Fly Tying the Swimming Isonychia Nymph

Lenny and I put our rods together and began working our way upstream through a mixture of pockets and riffle water toward Decker's Eddy. We enjoyed the morning by working the drift lanes between rocks and seams in the currents, and we caught fish fairly often as we hopscotched our way upstream. When I paused to take in the scenery, I couldn't help but notice the number of Isonychia nymph husks on the rocks along the edge of the stream, some of them barely protruding above the waterline.

At a spot where the stream negotiated a bend, and then broke into a long flat pool, a larger fish showed itself momentarily but showed no interest in my Ausable Wulff or the beadhead pheasant tail nymph dropper. It had been a great morning, and I was more than content with the fishing, but that larger brown inspired me to cut the flies from my leader.

"What are you doing?" Lenny asked when he saw me changing flies. I knotted on a Swimming Isonychia Nymph, attached a dropper of about 18 inches to the bend, and added a second nymph of the same pattern to the end of the dropper. In between the flies I clamped on one small split-shot, just heavy enough to take the nymphs down in the current.

"I want to see if this guy is hungry," I said as I made the first cast across and upstream, then made a mend and lifted the rod to work the drift with the rod tip as the flies began to swing across the current . . . too short.

My next cast was slightly longer, and again I teased the flies with the rod tip as they swung across the run. The third cast was 2 feet longer, and just at the point where the flies began to lift and swing, there was a flash and eruption of water as the fish hooked itself.

The rod doubled over and the fish wallowed in the current before taking off on its first hard run, taking line from the reel. It was a better fish than any I caught previously in the day, but I managed to turn it and work it upstream, pausing while it stopped to shake its head violently along the far side of the stream.

One more hard run downriver again stripped line quickly off of the reel until I stopped the trout on a threshold of tangled and sunken tree limbs, then slowly worked the trout back upriver into the mesh of the net. The husky Willowemoc brown eyed me with the Swimming Isonychia Nymph seated firmly in the corner of its mouth.

Underwater World

The world of a trout stream is one of incredibly diverse life forms. Small schools of dace, chubs, and trout fry flash as they turn in the calmer shallows, while more solitary fish such as sculpins move slowly across the streambed feeding on plankton and tiny macroinvertebrate organisms.

An array of nymphal stage insects exist here, from the minute larval and pupal stages of chironomids to the husky clambering nymphs of the various stonefly species. Caddisflies exist in some varieties as case dwellers; moving about freely on the stream beds in small structures composed of pebbles, sand, twigs, and other vegetable matter carefully cemented together, while other case-dweller species anchor themselves securely to stones in faster currents while they work to scrape food from the surfaces of rocks and stones along the bottom.

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In the crevices between stones, the net builders of the free-crawling Hydropsyche species construct small silky nets to seine food matter drifting in the currents, while the green wormlike Rhyacophila larvae anchor themselves securely to solid objects with their clawlike prolegs on the tips of their abdomens. A variety of mayflies represent themselves in several distinctive and diverse forms, from the flattened fast-water clingers of Epeorus, MacCaffertium, and Stenacron that seem to be almost pressed onto the rocks and stones by the swift currents they inhabit, to the crawling varieties of Paraleptophebia and the husky Ephemerella species that inhabit somewhat slower pools and runs.

The giant burrowing Ephemera, Hexagenia, and Litobrancha species tunnel themselves into the softer substrate of the streambed until they are mature and ready to emerge.

The last grouping of mayfly nymphs are the important swimming varieties, which are represented by the tiny olive-colored Baetis and Acentrella species, Siphlonorus, and the larger mahogany Isonychia nymphs.

Because of their wide regional distribution, and the idiosyncrasies of fishermen, Isonychias over the years have been referred to as Slate Drakes, White-Gloved Howdys, Dun Variants, Leadwing Coachmen, and Slate-Wing Mahogany Duns. They are found in many fast-flowing Eastern streams, with two peak emergences each year.

The first and the heavier hatch usually begins in early to mid-June and continues into early July.

A second, similar event begins when the water temperatures have returned to a more optimal levels in late September to mid-October. Some Eastern tailwaters with year-round cold water have these flies hatching sporadically all summer, making Isonychias one of the most important summer mayflies in the East.

Over the years entomologists have continually reclassified and renamed many of the mayfly species we imitate, and the Isonychia genus has enjoyed its share of review and revision by the scientific community. Historically the Isonychia genus was comprised of seven distinct species dating to J.R. Traver's research in 1930 which identified those species as I. pacoleta, christina, circe, fattigi, harperi, matilda, and sadleri, with another species bicolor identified by Walker.

Isonychia bicolor was traditionally credited as the earlier emerging species, followed by sadleri and harperi species which made their emergences in the fall of the year, and most angling texts written in the past 40 years have recognized all these species of Isonychia.

However, modern entomologists Dr. Boris Kondratieff and J.R. Vorshell have concluded from DNA analysis that all of these previously identified species of Isonychia are "synonyms." All of them are actually the same species Isonychia bicolor, and the mayflies that emerge in the fall are merely the offspring of those that hatched earlier in the spring of the same year. This multi-brooded behavior is known as "bivoltine."

Isonychia nymphs are unusual, not only from the standpoint of their physical appearance, but from a behavioral perspective as well. Most nymphs that inhabit our streams are inconspicuous for most of their underwater lives; living reclusive lives hiding under rocks on the streambed or tunneled into the bottom silt until they are completely mature and ready to emerge into an air-breathing adult mayfly.

Their visibility and vulnerability to predators such as trout is brief and relatively limited until the mayflies begin their migrations prior to and during actual emergence periods.

Swimming nymphs like Isonychias, on the other hand, spend a large portion of their lives in the open, surviving by being agile even in fairly fast water. For both the trout and fly fishers, this swimming behavior makes imitating the nymph stage of the Isonychia a very important part of our fishing strategies, and flies that imitate these swimming nymphs deserve a place in every fly box east of the Mississippi.

The nymphs are rather large and average 14 to 15 mm during the earlier emergences, which matches a size 12 or 14 2XL hook. Second-generation hatches are smaller and average 10 to 12 mm in length.

The nymphs have three tails which are short and featherlike in their structure. The thin cigar-shaped bodies of the nymphs are a claret-tinted dark gray or dark reddish-brown. They are heavily segmented and have oval-shaped gills arranged nearly perpendicular to the abdomen at each segment.

Isonychia nymphs in some watersheds have a light-colored median stripe, which runs the length of the top of the nymph. This stripe is nearly white in some instances, while in other rivers, this characteristic is not as prominent.

Another unusual feature is the presence of gill structures at the base of the forelegs—much like those found on stonefly nymphs—in addition to the gills located along their sides at each abdominal segment.

Isonychia nymphs dart about from stone to stone in quick bursts much like minnows. At rest, the sleek nymphs sit perched on rocks like praying mantises, maintaining their hold on the rocks with their mid and hind legs while holding their forelegs outward to collect food from the currents. The forward edges of the front legs are lined with long fibers along the forward margins that enable them to filter food drifting in the current.

When frightened, they swim rapidly, moving their abdomens in an up-and-down motion for propulsion, with their legs tucked close to the thorax.

It is somewhat ironic that crawling nymphs often drift toward the surface to hatch, while these excellent swimming nymphs crawl up rocks and riverside vegetation to hatch. Large stones along the edges of the stream often have a number of empty nymphal husks on them, a clue you should recognize while fishing.

This behavior of crawling during emergence minimizes the importance of dun imitations, especially on streams where there is an abundance of rocks for the nymphs to climb out of the water.

On larger rivers where protruding rocks are not always present, the nymphs sometimes hatch in a more conventional manner like most other mayflies. In every situation, the nymph plays the most important role in your approach to fishing the Isonychia hatch.

Creating an effective nymph imitation is not difficult to accomplish, and you can certainly catch fish on simple, traditional fly patterns. But one appeal of the Isonychia nymph is the size and the distinct profile of the naturals, which allows room to be creative in designing a nymph that is more appealing, and matches the natural nymph more precisely.

One of the best patterns I ever saw was tied by the late Chauncy Lively and was presented in his book Chauncy Lively's Fly Box, A Portfolio of Modern Trout Flies published in 1980 (Stackpole Books). The pattern presented in his book featured an abdomen made from a condor quill biot, which produced a nicely segmented abdomen, but also created the effect of prominent gill structure as well.

In creating my own pattern I used the same idea but substituted a turkey biot since condor feathers can be difficult to secure. The abdomen is constructed over a foundation of monofilament on each side of the hook shank to produce a wider, oval cross section.

The wingcase is made from black Thin Skin, with the same white wire stripe pulled over the top, and coated with five-minute epoxy to better imitate a mature nymph with its dark, swollen wingcase ready to split open.

To get more movement from the fly, I use the curved Daiichi #1770 Swimming Nymph hook. The pattern has been hugely successful for me, and has accounted for some impressive days on the river.

Subtle Twitch

Using the Swimming Isonychia Nymph takes me back to my teen years when swinging wet flies was still a popular and successful style of fishing, often with two or three flies fished on droppers. The style of presentation with this fly is much the same as fishing wet flies, and I usually fish two flies at a time with 18 to 24 inches of monofilament separating them.

Trout expect to see the nymphs swimming in quick bursts, so it's important to add movement to the flies. I swing the flies across the current on a tight line, and twitch and work the flies with the rod tip to dance the flies across the stream.

During emergence periods, the nymphs move toward the edges of the stream where they can crawl onto dry land to hatch into adults—and the trout will be looking for them near shore.

Cast your flies toward the center of the stream and mend the line so the flies have time to sink, then begin their pulsating journey toward and around riverbank rocks. Work the rod tip with 4- to 6-inch twitches to add movement to the flies and be ready for takes, which are often fast and aggressive.

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Henry Ramsay (ramsayflies.com) is a fly-fishing instructor, photographer, and presenter from Birdsboro, Pennsylvania. He is the author of Matching Major Eastern Hatches: New Patterns for Selective Trout (Headwater Books, 2010).