The Slate Drakes of Summer

The annual spring pageant of mayfly hatches may end with the Green Drakes of June, but don't think the fishing is over. The summer and early fall season offers its own mayfly specials, and one of the most popular – with anglers and with trout – is the Isonychia, which goes by the aliases Slate Drake, Mahogany Dun and Dun Variant, among others. Isonychia has never gotten the publicity lavished on the Hendrickson or the Green Drake. That may be because its initial heavy hatch often takes place at about the same time as the drakes and the sulfurs, which get all the attention. But the nice part about the Iso is it keeps hatching, albeit in smaller numbers, all season -- long after the drakes and sulfurs are gone. From mid-June through late September, it's one of the most important mayfly hatches on eastern and Midwestern rivers.

It's also, for my money, the most fun. Its nymphs, duns and spinners are big – their imitations are tied on size 12 and 10 hooks – and easy to identify, since there are no other mayflies of comparable size and color hatching at this time of year.

Trout in the swift-water sections of Catskill and Adirondack rivers will smash dun imitations on the surface, and Iso spinner falls can provoke outstanding evening feeding among trout that have gone the whole day without a concentrated hatch of anything. But much Isonychia fishing is done under water, with nymphs like the Zug Bug and wet flies like the classic, handsome Leadwing Coachman. And if you find dead-drift, high-stick nymphing tedious and taxing, you'll love fishing the Iso hatch, because you are free – you are, in fact, encouraged – to let your subsurface fly swing and zip across the currents. Iso nymphs are terrific swimmers.

There are some 25 different species of Isonychia, report Al Caucci and Bob Nastasi in "Hatches II," but two of them account for most of the excitement on eastern and Midwestern trout rivers: bicolor and sadleri. Iso bicolor's more common in the east; it may have an intense initial hatch period of three weeks or so, but will continue emerging afterward, more sporadically, for two more months. The sadleri is the dominant Iso in the Midwest, where its hatch has a duration more typical of mayflies, less than a month. The two species are similar enough that one description applies to both.

The nymph is large, one-half to three-quarters of an inch long, and has a distinctive white stripe running down the middle of its back. Its tail has a thick fringe. Unlike many mayfly nymphs, which can only get around by crawling on the rocks of the streambed or drifting helplessly in the current, Iso nymphs swim well. They can and do dart and zig-zag through the water.

A nymph that big that swims that well works up an appetite. Iso nymphs are not vegetarians; they'll eat small larvae and nymphs. They've been compared to stonefly nymphs in this regard.

But the most common comparison to stoneflies comes from the way Isonychia bicolor hatches. Generally speaking, it swims to shore and crawls out onto dry ground for the metamorphosis from nymph to dun. I've seen scores of freshly hatched and still-hatching Iso's on the rocks along the edge of New York's Esopus Creek. You'll see their dried shucks on rocks, just like the husks of stoneflies.

This tendency to migrate toward shore makes the shallows near the edge of a stream a good place to swim an Iso-imitating nymph. In pocket water, trout like to hang out near the very same protruding rocks upon which Iso nymphs crawl to emerge, so let your nymph swing through these spots and play in the slack water behind the rocks. However, some Iso's reject the stonefly routine and hatch in the water like most of their fellow mayflies. And even when they do hatch on the rocks, enough duns fall or are blown into the water to get the local trout in the habit of watching for them. So it's worthwhile to fish a floating dun, too.

The Isonychia dun is reddish-brownish gray, or maybe reddish-grayish brown, and has dark, smoky wings. In September, when you haven't seen much else in the air but little blue-winged olives, an Iso cruising by looks like a B-52 among a fleet of Cessna's. Despite its habit of hatching out of the water, the Isonychia inspired one of the all-time great heavy-water floating dun patterns, the Gray Wulff, designed by Lee Wulff to imitate the Iso of the Ausable. (The Ausable's prolific Isonychia are also imitated by another local pattern, the Haystack, popularized by Francis Betters, sage of the river.) Anytime you're on likely Isonychia water – which is to say, swift-flowing mountain streams – and you see a dun, flying or floating, it's worthwhile to try a Gray Wulff, Isonychia Comparadun, Haystack or similar pattern. Don't wait for rises; fish the water. Duns may be available to trout only on an occasional basis, but the spinner is a different matter. Sexually mature Isonychia gather over their rivers in large numbers in the late afternoon and evening. If the lone dun looks like a bomber, the falling spinners put me in mind of a battalion of ruddy paratroopers, descending to a very dangerous situation indeed – a stream full of ravenous trout.

Once during a heavy Iso spinner fall on the West Branch of the Ausable in the Adirondacks, I heard an older angler nearby mistakenly call them Hendricksons. Maybe he was just plain misinformed, but maybe too he came from a circle of old-timers who applied that term to all good-sized, reddish mayflies. He went off downstream, and I don't know whether he tried a Hendrickson pattern on the fish or not, but if he did, he may well have hooked up. Isonychia does sort of resemble Ephemerella Subvaria, especially during the spinner stage of its life.

The spinner is a brighter red than the dun. As with most mayfly spinners, its (two) tails are long, and its wing resembles clear cellophane. Art Flick's Dun Variant, with its red-brown quill body and oversized, pale gray hackle, is an excellent imitation. Then again, the Rusty Spinner, so useful for so many mayfly spinner falls, seems tailor-made for the Iso mating swarm.

If you know of a river where trout seem particularly fond of flies made of peacock herl, chances are that river is home to the Isonychia fly. The purplish iridescence of wet peacock is vaguely lifelike and attractive to trout in general, but seems especially well suited to imitate the body color of the Iso. What's more, the small, wavy "hairs" of peacock herl do a good job of suggesting the abdominal gills and fringed tails of the fly. These characteristics may come together more effectively in the Zug Bug than in any other nymph pattern. The tail made of peacock sword tips is a dead ringer for the flipper tails of the real fly, and the silver rib segmenting the body creates the illusion of the feathery gills extremely well.

The Leadwing Coachman, among the oldest and most dignified of wet fly patterns, is also a dynamite Iso fly. It has no tail, nor any rib to approximate gills, and its down wing of gray mallard flight feather doesn't imitate a stage of Iso mayfly the fish are used to seeing, except perhaps for drowned duns. Yet it's one of the most killing flies on the Esopus, a veritable institute of Isonychia research. I expect the Coachman works so well because it suggests crucial elements of the real fly – its colors and its mobility under water.

If you've got access to a cold, rushing stream with trout in it on a sunny summer day, you've got yourself a little slice of heaven. If that stream is also home to the Isonychia, you're doubly blessed. This big, tough, wild-water fly may just bring you some of the most rewarding fishing of the season.