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How Freshwater Fish Survive the Winter

December 10, 2007

By Dan Howley, Albany Times Union, N.Y.

Dec. 10–They don’t have little mittens for their fins and they don’t have scarves or gill muffs, but we don’t have to feel sorry for the millions of freshwater fish that live out the winter in frigid water under a canopy of ice.

Adapting to the radical changes our severe Northeast winters impose on their habitat comes as naturally to fish as jumping from a weed bed to snatch an unsuspecting insect from the glassy surface of a pond on an August dawn.

Yes, they are cold blooded and, yes, their physiology allows them to survive in hostile environments, but that makes it no less interesting to peek beneath the ice to see where they are and what they are doing while encased in winter’s frozen mantle.

The first thing you would notice is that the world of fish in winter is a world that swirls in slow motion.

“In many cases, fish literally slow down almost to the point of hibernation,” said Doug Stang, assistant director for fish, wildlife and marine resources for New York state. “They become very lethargic and sedentary. They don’t have any other way of warming themselves, no thermal system like mammals have.”

As the mercury winds down, Stang said, so does the activity level of fish. By the time much of their world is ice-bound, they are nestled where the water doesn’t freeze, in the warmest pockets the cold water has to offer, in still places away from anything that resembles rushing water.

“They don’t want to be continually in motion in winter, so they find places out of the current, behind rocks or logjams or in deep pools — any place they don’t have to fight the current,” Stang said.

For most of them, including the popular largemouth and smallmouth bass, their winter digs are near the bottom of rivers and lakes where the water is warmest, not warm by any human standard but warm enough to make their winter existence at least a measure more tolerable.

Stang pointed out that in bodies of fresh water without current, like lakes and ponds, an inversion takes place in the water temperature as late autumn drifts into winter. What happens is the cold water moves to the top nearest the ice and the warmer water gradually slips to the bottom, the opposite of the summer pattern where the coldest water is deepest down.

Stang explained that the warm water sinks at 4 degrees Celsius or 39.2 degrees Fahrenheit because, at that temperature, it is denser than the colder water that rises above it. The colder water layered along the surface eventually freezes at zero degrees Celcius or 32 degrees Fahrenheit.

“The physiology of fish is such that most seek the warmest water to spend the winter,” he said. “For them, that’s the best they can do, it’s the most hospitable habitat for them.”

But some fish are apparently more equal than others in the temperature tolerance department.

“Trout and salmon prefer colder water and are more active in winter,” he said. “So their winter place will be at a higher water level. So that if you were ice fishing for smallmouth bass on Lake George you would fish the bottom anywhere from 30 to 100 feet down, while you might find salmon or trout just three to six feet below the ice.”

In nature’s sometimes cruel script, some fish, especially the young ones, will not survive the ritual by ice, because their physiology is too undeveloped to handle a particularly severe season. Then, there are those that become prey for larger predator fish. Others even become disoriented in the ice formations near river banks and shorelines and get trapped in shallow pools.

The cycle will begin anew with the spring thaw and the gradual rise in water temperature that stirs them back to life.

Dan Howley can be reached at 454-5321 or by e-mail at dhowley@timesunion.com.

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Copyright (c) 2007, Albany Times Union, N.Y.

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