Wisconsin Fly Fishing March Brown Mayfly Hatch

March Brown Hatch
Maccaffertium vicarium (originally called Stenonema vicarium)

March Brown mayfly hatch is used to describe Maccaffertium vicarium mayfly and it's smaller relative the Gray Fox. Actually, the Gray Fox (Stenonema fuscum) used to be considered a different species than Stenonema vicarium, but in 1979 the entomoligists combined the smaller Stenonema fuscum with Stenonema vicarium. Then later on they changed the Genus from Stenonema to Maccaffertium. Many books (like Hatches) reference Stenonema vicarium which is now called Maccaffertium vicarium.

This is one of the great hatches found on many northern Wisconsin freestone streams and rivers. Look among the rocks in the riffles and faster runs for the flat, clinging nymphs. The mature nymphs will be about 15 - 20 mm long. The hatch generally occurs near the end of May through mid-June depending what stream you are on. Northwest Wisconsin generally sees the March Brown (Maccaffertium vicarium) mayfly hatch in early to late June. The nymphs are known to migrate towards the shallows just prior to hatching, making the the larger Maccaffertium vicarium duns less vulnerable to the trout. The duns are usually sporatically hatch all day along with the more dense hatching Emphemerella invaria (Sulphurs). In the cool June evenings the concentrated March Brown spinners may give you the best chance of finding some larger feeding trout.

Fly fisherman should carry a selection of size #10 & #8 nymph, dun and spinner patterns to match the March Brown hatch. The clinging nymphs will be migrating towards the shallows prior to emergence and the trout may follow. At theses times I have spooked my share of larger trout in the shallow riffled waters probably looking for the large clinging nymphs. Look for the March Brown spinners fly ing around near dusk over the riffles and faster water sections.