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Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Mayfly Species Stenonema vicarium (March Brown)

In the East and Midwest this is one of the most important hatches of the Spring. They are large flies which emerge sporadically, making for long days of good fishing.

This species contains the two classic Eastern hatches formerly known as Stenonema vicarium and Stenonema fuscum, the "March Brown" and "Gray Fox." Entomologists have discovered that these mayflies belong to the same species, but they still display differences in appearance which the trout notice easily. Anglers should be prepared to imitate both types.

Where & when

Time of year : Mid-May through early July

This hatch begins in Pennsylvania and the Catskills in the middle of May. It peaks there in late May and early June. Throughout the rest of June and early July the hatch moves into the Adirondacks, New England, and the Upper Midwest.

The best action lasts from 1-3 weeks in most places. The traditional March Brown duns originally in the vicarium species are more common toward the beginning of the hatch, while the former-fuscom Gray Fox variety take over in the latter weeks.

Hatching behavior

Time of day : Late morning through evening, usually peaking mid-afternoon; only late evening on hot days

Water temperature: 55°F

The nymphs are reported to scurry from their fast-water habitat into slow pools and marginal water before emergence.

March Browns take a long time (sometimes more than 30 seconds) to escape their shucks in the surface film, and then they ride the water for a long time and struggle through failed attempts to take flight. This hatch is exceptionally prone to producing cripples and stillborns.

These characteristics leave room for many types of flies: floating nymphs, emergers, low-riding duns, hackled duns, and cripple patterns all have their place.

Spinner behavior

Time of day: Late evening through dusk

March Browns return to the stream as spinners a few days after emerging. Spinners from many days of hatching typically mate together on select nights, and this makes for inconsistent but exceptionally good spinner falls.

In Hatches II, Caucci and Nastasi write that the females oviposit by making repeated runs at the surface, dipping their abdomens briefly and rising up for another run. This contradicts the behaviors listed in Mayflies of Michigan Trout Streams, in which the Leonards say the females may drop their eggs from the air or release them as they fall spent. Readers are invited to share their experiences in the comments and clarify the spinner behavior.

Nymph biology

Current speed: All speeds, but best in fast water

Substrate: Everything from boulders to weeds to leaf drift to silt

Environmental tolerance: Very tolerant of high temperature and slow water

In Mayflies, the Angler, and the Trout, Fred Arbona reports stomach samples indicating the importance of these nymphs.

Trout appear to be aware of this phenomenon, for their stomachs will often be crammed with as many as 100 to 200 immature nymphs a week before the hatches will begin for the season.

Stenonema vicarium Fly Fishing Tips

This hatch often occurs simulatenously with more concentrated species, and fish may be selective to those. Although March Browns provide good action throughout much of the day, you should not let them distract you from higher-density hatches of smaller flies like Ephemerella invaria which the fish might take selectively instead.

Specimens of the Mayfly Species Stenonema vicarium

2 Male Duns
1 Female Dun
Female Stenonema vicarium (March Brown) Mayfly Dun
Here's a classic March Brown dun from the Catskills.
2 Female Spinners
4 Male Spinners
11 Nymphs

Recent Discussions of Stenonema vicarium

2 tails or 3
1 replies
Posted by Snagy on Feb 5, 2010
Last reply on Feb 6, 2010 by Taxon
I notice that while the dun, nymph, and spinner photos on the page are all listed as March Brown (maccaffertium vicarium). While the coloration patterns all seem to follow other March Browns I have seen, I notice that the winged flies pictures all have 2 tails, but the nymphs in the photos have 3. I was under the impression this species was a 2 tailed mayfly. Is the nymph mislabelled?
diff between march brown &gray fox
7 replies
Posted by Jrcald on Mar 12, 2007
Last reply on Mar 13, 2007 by Troutnut
i know that gray foxs and march browns are classified in the same family but what are the differences ? what is the color differences are both bodies the same color tan ?

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