There are 4 main species of stoneflies that hatch out in the Black Canyon, each with its own Latin name of course. But, lets cut to the chase. From a pattern standpoint we have the the Salmon Fly,the Golden Stone and the Yellow Sally.
The big buzz for us as at the fly shop is the Pteronarcys Californica or what we call the Salmon Fly, Willow Fly, or Giant Stonefly. This "Rock Cow" lives under rocks and in cracks in and around riffles and runs, wherever there are significant amounts of oxygen generated. They can be 1.5" to 2.25" in length, and like cows, are herbivores which eat decomposed plant mater, vegetation, and algae. The Golden Stones, Perlid species like the Hesperoperla pacifica and Classenia Sabulosa, while somewhat smaller, are voracious predators and are extremely aggressive, even taking fingerling trout nearly their own size and piercing them with their sword like mandibles to kill them and then eat them. The Golden Hatch is not near as popular a draw for trout that their big cousins are. In general, Stoneflies have an exoskeleton much like grasshoppers. They grow by shedding this exoskeleton, growing larger during their molt, at which point the exoskeleton hardens again. By the time the migration occurs, a mature stonefly nymph will have gone through as many as 50 "instars" or molts in a 3 to 4 year span. Stoneflies are for the most part poor swimmers, and while they crawl easily between and under rocks, they mostly drift clumsily downstream. "Behavioral Drift" occurs when the river currents move them downstream to find a new home. "Catastrophic Drift" occurs when there is a river blowout, a flash flood, or huge, flushing flows, in which case the nymphs are dislodged from their homes and unintentionally tumble downstream where they relocate. By the way, don’t confuse abandoned, molted husks floating in the water as floating stones! These are in the water a lot of the time.
At three to four years old, the mature stonefly nymph gets the urge to migrate ashore. This annual phenomenon is dependent on flows, a temperature of between 50-52°, and phase of the moon (not as important). This normally occurs in early to mid June. In recent drought years where we’ve had low flows during the winter and little or no flushing flow/runoff the hatch has started as early as the end of May. In "high water", cold-release years like 2008, it was well into the third week of June! A "normal" start date would be for nymphs to begin to migrate to shore towards the first week of June in mass. You will normally have a small hatch early, like "advance scouts" checking out the terrain, but it’s the "Invasion of Normandy" that we’re looking for. Checking under rocks in the river’s edge can tell you a lot about where the process is, just prior to emergence or to see if there is another wave due to emerge. It is this "migration phase" that creates some of the most epic large-nymph fishing opportunities you’ll ever have, and you’ll get it by fishing deep troughs, and fast water right on the edge of highly aerated portions of the river. You may not want to even get your feet wet, because you’ll be standing where you should be fishing!
When people call into the shop they want to know about “the hatch�. “When’s the Hatch gonna be?� Check out the video “The Hatch� as an appetizer for a taste of the action, or watch “Bugs of the Underworld� by Ralph Cutter to watch the process unfold. . Unlike Caddis and Mayflies, this hatch is done out of the water. The real process begins as the mature nymphs crawl ashore late at night, where they latch onto grass, rocks, twigs, willows, trees, or whatever else they can lock onto and cement their forked feet to. Some anglers have reported that Yellow Sallies Water hatch, so keep and eye out for this.
Once their feet are locked on target, The nymph begins its transformation, as a slit appears behind the head and well down through their wing pad region, allowing the Adult Stonefly—head intact-- to wriggle out of its nymphal “shuck� and once out of its submarine like case the wings start to dry in preparation for flight.
Most of this action takes place in the middle of night, and the newly exposed and shivering adults will hunker down for the night. You’ll find them in the grasses, bushes and willows next to the stream, waiting for the warmth of day and first rays of sunlight to warm them up. Once they get warmed up and revved up they’ll start flying and take to the air, sometimes in mass..Â
It’s at this point that fly fishers get pretty excited. There are bugs literally everywhere-- in the bushes, in the grass, in the air, on their hats, crawling up their legs and shorts, down their necks, on their sunglass cords and hats—and to think, we waited a whole year for this! If you’re not fishing, you are going to want to get back away from the water, or you’ll be incredibly annoyed.
However, first day post emergence can be one of the most frustrating day of fishing you’ll ever have. Trout, freshly gorged on nymphs that were migrating shoreward the night before are sated and full. Their bellies are fat, lumpy, bumpy and distended. They have no interest in looking up to the surface and going though the effort of chasing after food. They’ve had their fill, and it takes a good part of a day or longer to digest their feast.
In the mean time, all those flying Adult stoneflies have begun to find each other. Males swarm all over the females to fertilize her and the eggs she is carrying. As you look in the trees and bushes you’ll see clumps, or balls of gyrating stoneflies in the midst of a mating “orgy�, for lack of a better word. This keeps them busy for another day of so.
Finally for the angler, the fertilized female says, “Enough, gotta go!�, and she flies out to the river to lay her eggs, or “oviposit� them by dipping the end of her abdomen, now full and dripping with heavy, sticky eggs through the surface of the stream so the eggs can sink to the bottom. Many times this is the last thing she ever does. GULP!
When the females hit the water in sufficient quantities, and trout have begun to get hungry again, its “Game on!� The females flap their wings and dab their eggs on the water and create a Feeding Frenzy of epic proportion. Normally sedate, controlled eaters, the Browns, Rainbow and Cutt-bows of the Gunnison River throw all caution to the wind, or waves, and slash at these bugs. It’s not uncommon to see Sandstone Grottos and rock walls with splash marks on them as greedy trout await egg laying Stoneflies or bugs that drop off of the walls. (defensively, they let go and drop when birds or danger approaches)
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It’s this “Feeding Frenzy� that most anglers are talking about when they ask about the hatch. The best feeding times are early morning to about 11, and it can shut off like a light switch. While trout may switch to PMDs, Golden Stones, and Yellow Sallies, the morning feeding period is mostly over in terms of the Big Stones. You’d better switch to another type of dry, or a nymph.  If you’re hanging around all day, or camping out, some evenings you’ll get another shot at it. Wind and cold really put them down, literally. They don’t tolerate cold well and they are poor flyers to boot. Look up in the sky. If it glistens with thousands of shiny winged bodies, all is well.
One good thing is that fish have a memory. Sometimes, weeks later, they will still hit orange strike indicators out of habit. We also love the stonefly hatch because while it’s a blast when you hit it just right, it also gets trout looking up towards the surface. This bodes well for the next hatches: the Golden Stone hatch (fair interest), Yellow Sallies and the PMD hatch (good to high interest). Then hopper season is just around the bend. What a great bug, what a great river!!