Mayfly Species Stenonema vicarium (March Brown)

Where & when

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

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

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

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

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.