Isonychia
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.
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.
Spinners
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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 (www.mayfly.com or 1-800-MAYFLY). He lives in Tannersville, PA.
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