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.
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.
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