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an excerpt from Fishing World, March/April issue 1979

The Dynamics of Nymph Fishing
by Gary LaFontaine

PART THREE

WHAT ARE THE TROUT
FEEDING ON?

Sometimes, when trout are grabbing anything, this question only has to be answered generally because it's enough to know where the fish are working. At other times, though, when trout are singling out a prevalent insect, it has to be answered specifically because the feeding zone, the fly, and the action imparted to the fly are all important.

Does a fly fisherman have to be interested in entomology? The information an angler acquires about the major insects of a stream provides a short-cut for catching trout.

The type of knowledge that's most valuable, an understanding of the life-cycle, pin-points the times that an insect is vulnerable to predation. These biologic facts alone answer the questions -- how, where, what -- about trout activity.

Trout are opportunists, gorging themselves at moments of plenty until insects spill out of their gullets. They prey on the easiest, most available concentrations of food, seeking the ideal situation for the feeding equation -- the biggest insects in the largest quantity in the most vulnerable position. And when this does happen, during one of the so-called "super hatches," every large trout in the river works the concentration zones.

The periods that insects are most available in a stream vary according to season. In the Spring the population reaches a peak, all the growth of various species during the previous Winter attaining a pre-emergence abundance. During late-Spring and Summer the daily hatches regulate the feeding of the fish, but they also deplete the bottom of mature nymphs and larvae. By the Fall, after the last of the major hatches, only those few insects that lead a 2-year or a 3-year nymphal cycle remain to provide a sizable food source, forcing trout to shift heavily to a minnow diet late in the year.

The relative abundance of insects determines what fish feed on. When there are neither drifting nor emerging nymphs available, a hungry trout scrounges on the bottom, but this grubbing is usually used more as a way of snacking than as a method for primary feeding.

The phenomenon that supplies the principal opportunity for feeding in a stream is the free drift or nymphs, called "catastrophic drift" by entomologists. For the insect this exposure to predation certainly qualifies as a catastrophe. When the nymph loses its grip on the rocks or vegetation of the bottom, it's swept into the buffer currents, temporarily lapsing into a state of shock. For a distance of 15 feet or more, until it reaches a suitable patch of stream bed again, it remains inert and helpless. Finally, if it has managed to escape the notice of a fish, the insect wiggles back to the safety of the bottom.

"For the insect this exposure to predation certainly qualifies as a catastrphe."
illustrations by Harvey Eckert

This catastrophic drift is neither a rare nor a random happening. During a 24-hour period, 3% of the total free-living, insect biomass of a river gets plucked from the bottom and carried off downstream. The process, directly correlated to overcrowding of habitat, is a natural method of redistributing excess population as the age group of an insect species matures.

Catastrophic drift also follows a predictable daily pattern. The amount of drifting that occurs depends on the amount of insect movement, the nymphs more likely to be knocked into the flow during moments of intense activity, and the amount of movement, in turn, depends on the amount of sunlight reaching the stream.

Aquatic insects regulate their feeding by the light. Many nymphs and larvae avoid it, a phototrophic reaction and feed during the dark. In the daytime they hide under stones or vegetation. A 1/2-hour after dusk and a 1/2-hour before dawn, as they come out in the evening or go back in the morning in hasty migration, these insects reach peak rates of catastrophic drift.

During the night the typical pattern of drifting is shown in a 4-point graph:

The opposite is true of insects on a diurnal feeding cycle. Their rates of drift are higher during the day, the peaks more flexible because of the changing cloudiness. These larvae, though, also drift heavily during radical shifts in the light.

The breakdown of which insects drift day or night is very simple:
Nocturnal Cycle:
mayfly nymphs, stonefly nymphs, free-living caddis larvae
Diurnal Cycle:
case-building caddis
Steady 24-hour Cycle:
chironomid midge larvae

For mature stonefly and mayfly nymphs, emergence has a major influence on this cycle. In the hours prior to hatching, because of increased restlessness, the nymphs drift in considerable numbers, regardless of the light intensity. Caddisflies, which go through a pupal stage in an anchored cocoon, refrain from this pre-emergence pattern.

If the drift cycle provides the regular meals, the actual emergence qualifies as a feast. At no other time are so many insects so vulnerable as when an important species begins to hatch. All the survivors from an entire age group suddenly become available, massed at the concentration levels, as they struggle from the water.

There are secrets to nymph fishing. Every so often in an article or book this is denied in an exposé piece (Nymph Fishing Is Easy), which claims that the technique doesn't really require very much skill or knowledge. And in an obvious way these articles are certainly correct -- it is easy to catch an occasional fish on an artificial nymph.

Although it's easy to be a mediocre nymph fisherman, there is still much more knowledge and skill needed to be a master. The secrets are the how, where, and what questions -- information that's so much harder to learn than with a dry fly, but the answers solve the problem of how to use a sub-surface imitation properly.

back to Part 1~The Dynamics of Nymph Fishing

back to Part 2~The Dynamics of Nymph Fishing (in-depth analysis of the How and Where)

As an extra feature, check out the full array of recipes for the Twist Nymph family
TIE IT • Twist Nymphs

"Fishing World" used this logo was a space-filler for ending articles.
The same is true today as back in 1979 -- whether we use on-stream etiquette, watch what we put down our kitchen & street drains, join a political cause for mine reclamation . . .

Our very first website readers tip from July 2000 helps us keep our stream banks clean.
Loose tippet and leader end up getting stuffed in pockets and reappearing at the most annoying times. And, if we simply discard them on the stream, they are potential hazards to small birds and mammals. Here's a great way to take care of those annoying pieces of tippet and leader. read the tip


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