Fly Fisherman Throwback: A Primer of Stream Entomology-I

Editor's note: Flyfisherman.com will periodically be posting articles written and published before the Internet, from the Fly Fisherman magazine print archives. The wit and wisdom from legendary fly-fishing writers like Ernest Schwiebert, Gary LaFontaine, Lefty Kreh, Robert Traver, Dave Whitlock, Al Caucci & Bob Nastasi, Vince Marinaro, Doug Swisher & Carl Richards, Nick Lyons, and many more deserve a second life. These articles are reprinted here exactly as published in their day and may contain information, philosophies, or language that reveals a different time and age. This should be used for historical purposes only.

This article originally appeared in the March-April 1972 issue of Fly Fisherman magazine. Click here for a PDF of the print version of "A Primer of Stream Entomology-I."

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Part One of a series of articles designed to familiarize fly fisherman with the practical aspects of stream­ side entomology. The series will cover the favorite fare of trout, smallmouth bass and even bluegill in all of their habitats, slow water and fast, from farm ponds to high mountain lakes. Mr. La Fontaine is both a scientist and fly fisherman who goes where the fish and the hatches are.

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 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 caddis flies which do not build cases) and many other insects, is great­ est at night, and the same applies in large rivers as in small streams.

"The rhythm, however, is more complex than a simple noc­ turnal maximum. Usually, the maximum follows 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 these 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 not be selective. Although I caught trout, including an 18-inch brown on a grey Wooly Worm, I was not catching 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 genus, alloperla, began to hatch in July, and I used 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.

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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, when the green caddis larvae (hydropshyce) became predominant in June, starting 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 an alloperla species with a Yellow Stonefly Nymph and divided 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 of the fly drew three more strikes than the normal pattern, but with the dark fly I hooked five out of nine striking fish and with the light 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 have been due to shape of the fly.

I slowly found the better areas to fish. Since the drift occurrence is highest in the riffles, I started fishing the choppy stretches, but in the dark it was hard to pick out the feeding stations and casting was in a random arc. My records showed a better strike ratio and a better hooking ratio at the heads of 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 slide and a timed guess, but an experienced nymph fisherman will not miss many more strikes in the deep dusk than in the 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.

Following is a list of stonefly genera that hatched last season on the Clark's Fork, and the period of emergence. The sequence of the emergence will hold true on most rivers, but the dates will vary in different streams and at different altitudes.