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Fly Fisherman 1972 – Imitating the Drifting Insect

A Primer of Stream Entomology — 1 

Imitating the Drifting Insect

The research of Aquatic Entomologists can be either interesting or worthless to the fly fisherman, but it is never useful unless the man can take the facts that affect the fish a step further and apply them to the catching of fish.

An article by H.B.N. Hynes, “The Entomology of Stream Insects,” contains a summary of the research done on the drift rates of aquatic larvae. “When night falls,” Hynes writes, “the insects wander out of the shelter and are more readily dislodged by the current. As a result, the drifting of mayflies, and also stoneflies, campodeiform trichoptera (the species of caddisflies that do not build cases) and many other insects, is greatest at night, and the same applies in large rivers as in small streams.

“The rhythm, however, is more complex than a simple nocturnal maximum. Usually, the maximum follow soon after sunset, and there is often a later one, or even two, before dawn on long nights. A full moon reduces drift.”

Since brown trout especially are known to feed at night, the information in the article seemed to be a gift, and I set a pattern of fishing early and late to see if I could catch fish on a nymph during those dark hours.

I began fishing without selecting an exact matching pattern of fly, figuring that with the hodge-podge of insects floating past, the trout would be less selective. Although I caught trout, including an 18-inch brown on a grey Wooly Worm, I was not catching the fish as fast as I should. I killed one trout on each night that I was not blanked, performing a stomach check on it as soon as I stopped fishing. In the stomach contents the insects were homogeneous in size, with usually one species dominant. With the stoneflies and the mayflies, when these were one of the preferred insects, the wing pads were dark in color, indicating that the insect was nearing emergence.

Since I was fishing on Montana’s Clark’s Fork River, a stream which like most Rocky Mountain streams contains major populations of stoneflies, I started choosing my nymphs to match the stonefly species that was prime to hatch. I collected samples in the day, telling by the development stage of the wing pads which insect was ready to emerge. In mid-June I matched the salmon fly (pteronarcys) with a pattern from Ray Ovington’s Tactics on Trout, using his variation of a Giant Stone Fly Nymph to catch a three pound cutthroat-rainbow hybrid in the evening. The small olive-grey fenus, alloperia, began to hatch in July, and I used the Schwiebert’s suggested imitation, the Yellow Stone Fly Nymph, to catch and release four browns over 17 inches while a family of tourists watched and listened to the splashes in the dark. Acroneuria californica became the predominant species hatching in late August. I finished my night fishing in September by matching a late nemoura species and catching the largest fish of the summer, a 4 lb., 11 oz. brown trout.

A similar pattern of matching the nymph could be followed on an Eastern trout stream with mayflies, starting with the Quill Gordon (iron fraudator) nymph that emerges soon after the opening day of the season.

I diverged from the stonefly pattern once, whe the green caddis larvae (hydropshyce) became predominant in June, starting with the large Grannom hatches on the river. The caddis fly enters a pupal stage before emerging, spinning a cocoon like a butterfly, so instead of a nymph, I fished a Grannom wet fly with a slight retrieve to simulate a pupa imitation.

Late in the season I made three adjustments in my fishing technique. I altered the exact imitation of the natural, tying a darker fly than the natural by using a darker shade of fur dubbing for the abdomen and thorax of the fly. I matched the fishing time into fifteen minute halves (night fishing in Montana is legal for only a half hour after sunset)I alternated the fifteen minute periods between a normal Yellow Stonefly Nymph and a rusty Yellow Stonefly nymph., The darker version fo hte fly drew three more strikes that the normal pattern, but with the dark fly I only hooked two out of six fish.

Maybe a black pattern would work even better, with the non-pastel coloring showing up better in silhouette in the dark. The relative failure of dark patterns in my early experimentation, when I was not matching specific insects, could haven due to shape of the fly.

I slowly found the better areas to fish. Since the drift occurrence is highest in the ruffles, I started fishing the choppy stretches, but in the dark it was hard to pick out the feeding stations and casting was a random arc. My records showed a better strike ratio and a better hooking ratio at the heads of the pools. I cast up into the riffle, trying to keep a nearly tight line as the fly spilled into the slower water. A trout’s pick up on the dead drift is slow and soft, and striking is a matter of feeling the line slake and a timed guess, but an experienced nymph fisher will not miss many more strikes in the deep dusk of that day.

I read Jim Quick’s Fishing the Nymph and I followed his suggestion to use a longer leader, changing to a 12 foot from a 9 foot, allowing the fly to settle deeper. The leader tippet of course must be light enough to let the fly move freely.


Note: (night fishing in Montana is legal for only a half hour after sunset) This is not true any longer, current regulations state fishing is open “all hours” for the season.