Little Yellow Stones

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Little Yellow Stones

Starting in mid June and continuing well into August little yellow stones take to the air to lay their eggs. Like tiny yellow leaves they fall to the water where trout sip them with leisurely confidence.

When it comes to summer my attention often turns to small streams hidden deep in the coastal mountains along the Oregon coast. These are not high mountains. The highest peaks barely reach 2,000 feet in elevation. But they are rugged and difficult to get around in, in large part because of the dense vegetation. Bushwhacking through salmonberry and devil’s club well above your head is common. The reward comes when you stumble out of the brush onto the bank of a hidden little stream underneath a canopy of Douglas fir, hemlock and cedar. Small riffles separate long glassy pools that reflect the overhanging vegetation like a house of mirrors broken only by the nose of wild cutthroat trout sipping some morsel fallen from above. It is rare to find another angler on these streams – larger quarry on more famous waters attract the masses like flies to a sweaty neck. Though the fish in these little streams won’t break many tippets, the fly fisher will find plenty of challenges. Limbs and branches wait to snag your fly on nearly every backcast. And a good natural drift through quickly changing currents is still necessary to entice even the unschooled cutthroats that hide below tree roots and behind boulders.

The hatch that defines these little streams for me is also small. In the mid or late afternoon find a seat on a large moss covered stump or trunk of a fallen fir. Sit quietly and watch the shafts of light that somehow find a way through the dense network of overhanging leaves and bounce off the shifting surface of water. If the time is right small delicate stoneflies will begin to appear ten or fifteen feet over the stream. They are females already mated and ready to lay eggs. The swarm often grows in number until the air is literally filled with their light yellow bodies drifting through shadow and light as they glide down to the water. If you watch even closer you might notice other insects flying in among the little stones. They are swift flyers that dart quickly to grab a stone in mid air, and then fly off with their dinner. These are dance flies of the family Empididae in the order Diptera. Much like falcons hunting slow flying prey they find the small stoneflies an easy target. There are many more stoneflies than dance flies, however, so plenty of stoneflies reach the water where they rest briefly on the surface and release their eggs. Of course any self-respecting cutthroat is quite knowledgeable of this behavior, and readily pop the resting stoneflies off the surface with energetic rises. Now it’s time to get off your stump and start fishing.

These small delicate stoneflies are commonly called little yellow stones, yellow sallies, or stripetails depending on who is doing the calling. They belong to a diverse family of stoneflies – Perlodidae – and most land in the genus Isoperla. The genus Isoperla has the highest species diversity of any genus of stoneflies with 57+ species currently known in North America. Some species are widely distributed occurring across most of the country, while others occur only in small regions. Altogether the diversity of species is evenly spread across the country with about 27 species in the east (Northeast to Southeast), 24 in the midwest, and 23 in the west (Rockies to the Pacific). Such a wide distribution indicates that wherever you fish, some species of these stoneflies are likely to be present, at least in small numbers, though they can be quite abundant.

These small stoneflies present a real identification challenge to the angler and professional entomologist alike. The great variety of species within this genus makes it difficult to find a few defining traits that apply to all species. The size of mature nymphs and adults ranges from 6-18 mm (1/4 to 3/4 inch) body length, with most species tending to the smaller end of the range. Adults appear larger than they really are as their wings normally extend past the tip of their abdomen; however, some species exhibit greatly reduced wings, a trait common to a variety of stonefly species. Color patterns, normally a poor character to rely on, can be useful for recognizing little yellow stone nymphs. Nearly all species show distinct dark and light longitudinal stripes on top of the abdomen. In addition Isoperla nymphs completely lack gills, and their tails are as long as or longer than their abdomen.

Adult Isoperla often show the same longitudinal stripes along the top of the abdomen as the nymphs, but not in all species. The head and prothorax (thoracic segment directly behind the head) also show distinct light and dark color patterns. Tails are well developed, but not unusually long. While known as little yellow stones, the color of adults ranges from light yellow to medium brown. Overall, the pale to dark yellow color, light and dark stripes on the abdomen, distinct light and dark color patterns on the head and thorax, and lack of all gills or gill remnants are the primary characteristics for recognizing species of Isoperla.

One of the key identification features, namely the lack of gills, strongly influences where and how these small stoneflies live. Without gills nymphs must obtain oxygen by diffusion directly through their exoskeleton. This normally occurs where the exoskeleton is naturally thin, like the base of the legs. Still, it is a rather inefficient approach to breathing, and to compensate species tend to prefer cold streams with high levels of dissolved oxygen. Riffle areas also contain higher levels of oxygen than slow quiet flowing reaches of a stream, and therefore riffles tend to be the habitat of choice for most nymphs. The need for cold water also results in a general trend of more abundant populations in smaller higher elevation streams than in larger lower elevation rivers. Some species, however, have adapted specifically to large rivers.

Riffles provide a variety of niches and food choices for small creatures like stonefly nymphs. Most little yellow stones will be found around the base of cobble and large gravel where the current is significantly diverted and slowed. These areas also trap leaves and small pieces of wood, which provide both shelter and the primary food for most Isoperla nymphs. Some species have been found to be predacious, eating a variety of small insects like chironomid or midge larvae. Even those species that are herbivorous or omnivorous early in life often switch to a more meaty diet in their last month of development as they put on a final growth spurt before adult emergence.

Mature nymphs normally find a large rock protruding above the water upon which to climb out of the water for emergence into the adult, or they crawl out of the water onto shoreline vegetation. There have also been reports of some Isoperla emerging directly in the surface film similar to mayfly nymphs. It is also true that such reports have been widely disputed and I for one have never observed this type of behavior. But neither have I seen all the species of Isoperla emerge, and given the resistance of insects to conform to one type of behavior I wouldn’t be at all shocked if somewhere this happens.

Depending on the species and geographic location, emergence may begin as early as May or as late as October. Peak emergence activity typically occurs in June, July or August in most coldwater trout streams. Newly emerged adults hide on shoreline vegetation. Mating takes place a few days after emerging, also on the shoreline vegetation. Like other stoneflies many male Isoperla adults attract mates by “drumming” the tip of their abdomen against a suitable substrate. Only virgin females respond, often with their own drum reply. Don’t expect to be overwhelmed by the sound of drumming stoneflies the next time you’re fishing along a stream during a good stonefly hatch, however. You need the ears of a female stonefly to easily pick up the frequency of these invertebrate percussionists. After mating females remain on the vegetation a few days while their eggs finish developing. At that point they take flight on some pleasant summer afternoon and lay their eggs on the water’s surface. The entire life cycle, from egg to adult, requires approximately one year.

Fishing tactics for little yellow stones vary with time of year and activity of nymphs or adults. The months following adult egg laying activity are spent as eggs (3 or 4 weeks) and very small immature nymphs, neither of which requires imitation by the angler. Growth of nymphs continues slowly through the winter, and then picks up dramatically in the spring. A month or two before adults actually start emerging is a time when nymphs are available to fish. However, even then these stonefly nymphs do not drift in the current as readily as many other aquatic nymphs, so they are not often important to the angler. The one time I find the nymphs worth imitating is during the early stages of adult emergence. The number of migrating nymphs peaks at this time, and good numbers get washed off the bottom into the current where feeding trout wait. Drifting a nymph pattern close to the stream bottom at the tailouts of riffles or heads of pools can be effective at such times.

My favorite, and I think most effective time to fish imitations of little yellow stones, is when adults are active. A simple dry fly works great when females are laying eggs. You can adapt a number of standard patterns to this task: a yellow sally, yellow renegade, or yellow elk hair caddis for example. I also use a simple dry fly with a light yellow body, white CDC wing, and light brown hackle. Fish the dries with a drag free presentation where you see the adults laying eggs and fish rising to them. I have also found a small yellow soft hackle effective. This works great for adults that get trapped in the surface film. Fish it dead drift in the surface or an inch or two below just like you would a dry fly. Sometimes fish find these swamped adults more enticing than those floating high on the surface.

I’m definitely looking forward to the next sunny afternoon that finds me sitting on a small stream under a canopy of firs waiting for the first flight of little yellow stones. Once they are in the air I know summer has truly arrived.

Rick Hafele has been fishing streams throughout the West, and many other parts of the world, for over 50 years, and has been sharing his experience and knowledge with anglers through books, videos, slide shows for almost as long!. You can find out what Rick is up to at his website:

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