The East and the West don’t exactly meet where March browns are concerned.

March browns provide a perfect example of “common confusion.” In the early 1800’s North America wasn’t exactly a hotbed of fly fishing knowledge. Most of the information and techniques were transplanted from Great Britain along with many of the early settlers. Even though there is virtually no overlap of aquatic insect species between North America and Great Britain, some eastern hatches were similar in appearance to those in England. Thus, an insect with similar size and color characteristics as one from England was quickly given the same common name without regard to scientific classification. As anglers slowly moved west, they took with them the same approach to naming insect hatches – if they looked the same they must be the same. Thus, common names like March browns, green drakes, pale-morning duns, willow fly, and many others, spread across the country even though the western species were quite different than their eastern (and British) namesakes. Entomologists have been battling this common name confusion ever since, and much confusion still exists among anglers today as a result. So, for the sake of history, let’s take a look at March browns East and West, and see how they are the same and how they are different.

In this corner we have the eastern species, Stenonema vicarium, commonly called the American March brown. The genus Stenonema belongs to the family Heptageniidae, and includes 20 other species, with all but one (Stenonema terminatum) restricted to eastern or midwestern streams. Stenonema vicarium is a well known and important hatch throughout the Catskills and Adirondacks. Other well known species of Stenonema in eastern/midwestern streams include the light cahill (Stenonema ithaca) and cream cahill (S. modestum, S. mecicanum integrum, S. mediopuntatum, and S. femoratum). A frequently mentioned species in early fishing entomology books is Stenonema fuscum or gray fox. Entomologists, however, now consider S. fuscum identical to S. vicarium and thus no longer a unique species. In addition entomologists have split the genus Stenonema into two, adding the genus Stenacron.

The nymphs of Stenonema vicarium, like all species in the family Heptageniidae, are referred to as “clingers” due to their flat, pan-cake like bodies in which the head is the widest part. Nymphs have gills on abdominal segments 1-7, but the gills on segment seven are very slender instead of flat plate-like gills. The tips of the gills (except those on segment 7) are rounded rather than pointed as in Stenacron. Stenonema vicarium is the largest species of the genus with lengths between ½ to almost ¾ of an inch, and nymphs are typically reddish-brown in color.

American March brown duns are best recognized from other heptageniids by their wing venation, mottled wing coloration, and size (½ to almost ¾ inch). Spinners are best recognized by their wing venation, size, and shape of male gentalia, a character that requires a good microscope to see. In general nymphs are easier to identify than duns or spinners, so to help confirm what species of duns and spinners are on the water collect either mature nymphs from the stream bottom or the empty shucks of nymphs left floating in the water after dun emergence. Besides the largest, the American March brown is also the first species of the genus Stenonema to emerge of the season.

Even though Stenonema vicarium is a clinger and shaped for living in fast water, nymphs of this species are most abundant in slow to moderately flowing sections of streams and rivers. They are found in a wide range of streams including free-stone streams, weedy spring creeks, and even slightly silted rivers. Hatches typically begin in mid to late May, when water temperatures reach the mid-50’s, and continue until early July. Prior to dun emergence nymphs migrate to slower, more placid margins of streams. Fish seem to pay special attention to this movement and target the migrating nymphs up to a week before duns begin emerging. When emergence begins, nymphs swim to the surface where they often struggle in the surface film as the duns slip slowly out of the nymph’s exoskeleton, creating even more opportunities for hungry fish.

Nymph behavior signals two important fishing tactics. One is to fish a nymph imitation deep along the bottom as nymphs migrate to the margins. Try this well before you see any sign of dun emergence. Then, when emergence is underway, fish a nymph in or just below the surface to imitate nymphs swimming to the surface for emergence.

Dun emergence is concentrated in the afternoon, and is typically best on cloudy overcast days when emergence may continue for three or four hours. When the weather warms in early summer emergence may begin earlier, say mid morning, or later, not starting until temperatures cool in the evening. Duns drying their wings may create quite a disturbance on the surface by flapping and fluttering before getting airborne. Other times they float quietly, but still remain on the water for long distances creating great opportunities for surface feeding. Spinner flights can also be impressive with large numbers forming mating swarms from early evening to dark. Surface activity can be fast when the females begin laying eggs in riffles and runs.

Because of the good surface feeding created by this hatch dry fly fishing is both fun and productive. The large duns fluttering on the surface should be imitated with a high floating dry that is twitched and fluttered as it drifts. When duns are more sedate use a lower floating pattern and fish it dead drift. Finally, when the spinner fall is on, use a flush floating spinner pattern and drift it without any action through the feeding lanes of rising fish. Clearly the eastern version of the March brown is a hatch not to be missed.

Now, in the other corner we have the western species, Rhithrogena morrisoni, commonly known as the Western March brown. The genus Rhithrogena is also a clinger in the family Heptageniidae. Rhithrogena morrisoni is one of 23 species of Rhithrogena in North America. Unlike Stenonema, which is almost entirely restricted to eastern or midwestern waters, about half the species of Rhithrogena occur primarily in eastern streams and the other half primarily in western streams. R. morrisoni is one of the true western species. It is widely distributed from the Rocky Mountains to Pacific Coast, but does not occur in midwestern or eastern waters. Other important and closely related species include R. hageni (Western black quill) and R. undulata (small Western red quill). Rhithrogena undulata occurs in some midwestern and eastern streams as well as the west. While all three species can be important, this discussion will focus on R. morrisoni.

Like other clingers, R. morrisoni nymphs have a flat body and a flat head wider than the thorax or abdomen. This shape is very effective for clinging to rocks in fast water reaches of rocky streams. Rhithrogena species, however, have taken clinging effectiveness one-step further; the gills (found on abdominal segments 1-7) are large and partially overlap each other, and on segments one and seven they are greatly expanded such that they nearly touch each other under the front and rear of the body. The result is that the gills form a nearly uninterrupted disc around the body that provides a suction-cup like grip on the surface of any rocks the nymphs crawl across. This gill shape, along with the presence of three tails, makes it very easy to recognize Rhithrogena nymphs. Rhithrogena morrisoni nymphs can be recognized from other Rhithrogena nymphs by their size (3/8 to ½ inch), reddish-brown color, and three pale, almost white tails.

The duns and spinners of Rhithrogena are more difficult to identify than the nymphs. Wing venation, a somewhat confusing feature, can be used to help identify the genus Rhithrogena in that the upper leading edge of the front wings have a small area in which the veins are highly branched. Duns and spinners also have two tails instead of the three found on nymphs (a trait common to most clingers), a light brown to reddish-brown body, and are about 3/8th’s to ½ inch long excluding the tails. Wings of duns are a mottled brown color, while the wings of spinners are clear. To identify R. morrisoni from other species of Rhithrogena, one must look at the male reproductive organs under a microscope, a step the average fly fisher is not likely to take. Fortunately R. morrisoni emerges a month or two earlier than any of the other western Rhithrogena hatches, so the time of year can help you confirm what species is on the water.

While Stenonema vicarium nymphs favor moderate to slow currents, Rhithrogena morrisoni prefer slightly faster water with a cobble substrate, but still avoid the swiftest riffle areas. They are also most abundant in the large, lower elevation segments of western streams and rivers. The other major western Rhithrogena species (R. hageni and R. undulata) prefer faster water in stream segments at higher elevations – usually above 5,000 feet. R. morrisoni nymphs cling so effectively to the stream bottom that they rarely find themselves in the drift where fish can feed on them. However, as emergence time nears they migrate from faster mid-channel areas to quieter areas closer to shore. This period of migration provides some drifting nymphs for feeding trout. There is some debate about exactly how western March brown duns emerge. Some authors have written that the nymphs remain on the stream bottom during emergence, and the duns then rise to the surface. Others have said that the nymphs start swimming to the surface, but the duns emerge from the nymphs several inches below the surface. Still others state that the nymphs swim all the way to the surface, and the duns emerge in the surface film. My own observations lead me to believe that emergence from nymph to dun occurs in the surface film to several inches below it. However, given the schizophrenic behavior of many insects, I would not be at all surprised to find some streams where the duns emerge from nymphs on the stream bottom.

The prime time of year for hatches of western March browns is late winter and early spring. In the Pacific Northwest (Oregon, Washington, & Northern California) hatches begin as early as February and last until early May. In the Rocky Mountain area peak hatches generally occur in April and May. In both cases western March browns generally emerge before heavy spring runoff, and provide an excellent early season hatch with good dry fly action. Because of the cool, sometimes downright cold, weather at this time of year, hatches are concentrated during mid-day, typically between noon and three o’clock. Overcast, drizzly days often produce the best hatches and surface feeding activity, so don’t stay home just because the weather looks crummy.

Because R. morrisoni nymphs only enter the drift in good numbers just prior to and during emergence, I only imitate the nymphs in the early spring when hatches are close to starting or during a hatch when nymphs are rising to the surface. Typical nymphing techniques work fine, but you want to adjust the depth of the nymph depending on where the naturals are in the water column. Wet flies also work well for imitating duns that emerge underwater or get caught and drown in the surface film – a common problem on cold windy spring days. When duns become abundant on the surface there is almost always good surface feeding activity and great dry fly fishing. However, if you are not getting many rises switch back to a wet fly or nymph fished near the surface. Many fish keep feeding just below the surface even though duns are plentiful. Finally, unlike the eastern hatch of Stenonema vicarium or other Rhithrogena species, Rhithrogena morrisoni does not produce significant spinner falls or spinner fishing opportunities. Instead, they appear to mate and lay eggs sporadically during the day rather than forming dense mating swarms and concentrated spinner falls.

While the east and the west don’t exactly meet where March browns are concerned, there is enough overlap that our forbearers can perhaps be excused for applying similar common names to these distinctly different species. After all entomologists are still trying to describe and understand the complex array of aquatic insect species in North America. Why should anglers be any different?

Rick Hafele is a professional entomologist for the state of Oregon. Besides studying and writing about insects he regularly gives programs on understanding hatches and fly fishing tactics. His newest instructional efforts can be seen in a four volume series of videos called
Fly Fishing Large Western Rivers: Winter, Spring, Summer, and Fall.