Angler Jay Bennet with a fine example of a reservoir rainbow taken with nymphing techniques during the winter months. When surface temperatures drop low on lakes, the trout will school in the deepest spots they can find that hold sufficient oxygen. It's the angler's job to put a nymph right on their nose. Photo courtesy of Keith Kern.

Metabolism: It's a fancy word for the engine that drives life. Fish are cold-blooded creatures and must follow fluctuations in temperature. When waters warm in the spring, trout become much more active. And so too, do the insects they feed on.

Spring, summer, and fall all have their differences, but there are active insects and equally active trout hunting them. When winter comes, trout don't hibernate, but do decrease their feeding. And there's usually very little insect activity on or near the surface for the fly angler to key on.

But the immature or larval forms of aquatic insects remain pretty active right through the winter. Imitating those bottom-dwelling creatures can help you catch trout, even when there's snow on the bank and water temperatures hover near freezing.

You may not find many hatches of winged insects in mid-winter, but you can still catch trout on the same basic nymph patterns that worked in other months of the year. You simply adopt different presentations and strategies.

Probably the most certain thing in winter nymphing is that the angler needs to slow down and be more methodical in covering the water. When trout -- or any fish for that matter -- are in very cold water, their responses slow down. They don't need to feed as often, so they won't go out of their way to chase food. You have to get your fly right under their noses.

Bass anglers refer to this as a small "strike zone." In warmer water, fish might move several feet to get some tasty tidbit, but in the middle of the winter, they may not move 6 inches to take the same food item.

Anglers casting wet flies or nymphs in such conditions need to slow down and work the water more carefully. There are times when a good-sized trout may take your offering on the first cast. Then again, sometimes it takes more than half a dozen casts to the same location to tempt a lethargic fish to snap at a fly.

In addition to the speed with which you fish, a change in fly sizes may well be in order. Fly-fishing guide Keith Kern owns K-Flies Guide Service. He fishes and guides on Big Bear Lake, a 3,000-acre lake in the San Bernardino Mountains of Southern California. At 6,700 feet, Big Bear gets cold weather and frequent winter snows, and ices over for short periods. Keith fishes it right through the winter. He ventures out in a float tube or kick boat when weather and ice conditions permit. His hard-earned experiences can help anglers on any winter lake on the West Coast.

"On Big Bear, we mostly fish small midges in dark colors in the winter. The lake will freeze over at times, but if we get a few days of warm weather, it will thaw and we can get our tubes in the water and fish," Kern said. "We fish very light leaders because the water is so clear."

In the middle of winter, there's very little insect activity in Big Bear Lake except for midges and daphnia, tiny crustaceans too small to imitate. There are also minnows and small baitfish to consider, but fishing nymphs slowly over the bottom is probably the overall best technique.