Understanding the Rocky Mountain West’s late-season emerging insects can help make fall outings the highlight of your year.
[by Boots Allen]
Yahoo! Autumn is here! If you are fishing the rivers of the Rocky Mountain West, the months of September through November are recognized by many as the best time to be on water. Crowds have dissipated from the summer onslaught, air temperatures are cool, streams are low and clear, wade fishing is generally easy, and trout are hungry as they feed in preparation for the approaching winter.
Part of what makes this time of year so enjoyable is the appearance of an entirely new set of bugs on which trout can feast. Gone for the most part are summer’s pale morning duns, Isoperla stoneflies, golden stones, drakes, and terrestrials. Instead, we see the reemergence of blue-winged olives, chironomids become the bread-and-butter insects on many western trout streams, and big bugs like October caddis dominate the scene on some waters.
Nonetheless, there are certain times and certain waters where
these standby bugs play second fiddle to other invertebrates—insects
that are far more crucial as trout foods. These include the mutant, or
short-winged, stonefly (Claassenia sabulosa); the great blue-winged
red quill, or hecuba (Timpanoga hecuba); and the big mahogany duns
(Paraleptophlebia temporalis and bicornuta).
On the streams I regularly fish in the greater Yellowstone area, these aquatic insects are well known by the resident fly fisher. They are certainly favorites of mine. Yet despite the relatively strong documentation in fly fishing literature (both print and online) of their presence, these bugs are often ignored by visiting anglers. That’s a mistake. Knowing their behavior, when they emerge, and the best tactics for fishing their imitations can make the difference between consistent action and a potential shutout.
The Mutant Stonefly
Also referred to as the short-winged stonefly, the mutant stone is a popular midsummer emerger on Pacific Coast streams like the Yakima and Cle Elum Rivers in Washington. They exist on waters throughout the greater Yellowstone area, but don’t begin to appear until the end of August and through September (although some emergences can occur as early as the end of July every now and then). The term “short-winged stone” is rarely used in the Rockies. Instead, it is primarily referred as the mutant stone or by its genus name, the Claassenia.
The Claassenia is an important autumn stonefly on Yellowstone area streams like the South Fork of the Snake River in Idaho, the Snake River in Wyoming, and the Yellowstone River in Montana. At over 30 millimeters long, it is a large stonefly with characteristics similar to a golden stone. One physical trait that sets it apart from almost every other stonefly is its wings. The males, which significantly outnumber the females during each emergence, have undeveloped wings that are one-half to one-third the length of their bodies (hence the name “shortwinged stonefly”). This trait renders the males unable to fly.
Another unique attribute of mutant stoneflies is their tendency to emerge nocturnally. You will be hard-pressed to find bugs crawling through their exoskeletons in the course of a day of fishing. What you will find is the cobblestone banks littered with shucks from the previous night’s hatch.
The late-season emergence of mutant stones allows anglers to fish large attractors used for more summer-oriented stoneflies and terrestrials well into October. Strategies and tactics matter in this regard. Keep in mind the underdeveloped wings of the males. Because of this, they have no choice but to scurry and run on the surface of the stream. Movement of your fly is key. Don’t shoot for the perfect drift. Skittering your pattern with authority across seams, eddies, bankside troughs, and flats, as well as along banks and structure, can reap benefits.
Pattern construction is important. The size of Claassenia requires size 10 to 8 long-shanked hooks, and many anglers targeting mutant stone hatches will gravitate toward modern Chernobyl patterns like Will’s Winged Chernobyls, Chubby Chernobyls, and Dornan’s Circus Peanut (not to be confused with the popular articulated streamer). Another very imitative pattern is Kasey’s Creature, developed by World Cast Angler guide Kasey Collins.
But old-school attractors have a role in this game as well. I have had tremendous success on Stimulators and Double Humpies. These patterns may not have the legs and foam like so many contemporary attractors, but when tied correctly, with generous amounts of hackle, they float like a cork in all but the most chaotic currents. More important, the hackle creates a wake that is almost a mirror image of a mutant stone’s trail when it
skitters along the surface.
Fish these imitations confidently throughout the day. Because of the nocturnal emergence of Claassenia, the morning hours between dawn and 10 a.m. are a great time to be fishing large attractors suggestive of mutant stones, which on some days can outperform other surface patterns that imitate mayflies and caddis.