FISH UNDER ICE.

What exactly happens to trout and other fish when streams freeze over? "Over" is the key word here. Even when the top side of a stream is frozen, water (and fish) are still moving underneath. But that doesn’t mean the living is easy.

"Winter is the most severe time for trout, aside from running out of water during the summer," says Tom Annear, water management supervisor for the Wyoming Game and Fish Department. "They basically go into a state of torpor [extreme sluggishness] until it’s safe."

Because fish are cold-blooded, their body temperatures change with their external environment. Metabolism and activity levels slow as water cools. Below about 38 degrees (F), if a fish is forced to move frequently to escape predation, to go after food, or because the habitat is unstable (ice forming or breaking up), the energy that fish expends could leave it mortally weakened. When there’s an ice roof overhead, their surroundings are more stable so fish are less active, greatly increasing their chances of survival.

"A solid ice cap, with snow on top that keeps the light out, is best for the fish," explains Annear. "It encourages fish to stay put."

However, as food floats by, fish instinctually move, whether they’re hungry or not. In winter, the buffet in the water column is mainly "instars" – immature insects at their smallest developmental stage – which provide very little nutrition due to their tiny size. "Fish might not starve to death trying to eat instars, but they lose body mass," says Annear. "Their body condition deteriorates until they can’t resist the current any more, then they drift downstream, hopefully to a better spot."

What qualifies as a better spot depends on the size of the fish. A fingerling will look for a crevice between two stones, but bigger trout seek out the slowest moving, warmest water, usually at the bottom of a deep pool. A half-degree change in water temperature can make a difference. It’s common for large numbers of trout – often multiple species that wouldn’t tolerate each other during the summer – to aggregate in one hole during the winter.

The most dangerous conditions involve frazil ice – a type of slush that forms when water is too fast-flowing to freeze solid. It is made up of randomly oriented, needle-like crystals and typically forms during the coldest part of the night, often disappearing by mid-morning. Frazil ice creates ice floes and clogs pools, accelerating the current.

"If there’s frazil ice, the fish move," says Annear. "The smaller fish might get under an ice shelf, but that only works to a point. In one study, we tagged 40 fish to find out what they did under these conditions. We found 20 tags up on the bank. The mink had grabbed them. Mammal and avian predators are both factors under these conditions."

One mammal, however, can be an inadvertent friend to trout during the winter. Fish seek shelter where riverbanks are undercut, especially where there are piles of brush, which creates a stable environment. Beaver dams provide such shelter.

Bottom line: Fish need shelter and stable water conditions to have the best chance of surviving the winter. The types of conditions found under solid ice.

ICE FISHING 101.

"What’s nice about ice fishing is you don’t need a lot of gear to get started," says Jarrid Houston of Houston’s Guide Service in Duluth, Minnesota. A lifelong ice fisher, Houston says you only six items to start reeling in fish on a frozen lake.

1. Auger: Ice drills come in a variety of styles, from hand-cranked to propane-powered augers. Whichever your pick, you’ll need an auger to drill a six- to eight-inch-round hole through the ice.

2. Ice Suit: An insulated, weatherproof ice suit is critical to staying warm on the ice. With built-in flotation devices, these suits also provide an extra level of safety.

3. Portable fish house: While ice fishing might conjure up visions of heated wooden shanties, Houston is a proponent of the modern "hub shack" (a pop-up canvas tent with built-in chairs) and flip-style shack (a sled with canvas that flips over you). "You can still go out and sit on a bucket, but you may only last an hour," says Houston. "With a fish house, you can stay out all day. And if you’re not catching anything, you can move it quickly."

4. Rigging: How you rig your line depends on how active you want to be. With a "tip-up," you just drop your line and leave it until the flag signals a fish hit. The line and hook attach directly to the tip-up, and you reel fish in by hand. "Jigging" is a more active approach – twitching the line to give the bait some action, which hopefully attracts fish. You can use the same reel you’d use in the summer but with a shorter rod, about 26-inches long.

5. Bait: The trend in ice fishing is to use soft plastic bait instead of the real thing because you can reuse it, you don’t need to keep it alive, and it’s easier to handle. "Smaller is better in winter because the fish are eating microorganisms," says Houston.

6. Electronics: Houston is a huge proponent of using sonar (a device that uses sound to "see" underwater) for ice fishing. "It’s a game-changer. You’ll catch 90 percent more fish because you’ll know what depth they’re at, whether it’s hard bottom or vegetation below you, or if there’s a drop-off."

If you’re wondering how to get a 30-inch walleye through an 8-inch hole in the ice, Houston says to be patient. Typically, a fish will dive three or four times as it gets near the hole. When it’s finally tired, you can reach down and grab it by the gill plate to pull it vertically out of the hole.

If you’re planning to let the fish go, do it quickly – within 30 seconds of landing it. Hold its tail as you circulate water through its gills. As the fish revives, it will kick, then go. "You pulled the fish out of 32-degree water, but the air might be zero degrees with a 20 mph wind," says Houston. "It’s important to get a fish back in the water as soon as possible, otherwise its eyes may freeze. That’s another reason for a shelter. It’s gentler on the fish."

Mind your own safety, too. Go ice fishing with a buddy. Bring a rope in case one of you inadvertently goes through the ice. And store your cell phone in a waterproof container in case of emergency.