The Dolly Warden trout adapts its body to the food supply (Image: M Bond / University of Washington)
Read aloud A species of fish found in Alaska turns out to be a master of gluttony: Every year in August, Dolly Warden trout inundate tons of salmon spawn - so much so that their stomach and intestines swell to four times their size. However, when this megamahl is digested, its digestive tract shrinks again and stays so small the rest of the year that hardly anything fits. For a snake that would be nothing special, in a wild fish, US researchers have such an extreme adaptation to fat and lean times but observed for the first time. The Dolly Warden trout (Salvelinus malma malma) belong to the close relatives of the salmon. The approximately 60 centimeters large fish live in the rivers of the far north, especially in Alaska and also in Siberia. While the juveniles migrate downstream into the sea every summer to eat there, the adult trout stay fresh all year round. There they feed on insects and small aquatic animals for most of the year - food that is rather sparse in the polar winter and the cold of Alaska.

However, once a year, in August, there is a sheer abundance in the home waters of Dolly Warden trout. Because then the full-grown red salmon from the salty Pacific return to their birth waters in western Alaska. Having reached the rivers and streams, the salmon females dig shallow pools and nests into the river bed and lay their spawn in it. There are so many in the Alec River near Chignik Lake that the females accidentally dig up the clutches of their conspecifics, according to Jonathan Armstrong and Morgan Bond of the University of Washington in Seattle. For the Dolly Warden trout living in these rivers, the buffet is opened: like living vacuum cleaners, they swim down the river bottom and slurp one egg after another. About half a pound of spawn can devour a fish in this way during the day, the researchers explain.

Stomach and liver enlarged fourfold

As the Dolly Warden trout adapt to this extreme gluttony, the researchers have now studied for the first time more closely. They caught 20 trout each for three consecutive years before the start of the salmon season and immediately thereafter. All animals were killed and their organs accurately measured and analyzed. "The fish that were caught six weeks before the salmon season had heavily atrophic stomachs, intestines and livers, " Armstrong and Bond report. The muscles of these fish also showed signs of low supply of nutrients and energy. Unlike, however, after the "big food": The trout intestine had doubled in size and weight, stomach and liver were even four times larger than before. Even the heart has increased slightly in mass. "This is the first time you have demonstrated such flexibility in the digestive tract of a wild fish, " say the researchers. display

For the Dolly Warden trout, this adaptation is vital, because without them, they would not survive the low-food time, as Armstrong and Bond explain. A normal-sized digestive tract consumes around 30 percent of the energy even in a resting animal. The fish needs to spend more calories to maintain their gut than for the muscles or brain. However, when the Dolly Warden trout partially absorbs the tissues of their stomach and intestines, shrinking their digestive tract down to the minimum, they save much of that energy. This austerity allows fish to thrive in barely five weeks of food the rest of the year with just five weeks of food. "But they live hard on the limit of what's possible, " says Bond. The strategy of these fish is similar to that of some extremely adapted desert plants, which survive dry and hot all year round after a short lush rainy season.

Jonathan Armstrong and Morgan Bond (University of Washington, Seattle), Journal of Animal Ecology, doi: 10.1111 / 1365-2656.12066 © science.de - === Nadja Podbregar

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