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There are, fortunately, only three important ways that trout feed on subsurface items in moving water. They grub insects from the bottom, pick insects from the drift, or suck insects from the underside of the surface film. The dynamics of the feeding methods remain the same whether they main prey is mayflies, stoneflies, caddisflies, or midges. The mobility of the nymph determines where it stays, but no matter what type it is, the flow characteristics of the water it is in determines the way a trout feeds.

Grubbing is either a vacuuming or a rooting action. The fish holds at a downward slant, snout pressed into the rocks or vegetation. During vacuuming the trout flares its gills, pulling in water and insects, and then squeezes out the water. During rooting it actually scrapes nymphs off the rocks or eats them along with bits of vegetation.

Either way its head is down in the still water and its tail is up in the faster water, forcing it to constantly struggle to keep this position. The turning body causes the flashes of gold or silver, reflections the fisherman can see, that identify this form of feeding.

The trout anatomy really isn't ideal for grubbing. Whitefish, suckers, and sculpins are much more adept at feeding nose down. Rather than forage in this manner, trout often stir up the bottom to loosen nymphs, and then swim quickly downstream and turn around to wait for the debris. This tactic allows them to feed with their tail down and head up.

Picking nymphs from the drift like this, those either carried naturally or stirred up by rooting, a trout holds comfortably in the quiet water and pokes its nose upwards. It sidles back and forth in the current, performing gracefully as it intercepts passing fare.

This type of feeding often continues for hours when insects are abundant in the drift. Most of the trout's movement is from side to side, seldom upstream except to establish a new position. Sometimes, though, a fish hastily swims downstream to grab a nymph that it let pass, not the result of a sudden change in mind , as it seems, but because an insect slid into focus at the side.

The event that regulates the entire cycle of a trout stream is an insect emergence. During the late Spring and early Summer, when major hatches occur daily, fish begin feeding heavily a few hours before the hatch and continue gorging until the activity peters out. Afterwards, they settle to comfortable water to digest the feast.

Even during emergence the nymphs or the pupae of the insect remain more important than the adults. With mayflies, a trout takes four nymphs for every adult. With caddisflies the ratio is unfigurable, so few adults are actually ingested. The reason for this difference is that mayflies ride the surface to dry their wings after discarding the nymphal shuck, but caddisflies keep their wings dry enough to pop from the underside of the meniscus right into the air.

When a trout actually feeds on the emergent stage, plucking an insect as it struggles to shed its skin, the way it works depends on the speed of the current at the surface. In slow-moving streams like spring creeks a fish holds a few inches under the top and sips gently, but in faster water it rises all the way from the bottom. The method of take is the same either way, but the tuck downwards right after the capture controls the amount of bulge in the surface.

There's a statement that's repeated frequently, "Trout feed either at the top or the bottom of a river; the water in between is barren of both insects and fish."

This phrase has been written so many times, in one form or another, that it's become a cliché. Like many clichés, though, it's not only an over-simplification, it's wrong. There aren't just two feeding levels in a trout stream.

There are actually four separate feeding strata, areas where insects become concentrated enough to trigger heavy trout activity. Each of these levels is a trap that holds the insect, at least momentarily, and allows a fish to key on the point of hesitation.

The surface tension of the water, the rubbery film called the meniscus, forms two of the feeding edges. The top of the film, the realm of the dry fly, supports insects until they either fly away or drown. The underside of the film also supports insects, usually emergents breaking out of a shuck, by providing a tangible roof on the stream for the escaping pupae or nymphs to hang on to. Both of these edges form distinct points of hesitation, each of them creating a temporary environment that gathers insects.

How important can the microscopic distance between the top and the bottom of the meniscus be to the fly fisherman? Being able to recognize the difference between a true rise to an adult insect on top and a breaking roll to a pupa or nymph underneath is usually crucial. There's nothing more frustrating than fishing a dry fly among working trout, only to have it be completely ignored; at the same time a correct pupa or floating nymph imitation would fool these fish.

The same, oddly, is not as rigidly true in the reverse situation. When trout are rising to take adult insects the proper dry fly is the best imitation, but a damp fly hanging in the film will still catch some fish. The reason for this is that enough of the floating naturals drown and become mired in the meniscus to cause a secondary concentration of insects.

There are two other important feeding levels in a stream, quite different from each other but both created by water flowing over a broken bottom. River currents are not uniform, of course, because of the friction of water against the solid bed. Even in a smooth, U-shaped sluice the currents are slower at the sides and the bottom, gradually becoming faster at the middle and the top.

As water breaks over and around rocks, pushing out squiggling eddies, there's a quiet cushion left among the crevices and chinks of the bottom. This buffer space in the riffles serves as the primary production area in a trout stream, generating 80% of the insect life. It also provides the haven that allows a fish to hold and feed within a fast water environment without exhausting itself.

This quiet water right at the bottom, harboring almost the entire insect community, is not the primary feeding zone for trout, though. It's an important level when trout grubnose down to vacuum or scrape insects from crevices, but grubbing is not an easy way for a fish to gather food. Every natural defense mechanism an insect possesses -- protective coloration, inaccessibility, mobility -- works to frustrate those feeding efforts.

The most important strata for insect concentration in a river, well known to aquatic entomologists, is a level that can vary from a few inches to a few feet above the bottom. This zone is an “interface,” a joining of two currents of different force, that is formed when the unobstructed flow of the main stream meets the obstructed water of the bottom. When the eddies that swirl and break of the rocks hit the faster current above, it, there's an intermediate buffer formed between them.

This “drift-line” interface captures insects that are involuntarily swept off the rocks. These insects, trapped in the current, are completely vulnerable, denied the chance to either hide or escape. The flow acts almost like a conveyor belt, carrying helpless prey to the fish.

This feeding level is the most important in the stream for nymph fishermen because of the amount of foraging time that trout spend there. In general, the feeding percentages for each of the zones are:

The statement "Trout feed either at the top or bottom," by ignoring two layers in between, fails to explain 70% of trout activity. The fly fisherman, though, can't afford to ignore these two crucial zones when trout are foraging on the concentrations of insects that gather in these areas.

Whenever there's a concentration heavy enough to trigger a spurt of selective feeding, there are two problems for the fly fisherman to solve -- not only do trout become selective to pattern with nymphs, but they also become selective to depth.