Anticipating a Caddisfly Hatch

Gary LaFontaine on Caddis Hatches

Which pattern should be used? Which technique is most effective? Which part of the steam is best? These are questions the fly fisherman must answer before he begins casting. Some knowledge of aquatic insects can provide answers to these questions.

HATCH — a magic word. What is hatching? Many fly fishermen travel hundreds of miles to fish a stream or lake when a particular insect is emerging. The appearance of a major mayfly, stonefly, or caddisfly becomes an event, even a ritual, that anglers in an area wait for each season.

The change from larva or nymph to adult (with an intermediate pupal stage for caddisflies) marks a period of abundance. It is almost as if nature deems that the bulk of a particular insect population is suddenly expendable. The insects that have been so well camouflaged expose themselves completely to the fish. Caution is possibly unnecessary at this point in the insect life cycle because, with the large number of eggs produced by each female, most of the immature survivors are not needed to replenish the stream. The emerging insects gather at specific levels in the current and create an immense food supply. For many species, forty to seventy percent of the fish predation for the entire life cycle occurs just prior to or during hatching.

A trout in a stream can gain as much as fifty percent of its yearly growth during two months of spring or early summer. The heavy hatches of these months provide an abundance of easy prey. The fish settle into a daily schedule and feed actively only when they can forage with the greatest efficiency. Often they lie semidormant except during the emergence periods. When they do begin to work a hatch the trout concentrate on the specific areas in the stream where the insects appear.

Is there any doubt why the emergence of a major insect is so important to fly fishermen? The activity is going to control not only what the fish feed on, but also where, when, and how they feed. Nearly every healthy trout stream contains insect species that reach high population densities and create these opportunities.

RESISTANCE BY ANGLERS to the entomology of fly-fishing is baffling for two reasons: one, because entomology is so vital to angling success; and two, because it is so easy. It is the way for even a beginner to understand the movements and preferences of the fish.

Entomology and fly-fishing for trout are inseparably linked. Certain flies simulate food types other than insects, such as minnows, crustaceans, leeches, or land creatures, but insects are still the major food for the average-size trout. Too many fly fishermen choose to ignore entomology completely because they misunderstand the value of it. Perhaps they see the more complicated manifestations of angling entomology, such as the exact-imitation theories, and do not see the more basic functions of entomology.

As a fly-fishing friend once stated, “Entomology will let me know if the thorax of my fly should be reddish brown instead of olive brown, or if the hook on my fly should be a number twenty-eight instead of a number twenty-six, or if the wings on my fly should be forty-five degrees up instead of seventy-five degrees up; none of which I want to know.” Such minor refinements of imitation, of course, can be critical during periods of selective feeding, but situations when the trout are that fussy are uncommon — less than ten percent of the feeding time in most streams — and even then the presentation of the fly is as important as the choice of fly.

The major benefit derived from even a basic study of entomology is not the ability to solve minor problems of imitation. The important advantage a fly fisherman gains with that knowledge is the ability to predict trout feeding patterns — and consequently the ability to suit his techniques to those patterns.

A sad fact of modern fly-fishing is that so much of the lore is geared to one insect, mayflies, that the typical angler has difficulty adapting his methods to the feeding that occurs during a caddisfly hatch. He is conditioned to fish his flies to simulate the typical habits of a mayfly, not a caddisfly.

The information that has been written about caddisflies, however, is not particularly accurate either. The typical angling description of an emerging pupa usually paints a picture of a super insect: “The pupa rips free of the cocoon and rises like a rocket through the water, popping through the surface and flying off immediately.” Pity the poor trout trying to capture such an energetic creature. And pity the poor fisherman trying to imitate such insect behavior. This account of the emergence, fortunately, is an exaggeration.

The speed with which caddisflies, the swimming type, ascend to the surface varies with the species, but it is doubtful if any of them rise like a rocket and shoot out into the air. A few accounts by entomologists describe the struggles the emerging insect goes through. For example, Dr. Cornelius Betten, in The Caddis Flies, or Trichoptera, of New York State, in a section written in approximately 1915, states about a common Spotted Sedge (Hydropsyche sp.): “I did not find the larvae but observed the pupae transforming on the surface of the water alongside of the government breakwater. . .” and in another section, “The pupae were caught as they were coming up for emergence alongside the government breakwater, but these specimens had doubtless been carried some distance by the swift current since they left the rocks.”

My studies of how trout feed on emerging pupae and my observations of the naturals also shed considerable doubt on the “rocket” concept of a caddisfly hatch. At least for those swimming emergers observed, including six important trout-stream families, my studies show that the ascent has definite periods of hesitation. It is the insect during these periods of hesitation that fly fishermen must imitate with their flies. They have to know where the pupae will pause and struggle before they can begin to fish a caddis fly hatch successfully.

Caddis Flies

The first area of concentration: Trout begin feeding on drifting pupae near the bottom.

What is the secret? Efficiency. The principle is simple; the actual attainment of it is not. Many anglers flail randomly, their fly occasionally crossing those areas where vulnerable insects concentrate, catching fish only when their fly is in a prime area. The expert, however, changes his tactics as the prime areas change, and keeps his fly for as long as possible in the productive zone.

The key to anticipating, or “ambushing,” a caddis fly hatch requires breaking the common notion of what it is. Too many fishermen only recognize the peak of the action, the frantic surface feeding coinciding with the heaviest concentration of insects on or under the surface film, but these fishermen miss out on fishing before or after the peak — fishing that is sometimes even better.

The first time an angler encounters heavy insect activity, he cannot anticipate it. It is a blind situation — he is unprepared for the ensuing feeding spree. He fumbles in his fly box for some kind of a matching fly and casts to the rising trout with various techniques. If he fails to find the right combination with his hasty attempts, he probably ends up frustrated and fishless.

Even a regular on a stream, lacking an understanding of entomology, cannot fully master such a situation. He might have enough experience with a particular insect to use proper flies and tactics during the main hatch, his methods worked out by past trial and error, but he can still only take advantage of the activity he sees, the hour or so of actual surface feeding. He cannot take advantage of the subsurface activity he does not see.

The fly fisherman who understands the typical life cycle of stream caddisflies, however, knows the vulnerable subsurface stages. He discovers where, when, and how the concentrations occur during an emergence, which allows him to anticipate and prepare for the appearance of the insect. This knowledge also allows him to take full advantage of the predictable daily feeding schedule of the trout. Such an angler is not a member of a scientific cult, but simply a fly fisherman who is prepared to match his tactics and flies to the changing concentrations of insects. There are three areas in which caddisflies concentrate during a hatch.

The First Area of Concentration

Usually, hours before the main hatch, some caddisflies begin popping out. The first of these random emergers often reaches the surface safely because trout are not conditioned to the occurrence, but soon fish take notice of the hatch. Even when they do start feeding, however, the trout seldom rise to grab a natural from the surface.

The emerging insects, fully formed adults inside thin and flexible pupal skins, cut their way free of the cocoon. Most species do not begin to rise immediately, though. Before they can start swimming they must generate air bubbles inside the transparent pupal skin. They drift momentarily with the bottom currents, enough of them carried along to create a concentration of helpless prey.

This is the first hesitation in the emergence of caddis flies; and preliminary to the main hatch the fish seem content to stay on the bottom, plucking the freely drifting pupae. Trout can feed for hours on these forms without breaking the surface or chasing an active insect.

A stomach sample from a trout caught just before the main caddisfly hatch will often be jammed with hundreds of emergents. These stomach contents are usually a complete surprise to the angler, but they demonstrate the extent and intensity of this unseen bottom feeding.

The tactic for matching this early hesitation of emerging caddisflies is seldom associated with the pupa. All kinds of techniques are recommended for a hatch, usually lifts, swings, or retrieves, which may be suitable at a different time, but when trout are keyed to the inert insect near the bottom the way to fish an imitation is dead drift with the standard upstream or across-stream nymph presentation. An occasional tightening of the line, kicking the fly into brief motion, followed quickly by a mend that drops it back into a drift can be added to the basic method.

Imagine, then, being able to fish successfully for hours before the main hatch and the visible surface feeding of trout. It is not difficult to capitalize on this opportunity if the angler will only take the time to learn the approximate emergence dates of the important caddisflies in the streams he fishes.

The Second Area of Concentration

Once out of the silk-lined, stone or vegetable cocoon, drifting freely in the stream, the swimming caddisfly emergent begins inflating its surrounding skin with gas bubbles and beating with hair-fringed legs, both of these actions lifting the insect up through the water. At the surface the adult hesitates, pushing against the underside of the meniscus (surface film) and struggling to shed the pupal skin.


The second area of concentration: Trout break the surface with porpoiselike rolls, taking pupae from the underside of the meniscus.

Most caddisflies do not ride on top of the water for great distances, but they do drift, some longer than others, hanging half in and half out of the surface film and invisible to all but the most careful observers. The emergent pulls out of the pupal skin with a series of wiggling motions. Once free, it takes off after a preliminary hop or two.

Ken Thompson, an aquatic biologist, puts the role of the surface film during emergence into perspective, “The meniscus, or surface film, poses an unbelievable barrier to insects trying to pass through it, either from above or from below. It is the result of the molecular bonding of water molecules, which have areas of positive and negative charge. Under the surface the charges are equal, but at the surface the molecules are ‘unsatisfied’ and under tension.

“A great amount of physical energy is required for an insect to break through the resultant surface tension (meniscus). An example, in human terms, would be the amount of energy needed for a full-grown person to escape if he were covered with three feet of dirt. Aquatic insects have evolved (through necessity) various ways to overcome this barrier. Swimming emergents swim to the surface and push against it. When the top part of the pupal thorax protrudes through the surface and splits open, a meniscus forms around it, thus creating an escape hole in the surface. As the pupal skin (nymphal skin in the case of mayflies and stoneflies) splits open, it is actually aided by the force of the surrounding meniscus, and the adult insect passes through the hole, never even touching the water. Once an insect is on the surface, its structural hairs and water resistant skin help to keep it floating high until it flies away.”

During a major hatch this second period of hesitation creates a tremendous supply of helpless prey in the surface film. This is the stage that triggers the visible feeding. Trout break the surface with porpoising rolls, taking an insect and turning down. Occasionally a trout jumps into the air, its momentum carrying it out of the water.

The fish usually switch over very quickly, abandoning deep feeding, once the main hatch erupts because pupae hesitate and drift longer at the surface than they do near the bottom. The concentration of insects becomes much heavier in the meniscus, and once this happens the upper level is where fish can feed most efficiently.

A trout can feed in one of three ways when it begins taking pupae from just under the film: it can hold at the bottom and when it spots an ascending pupa it can follow the insect (seldom trying to grab it while it is swimming) and suck the pupa in as the insect begins struggling to shed the shuck; or the trout can hold at the bottom and watch for emergents already in the surface film and rise for the pupae as the insects pass over; or the trout can hold just under the surface and sip emergents drifting toward it. Which feeding mechanism a trout uses is determined by the quickness of the insects and the speed of the current. A trout only feeds the third way, holding under the surface, in the gentler flows. It feeds in either the first or second way in faster currents.

The peak feeding, with fish rolling and jumping all over the stream, should be a time when fly fishermen master the fish, but it can be a time of total failure and frustration. Too often anglers fail to realize that the insects are hesitating and concentrating under the film, not on it, and as a result they mistakenly assume that it is an occasion for dry flies.

Again, an effective tactic for fishing this stage of the hatch is one not usually associated with caddisflies. Especially when trout are holding just under the surface or rising to pupae already in the film, an emergent imitation designed to ride semidry generally outperforms any sunken fly fished with an active presentation. Used either dead drift or with a twitch and swim, the “damp” imitation stays in the surface film for the entire time, not just for a moment at the end of a swing or lift.

A technique that kicks the fly into a pulsing swim toward the top, the Leisenring lift, is generally effective when trout are rising all the way from the bottom to take emergents. A sinking pattern is cast upstream and allowed to drift near the bottom, but when it reaches a likely holding spot it is teased to life, hopefully in front of a trout. It is a method that demands skill in reading water if the angler is not casting to visible fish.

The feeding that occurs during this second concentration of pupae takes place in plain view, and a fly fisherman can figure out what to do when he sees the riseforms, but some anticipation of a particular hatch is still valuable. Possibly no other insect type inspires such strong selectivity in trout as a caddisfly pupa, mainly because the bright bubbles of air inside the pupal skin form such a visible characteristic. In this situation the proper method of presentation is not the overwhelming part of success that it is at other times. A fly fisherman cannot just pull out a general imitation and depend on his skill at mimicking the action of the natural. He needs both the proper presentation and the precise fly to consistently fool fish, and only a knowledge of local hatches can give him the chance to acquire or tie patterns beforehand that match a specific caddisfly.

The Third Area of Concentration

When the peak hatch is over and the surface of the river is blank, most anglers quit fishing, or at least stop trying to match caddisflies but there is still an hour of so of very exciting action left. There is one more concentration of insects that pulls fish, often the largest, into specific areas of the stream.

During any hatch there is a low percentage of natural cripples. Seldom, if ever, are these cripples numerous enough to cause selective feeding during the main emergence period. They would have to make up a much higher proportion of the hatch to create such a response. When fish feed selectively they feed with a regular rhythm, and they cannot feed regularly on a component of the hatch that appears irregularly.


The third area of concentration: In backwaters trout gather under the foam to take drowned emergents.

The purpose of selective feeding, a set of patterned responses to the stimulae of the insect, is efficient use of energy. By moving with rhythmical repetition a trout ingests as many calories as possible with the least expenditure of energy. The stronger the selectivity to a food item the more regular the feeding motion. There is no way a trout can determine if an insect partially out of its pupal skin, struggling to complete emergence, is in that condition temporarily or permanently. Certainly the cripples are eaten during the hatch along with the healthy insects, but considering them important at that time contradicts some basic principles that are controlling the trout.

Cripples are usually not important during the main hatch because they are not concentrated. During a heavy hatch, over a prolonged period of time, there are nevertheless quite a few of them, and if they could be gathered into a small area they would become a significant food source. Fortunately, for both trout and trout fishermen, that is exactly what happens after the hatch. Even after they drown, these crippled insects remain quite buoyant and are swept along with the current. Eventually many of them join with bits of flotsam or foam and collect in a backwater of the stream to become a great, swirling cafeteria for the fish.

It is after the main hatch that the leftover cripples become the center of attention. The trout abandon the regular feeding lanes and gather under the foam. In the gentle eddies they sip the inert insects, their snouts and backs breaking or bulging the white covering and leaving a momentary spot of clear water in the foam.

The best patterns for mimicking cripples are adult imitations that not only look like, but also act like, bedraggled, half-emerged caddisflies. They sprawl flat in the surface film, wings fanning out along the sides, and quiver with the slightest twitch. They recreate the last, feeble struggle of the natural because they are tied with soft materials.

In backwaters that have a mixture of open and covered water an angler can see his fly floating if it is in a clear area and the casting and striking is quite normal. But in the foam, he has to slap it down hard enough to force it through the upper layer. The angler has to fish blind, striking when a bulge or gap occurs where the fly broke through.

Does the post-hatch feeding attract big fish? Graham Marsh rose an eight-pound rainbow three times on the lower Clark Fork River from a deep backwater. He watched the fish come slowly up out of the hole, but each time the drag on the line snatched the floating flyaway just before it got there. Even with his considerable skill, Graham could not stretch out an extra half-second of free drift with the standard techniques he was using, but it was a glorious sight anyway watching that huge trout rise to a dry fly.

AMERICAN FLY FISHERMEN have not begun to tap the potential of the caddisfly hatches popping out all around them, and they never will with general fly-fishing skills. They will need to understand the habits of the caddis flies in their home trout streams and the unique tactics for simulating the actions of those insects before they can work the full period of the hatch. To benefit from the current revolution in caddisfly knowledge, fly fishermen will have to collect and identify the important groups, at least to generic level, that they encounter regularly. Then they will have to study the biological information in angling entomologies for these insects so that they can anticipate and properly fish the hatches.