By some estimates, trout take 80 to 90 percent of their food below the water's surface.

During the winter and early spring, when insect hatches are rare and trout stay deep and immobile, this number is closer to 100 percent.

Trout, like all fish, are opportunistic feeders and will take an easy meal over an equally tasty but more difficult one. For the fly angler during the winter months, this means fishing nymphs deep to where the trout are holding.

A nymph, as I'll use the term, is an artificial fly that imitates the sub-surface stage of an aquatic insect - most notably the Mayfly, caddis fly or stone fly. Technically, the immature stage of a caddis fly is the larva and pupa, but the catch-all term for artificial flies that imitate these three is the "nymph."

Whatever the terminology, trout love them.

What are conditions that make nymph fishing productive?

1) When a hatch isn't evident;

2) late autumn, winter and early spring when hatches are rare or occur only during the warmest part of the day, limiting feeding at the surface;

3) during a hatch when fish are taking emerging insects.

Nymph fishing, like any other form of fly fishing, can be as varied in technique as the number of anglers doing it.

However, most anglers would agree a longer fly rod has an advantage with respect to line control than a shorter one.

A 9-foot, 5- or 6-weight rod has plenty of length to keep as much of the fly line off the water as possible. It also has the stiffness to be able to cast the weighted rig of flies and shot. A leader of 7 ½ to 9 feet is appropriate, tapered down to 4X.

Nymphs come in the weighted and non-weighted variety. I prefer the latter.

Weighted nymphs have added weight built into the body of the fly that helps them sink faster. However, a non-weighted fly with a split-shot or two pinched onto the leader 8 to 12 inches from the fly will have more of a natural action and movement in the current than a weighted nymph.

Some anglers swear by strike indicators while others swear at them. I happen to like them and find them extremely useful.

A strike indicator is like a small bobber attached to the leader and can be made of cork, yarn, wood, or foam that floats on the surface and suspends the weight and nymph just off the bottom. It also helps telegraph the strike to the angler.

When the strike indicator is pulled underwater, stops in the middle of the current, gets pulled sideways, or does anything else that an ordinary floating object wouldn't do, set the hook. The distance between the fly and the strike indicator should be about 1 1/2 times the depth of the water you're fishing.

The easiest and most productive technique of nymph fishing is the upstream cast with a dead drift. Cast far enough upstream of the suspected lie to allow the fly to sink near the bottom before it reaches the trout.

The strike indicator will let you know if your unseen fly is moving at the same pace as the current. It'll also indicate if a trout takes the fly or if you're hung by the bottom.

Anglers who don't snag a piece of bottom every four or five casts probably aren't fishing deep enough. The nymph must be bouncing along the bottom of the stream bed or just above it to be effective during the colder months of the year.

At the end of the drift, I usually allow the fly to swing back toward me, then I lift the rod tip. I often get strikes as the fly begins to rise from the bottom at the end of the drift.

Fly selection during the winter doesn't differ much from other times of the year when it comes to nymph fishing. Unlike having to match a hatch with dry flies, the same aquatic insects are on the stream bottom year-round.

During winter, I tend to use smaller sizes of flies, but the same successful flies that worked in the spring and summer will catch fish during winter. Pheasant-tail, gold-ribbed hares ear, Tellico, prince, and zug bug nymphs in sizes 16 to 20 will catch fish at most rivers and streams in South Carolina.

The majority of winter nymph-fishing opportunities will come from the deeper, slower pools, especially at curling eddies near main flows. Drifting nymphs through these slow pools, using a longer leader and a split-shot heavy enough to get your fly down quickly, usually is the most effective way of taking trout.

Wintering trout also will use cover just as they do during warmer months.

Let your nymph drift through current seams between faster and slower water, beneath undercut banks, and behind rocks. Having a minimal section of fly line on the water is essential for good line control and detecting the light, non-aggressive strikes of winter trout.

Being cold, wet and uncomfortable can ruin a day of fishing and also is dangerous. Dress in layers of clothing to deal effectively with winter weather conditions while fishing.

Start with a base layer of long underwear. In addition to keeping you warm, it'll keep you dry by wicking moisture from your skin. Put on an insulating layer of fleece over that to help retain warmth for the upper and lower body, as well as wear a good pair of thick wool socks. Waders and a wind-resistant Gore-Tex wading jacket should be the outer clothing shell.

Because 70 percent of body heat loss can come from an uncovered head, a warm cap will go a long way towards keeping anglers comfortable and safe. Fingerless gloves also will make a winter fishing trip more enjoyable.

With a few exceptions, trout water in South Carolina is open year round to fly anglers. By preparing for the cold and using proper nymph-fishing techniques - plus having some intestinal fortitude - the weather shouldn't keep you from catching trout all winter long.