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Taking Advantage of Isonychia Hatches

When fewer anglers are crowding your favorite pools and pocketwater stretches, trout key in on these large mayflies.

Taking Advantage of Isonychia Hatches

Freshly hatched duns have the silver sheen that may have helped earn them the common name Leadwing Coachman. These large duns have yellowish middle and hind legs and reddish-brown extended forelegs, making them easy to recognize at streamside. (Ted Fauceglia photo)

By mid-June, most trout anglers have forsaken their fly rods for baseball and barbecues, but the summer action is just beginning on trout streams across the East and Midwest. At a time when there are fewer anglers crowding your favorite pools and pocketwater stretches, trout key in on the large Isonychia mayflies hatching sporadically over a four- to five-month period (June through October). Because this hatch is sparse but steady, the fish are usually less selective, which provides for some easy fishing–made even easier because the imitations are large and easy to see.

Common names for the large Isonychia mayfly are Mahogany Dun, Slate Drake, Dun Variant, Leadwing Coachman (in the East), and Maroon Drake and Mahogany Drake (in the Midwest). Most veteran fly fishers in my neighborhood just call them "Isos." They are a staple on quality trout streams in this part of the country.

Trout, especially wild and holdover fish, seldom feed on the surface during sporadic hatches–the energy a trout must expend bucking the current for only a few bugs is generally not worth the effort. Not so with the Isonychia mayflies. I can't count the number of times I've stood hip deep in a favorite run or pool on the upper Delaware and watched in amazement as good-size wild trout took these duns in broad daylight during a sparse hatch. My theory is that the trout key on them because they are around so long. This is one of those rare times on the upper Delaware when you can blind-cast the water with a dry fly during hatchless days and still catch fish!

There is great fishing to the Iso hatches throughout the Appalachian Mountains, the Poconos, Catskills, Adirondacks, and New England (in the East), and northern Michigan and Wisconsin (in the Midwest). There are concentrated hatches in late May and June in the Poconos, Catskills, and central Pennsylvania streams, followed by sporadic hatching into the fall, when water temperatures are below 66 degrees F.

The best Isonychia hatches in the Midwest occur between mid-June and mid-July. The insects hatch on Michigan's Au Sable and Wisconsin's Namekagon and Brule Rivers at about the same time. Unfortunately, they compete with the action of the giant mayfly "Hex" hatch (Hexagenia limbata), but the hatches continue throughout the long season.

My home waters on the upper Delaware have great Iso hatches. Isonychia begin to hatch in late May or early June (sometimes with concentrated hatching activity) on the West Branch, then continue sporadically through late October. The upper East Branch (a smaller tailwater) activity is similar.

The lower East Branch (and the Beaverkill) usually warm up in mid-June, but the Iso hatches come on strong in late September through October, when water temperatures fall into the 50s. The main stem of the Delaware, especially the upper 10-mile section that benefits from the West Branch's large coldwater releases, has periodic hatches throughout the summer and the fall.

Isonychia Nymphs

Bob Nastasi and I list six species of Isonychia in Hatches II bicolor, sadleri, and harperi in the East and Midwest, and velma, campestris, and sicca in the West. According to the latest entomological texts, sadleri and harperi, widespread throughout the East and Midwest, are now considered synonymous with bicolor. All three range in size between 12 and 16 mm and are similar in color. I. velma, campestris, and sicca are Western species and are considered uncommon insects, confined to a few Rocky Mountain streams, the Yellowstone watershed, and some Pacific Coast rivers. The most important species, I. velma, is the largest, reaching about 20 mm (equivalent to a #6 or #8 hook).

The nymphs are agile, swift swimmers. They are predacious and feed on tiny larvae and nymphs as well as detritus and algae. All in all, the nymphs are easy to recognize. They are large (12 to 16 mm), brownish black with a whitish mid-dorsal stripe, and have explosive starts when swimming. The gills are oval and platelike and located on the first seven abdominal segments. The forelegs have long, basketlike hairs on the femur and tibia. Their three tails are heavily fringed on both sides of the middle tail and on the inner sides of the two outer tails.

My aquarium observations, as well as our experience with the live bugs in my fly-fishing schools, show that these nymphs rank among the swiftest of all mayfly nymphs. Aside from being quick, they are also among the shyest, streaking for cover as soon as the aquarium is disturbed. In spite of their lightning speed, these nymphs are extremely agile and can change directions as quickly and unpredictably as a car in a demolition derby.

A few days before emergence, these fast-water-swimming nymphs migrate to the shallows where they congregate in a stream's quieter sections. As emergence approaches, many nymphs crawl out of the water to hatch like stoneflies, but on larger rivers most will hatch in the surface film. Look for their empty shucks on boulders and logs, especially in late May and June when the hatches are more concentrated, for evidence of an out-of-the-water emergence.

On quieter stretches, such as long, wide flats with a steady current, look for emergence activity around undercut banks, deadfalls, or manmade abutments. This is also a great hangout for lunker trout that take advantage of the nymphs' hatching traits.


On streams with pocketwater stretches, look for the empty shucks on the lee side of protruding boulders. These boulders create tiny pools within these turbulent whitewater stretches where the nymphs can emerge safely. They also provide ideal holding stations for trout that take full advantage of the nymphs' launching sites.

Trout are very conscious of all Isonychia migrations and follow the nymphs into these shallow areas when safety permits. Anglers should present their flies softly and keep out of the trout's sight when fishing in the shallows.

Nymph patterns can be effective, especially during concentrated hatches. Patterns should be weighted with lead or include a bead head. Unweighted patterns are effective in shallow water. My favorite nymphs are the Iso Compara-nymph, Zug Bug, and Beadhead Zug Bug. All are tied with peacock-herl bodies that, when wet, have an iridescent quality not unlike the dark, purplish or blackish brown of the natural insect. The Pheasant-tail Nymph is also effective and has similar qualities.

Taking Advantage of Isonychia Hatches
Isonychia nymph. (Ted Fauceglia photo)

Weighted nymphs should be fished dead-drift or bounced along the bottom in runs and pocketwater. Use unweighted nymphs in the shallows and tails of pools when casting to feeding fish. During nonhatching periods on the upper Delaware, we fish the weighted nymphs dead-drift through the gentle riffles or we'll cast across- and downstream and retrieve the fly with a short, stripping action to imitate the natural's erratic swimming motion.

Isonychia Duns and Emergers

The duns have slate-gray wings and dark-brown bodies with purple or olive highlights, according to the species. Isonychia have three tails as nymphs and only two in the dun and spinner stage. The middle and hind legs are yellowish, while the extended forelegs are reddish brown, the same color as their elongated bodies. These features make these handsome, dark-winged duns easy to recognize at streamside.

The adults usually hatch in late afternoon or evening because of the higher water temperatures. During cloudy or rainy days, they may emerge sporadically throughout the day if the water temperature is in the lower 60s. On cold tailwaters, you can expect hatching to start around midday and continue into the evening.

The naturals often crawl out of the water to emerge. A good number are blown back by wind gusts. Many emerge in the surface film in the traditional manner. On larger rivers that lack boulders and other convenient perches, such as the West Branch, East Branch, and main stem of the Delaware and the Au Sable in Michigan, they will emerge on the stream's surface in typical mayfly fashion. The duns seem to struggle more to escape their nymphal shuck than do smaller mayflies. Even after a dun is free, it will ride the currents for an exceptionally long period, struggling to get its extra-long body airborne.

Because the duns and emergers are vulnerable due to the extended time on the surface and in the film, both emerger and dun patterns are necessary. The deerhair Iso Compara-emerger is deadly due to the trailing shuck and low silhouette. The CDC Compara-emerger is also a great choice and is more visible, but both are highly visible because of their size, which is duplicated with a #10 or #12 dry-fly hook. (I like a #14, 4X long–it has less steel and floats better.) The deerhair Compara-emerger is more durable than the CDC pattern.

When possible, the emergers should be fished across- and downstream for proper drift. On smaller streams this may not be possible and upstream casts will be required. Use curve casts or angled reach casts in these situations to avoid lining the fish with your leader or fly line, and be prepared to set the hook, as slack develops quickly when your fly is floating back to you.

The Comparadun is my favorite pattern for this hatch. It floats flush in the film, providing the appearance of a vulnerable emerger or dun, and it's durable enough to easily take a dozen fish or more before you need to tie on a new fly. A couple of swishes in the water followed by a few false casts will keep it fresh and effective.

The Iso Parachute in the same hook size is also effective, but less durable. Again, I like #14, 4X long and #12 hooks. Carry a few standard-size #14s also if your river has the smaller species previously mentioned. A conventional Catskill-style pattern like the Dun Variant can also work at times, especially during a sporadic and sparse hatch–the fish are much less selective at this time.

Not all Isos are #10s. I can recall an early afternoon on the East Branch when I received refusal after refusal to my emerger and dun patterns. Seining the surface, it became apparent that there were a lot of smaller (and lighter) duns (#14s) on the surface, mixed in with the less prevalent, darker and larger Isonychias. Luckily, I had a few #14 Dark Hendrickson Compara-emergers and Comparaduns in a spare fly box, and it was just the ticket. The trout were wired to the smaller naturals that day, and I took six good fish in the 16 to 20-inch range before the action subsided. This is not an unusual situation on the upper Delaware tailwater rivers. I find that most of the major hatches seem to have sibling species with variations in size and color.

Isonychia Spinners

The mating ritual is often first sighted high above the stream about an hour before dark. The spinners are large and their silhouettes are usually quite distinct against the twilight sky. I like to use the time before they fall to the water to get into the proper casting position and tie on my spinner pattern. Close to dusk, the females flit to the surface and release their egg masses after mating.

The spinner action, though brief (30 to 60 minutes) can make the day. You'll be surprised by the size of the trout attracted to these highly concentrated spinner falls. Large trout usually migrate to shallow-water feeding positions during these falls, and hooking a heavy "slob" in knee-deep riffles is exciting stuff.

The best spinner falls occur when the air temperature is between 60 and 70 degrees F. The ideal air temperature may occur at various times of the day or night throughout the long period in which Isonychia hatches. In late May and early June the weather can be hot or cool, and it can also be variable (cloudy or rainy), especially in the mountains and the more northern latitudes. If it's cool, look for spinner falls to occur in the late afternoon; if it's hot, evenings and mornings.

As the summer progresses, the midday temperatures can approach 90 degrees F or more. This is too hot for the spinners, so they hang in the cool foliage where the heat cannot sap their strength before their strenuous mating flight. Once a spinner's strength is gone, it can't be replenished because their mouth parts are atrophied. So they make their appearance in the mornings or evenings when the air temperature drops toward 70 degrees F. During heat waves, I've witnessed spinner falls well after dark and as late as 1 A.M.

In late August and September, the evening air temperature may be too cool–and the afternoon temperature too warm–for spinner flights in many areas, especially in the northern mountain sections of the East. Mating flights then generally occur in the morning. In late September and early October, the best air temperature for spinners is in the afternoon when the air is warmer, especially in the higher elevations.

Rain, mist, high humidity, and heavy gusts of wind can spell disaster for the spinner flights, so during this time, the flights are usually deferred. Keep in mind that the mayfly's whole existence has been in preparation for this mating activity, and they will instinctively hold off the flight until the conditions for successful mating and egg laying prevail. When undesirable conditions continue for several days, the insects may become desperate, and anglers can expect exceptional spinner falls with the first break in the weather.

It's easy to determine when trout are on spinners by the "sip rise" form. Sometimes the ring is more pronounced, other times almost imperceptible. Spent spinners are extremely vulnerable and the trout sip them leisurely.

Presentation is always critical. You could select the perfect fly in the perfect size, but it means very little if the fly is not delivered on target and absolutely drag-free, especially on the smooth, clear pools, flats, and gentle riffles. A slack-line reach cast, quartering downstream, is the most effective one to use to eliminate drag and micro-drag (drag that you can't see, but the trout can). This cast also presents the fly, not the leader or the line, to the trout first.

My favorite spinner pattern is the Compara-spinner in sizes 14 (4X long) and 12. Bob Nastasi and I created this pattern in 1971 before the publication of Hatches. It has a dubbed body, split tails, and a hackled wing trimmed top and bottom. A grizzly hackle feather mixed in with a light dun or cream hackle is even better than just a plain color. Z-lon or poly-winged spinners are also very effective. Use 4X or 5X tippets for these large spinners. A 10-foot leader, including a 2- to 3-foot tippet, is sufficient.

I still marvel at the size of some of the trout I've hooked during an Iso spinner fall casting spent-wing patterns to a sip rise, only to find that I'm fast to a heavy trout. It's one of the most exciting aspects of fly fishing.

Large flies, great weather, eager wild or holdover trout, and having the stream all to yourself–it's hard to understand why more fly fishers don't take advantage of the summer and fall fly fishing to the Isonychia hatch. By understanding and imitating the natural's unique behavior, you'll be able to extend your season long after the spring bursts of the more famous bugs.

Al Caucci is the author of five fly-fishing books, including Hatches II, and the owner of the Delaware River Club Flyfishing Resort and Al Caucci Flyfishing. He lives in Tannersville, PA.

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