population in the Chignik Lake drainage appear to be anadromous during their first years of life or until they mature. After they reach maturity, they stop being anadromous and feast on salmon products seasonally and simply fast most of the year in freshwater where there are fewer predators than in saltwater.

One can only speculate how widespread this habit may be among other populations.