Fly Fisherman Throwback: A Primer of Stream Entomology-IV

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 October-November1972 issue of Fly Fisherman magazine. Click here for a PDF of the print version of "A Primer of Stream Entomology-IV."

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Part four 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.

Perhaps the fascination of angling lies in the fact that we can learn only the generalities of catching fish. We find that it is a game of specific moments and unique problems, and that the trout vary so much in habit and feeding routine from stream to stream that it is impossible to make hard rules. How does the fly fisherman cope with this challenge? As beginners we see that to attack each fish as a singular and limitless challenge is also impossible, but that even if there are not rules, we can eliminate most plans of attack. Soon we learn from study and experience to narrow the probabilities and try to guess the specific approach for each fish. This is what might be called "problem fishing."

Picture a day in early June on the West Branch of the Ausable. For Fred Rapp and me it was the last day of a three-day fishing trip. We stood spaced in the long slick, not wanting to flail the water before the trout began to feed. Deep at the head of the slick we both saw the flash of a fish moving underwater. Fred soaked his nymph in his mouth and cast upstream. It was too soon, however, because it was half an hour before either of us had a clear hit, and then Fred muttered at a miss.

The first dun, a Rhithrogena mayfly, popped from the water. Fish began to feed on the rising nymphs and I hooked a small brown as my fly swung on a taut line and rose. The fish splashed and dove, possibly putting down better fish in the area, but I let him have his head before I released him.

Casting faster to try to catch fish before the flurry ended, I missed two strikes on the nymph. Irritated at the concentration that the artificial nymph required, I changed to a dry fly, a dark Quill Gordon. Farther down the slick I hooked a fish, the best of the day, which pulled off after a deep dive.

The hatch ended and we stopped fishing. We laughed as we asked each other how we had done. "Three," Fred totaled, "all small."

We had not done very well. The hatch was sparse and the fish were cruising and hard to point, but we had known when to expect the flurry and it was like missing an ambush. If we were not too disappointed, it was because there would be a hatch of sulphurs (Ephemerella dorothea) in the early evening and that was the main feed of each day. The short, sparse hatch for the past three days had given us some extra dry­ fly fishing to rising fish. The first day the small emergence caught us by surprise, but trout were rising and we captured specimens of the subimago (dun) and matched the insect with a Quill Gordon. We snipped off weighted nymphs and caught a few 9- to 11-inch browns on the dry flies.

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That night we let a dun molt into an imago (spinner) and identified the in­ sect as a member of the genus Rhithrogena. With this information the next day it was possible to be on the right water, the slick or pool beneath the fast riffle, and to have a representative nymph to fish the pre-hatch. With the selected nymph Fred caught the best fish of the three days, a 16-inch brown.

Studying the insect that the trout were rising to had put us on the right spot with the right fly. Even if we had not capitalized on our extra information, our chances of success were higher, and to the modern angler trying to catch more fish, this is the value of studying the insects, aquatic and terrestial, that are important on the trout stream.

The moments of super-selectivity are not the main concern of the beginning fly fisherman. An angler could look at the dun and match it without knowing the scientific name. The selectivity of the fish to a single pattern is usually not insurmountable (the aptly named fly, called the Ner-e-Nuff also caught fish during the Rhithrogena hatch).

A basic knowledge of the life and habits of the aquatic insects, without the necessity of learning scientific nomenclature, will help the beginning fly fisherman in two ways. He will learn the peaks of insect vulnerability, those moments in the life cycle when the insect is most exposed to fish predation, and the method of angling that is best for approaching the trout feeding on the exposed insect.

The emergence period of the mayfly, with the intense activity of the nymph in the pre-hatch stage, is a time of heavy predation by fish. Many anglers who point out the hatching of the mayfly as a minor percentage of the in­ sect's life as compared to the great time spent as a nymph, and cite this percentage as a reason for greater effectiveness of the nymph over the dry fly, may not realize that if the entire length of nymphal life was preyed upon as heavily as the insect is preyed upon during emergence, few of the specie would live to adulthood. The dry fly is still the prime artifice when used at the right moment, and a proper nymph cast to a bulging fish is one of the most deadly of enticements.

The stages of the life cycle of the mayfly are easy for the observant angler to learn to recognize. The nymph, in most species, splits its nymphal shuck at the surface and emerges as a dun. Trout will feed on the rising nymph during the first part of the hatch (although there are trout that will feed entirely on the nymph), and on the floating dun as more insects reach the surface.

The beginning fly fisherman can also quickly learn the half dozen or so major mayflies along his favorite trout stream. One season of stream notes, in which specimens are gathered and dates recorded, are enough to set up an emergence table for an area. Careful study of the water during gathering will allow collection of both nymph and dun, and he can match the insect to a fly by sight.

These notes become a table of pre­ diction for the season of average rain­ fall and temperature range. The same specie of mayfly will emerge yearly within the same span of dates, and as the angler becomes attuned to the pulse of his stream he can wait in readiness for the successive mayflies to appear.

Later if the angler wishes to learn the scientific name of the specie, he can have an insect specimen classified at a nearby university, or he can learn with a little study to classify the insects himself. R. Usinger's Aquatic Insects of California contains the keys in the form of couplets of physical features that narrow down to the individual specie. Once the names of the physical parts of the insects are known, it is not difficult to identify with a microscope.

On the Ausable our study helped us to catch a few fish that we might not have caught otherwise. On our third day, after the Rhithrogena hatch, Fred and I went up to a small inn to await the evening flurry. The bar-man asked, "How'd you do?"

"Just a few small ones today," Fred said, "but that's better than nothing, and, you know, us dubs have to fight for everything we can get."

Life Stages of the Mayfly

• Egg: Female deposits eggs by drop from air to water, by dap from abdomen to water, or by submersion into water as she crawls down rock or stick. Eggs adhere to bottom rubble until hatching.
• Nymph: Aquatic stage of the insect, usually lasting one year. The nymph splits its skin as it grows (instar). Different species cling to rocks, crawl over detritus, or burrow in silt. They feed on vegetable matter or on animal prey such as midge larva. To hatch, the insect crawls out on stones, swims to surface and splits shuck, or splits shuck on the bottom and swims to the surface.
• Dun: Bright, agile stage of insect. Mating is completed in the air and the female deposits the fertilized eggs. Spinners die and fall to the ground or water, at times in heavy numbers that cause important trout feeding.
• Spinner: A short lived stage. Flies from the water surface to nearby tree or brush. Hides in foliage until opaque sheath splits and adult spinner emerges.