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How trout survive winter in a frozen river

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The snowcapped Absaroka Mountains tower over the Paradise Valley as the Yellowstone River flows through it. PHOTO BY SILAS MILLER


On a crisp winter afternoon, I step onto the sheath of ice that lines the canyoned banks of the Gallatin River. The sun radiates off the fresh snowpack beneath blue skies. No insects buzz, and no critters crawl or hop about the terrain. Life seems as frozen as the air. But beneath the seemingly forbidden icy surface of water, trout maintain their survival. How they do so is a question worth asking for the fly-fisher eager for a chance to catch them, as well as for anyone with a capacity to marvel at their adaptive capabilities so alien to our own.

As a warm-blooded fly-fisherman, I must maintain a constant body temperature. My methods of adaptation for a cold river involve the thickest beanie I can find on the market and two layers of wool socks tucked inside of expensive Gore-Tex waders. Such insulating items are useless for any of the Gallatin River’s cold-blooded rainbow and brown trout. They belong to a group of animals called poikilotherms, whose body temperatures fluctuate in correspondence with their environment.

But a trout can still freeze to death in a river, so to speak, amid conditions fraught with relatively little food and limited space for shelter from predators and fast currents. In response, they acclimatize their biological processes. A trout’s metabolism slows at a corresponding rate to decreases in water temperature. As a general rule, their metabolic rate halves for every temperature drop of 10 degrees Celsius. The metabolic compensation grants them imperative energetic advantages to consume less food at a time when resources are scarce. 

It comes as no surprise that a trout’s behavior changes with less energy. With reduced needs for oxygen and food, they relocate to areas of minimal current where less exertion is required to maintain their position. They might migrate to a lake if nearby, or they may swim up narrow, frozen-over contributory streams that are shielded from attack by bald eagles and belted king fishers. The smaller the fish, the slower its metabolism, which means it can last longer without a meal. It is thought that a small trout might nestle itself within the crevice of a rock where it remains for the duration of winter, shielded from the current and hidden from feisty river otters and mink.

Larger trout with a need for occasional caloric intake head for the depths of the deepest and slowest pools that also happen to be in a primary current where the river naturally gathers food. They do not move far to strike prey, which usually accounts for no more than occasional stonefly nymphs or larvae hatched from the bottom of stones or dislodged by shifts of anchor ice along the riverbed. They also don’t fight as much, which grants proximity for a larger number of trout. Smaller trout, however, remain vulnerable as prey.  

So I walk along the frozen banks of the river with an eye out for the deepest and laziest water. Those shallow riffles and speedy runs that I’d trade my tax return for a chance to fish at dusk come summer are no longer good for catching anything other than a cold. I didn’t always know this, but learning the nuances of river ecology to trick its inhabitants brings forth a connection with place. And as we also change our behavior for the cold, I am almost always alone when I fish amid the stillness of winter, making the connection all the more enjoyable. 

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