Western March Brown

Rithrogena morrisoni.

Henry's Fork Season:                April 15^th - June 15th

Madison (YNP)                         July -August              

Yellowstone River                      July 15^th - September 5^th

Habitat:                                    Require clean, cold water. The nymphs are clingers and spend most of their nymphal stage living in riffles and runs. At maturity they often move out of the faster waters into more moderate flows where they will emerge.
Western March Browns hatch during the warmest part of the day during March and April. Look for them from mid-morning to late afternoon.

Nymphs:                                   #14-#16 - olive/brown body, with a head wider than the abdomen.

                                                Three tails with large overlapping gills under the abdomen.

                                                Gold-ribbed Hare's Ear

Emergers:                                 Emergence in underwater

                                                Cripples not common

                                                Brown CDC Nymph/Emerger.

                                                March Brown Soft-hackle

Duns:                                       #14-#16 - Two tails, mottled-brown/olive color (tan underneath) body

                                                March Brown Sparkle Dun

                                                March Brown Comparadun

                                                Hairwing March Brown

Spinners:                                  #14-#16 - Two tails, Clear wings, brown body     

Time:                                        Duns - Mid-morning to mid afternoon emergence

                                                Spinners - Calm, warm evenings

Strategy:                                   On warm sunny days, the duns normally emerge closer to the riffles and margins where the nymphs were holding.

                                                On colder days, fish holding water further down the pools.

WESTERN MARCH BROWNS are the first large mayflies to emerge in the spring. For many Western anglers, this hatch signals the start of a new season. In rivers with good populations, these mayflies often produce some of the best dry-fly fishing of the season, once trout have keyed to feeding on the emerging duns.
This mayfly nymph has large gill plates along the lower exterior edges of its abdomen that form a suction cup, enabling it to cling to most objects. This holding ability, along with its flattened body shape, allows the nymph to move around with ease on rocks or woody debris even in the fastest flows of a stream. For this reason, nymphs are seldom available for trout to feed on until just before emergence.
About a week before emergence, however, the nymphs move out of the faster currents to more moderate flows, where they wait until they are ready to emerge. This is the prime time to fish a March Brown nymph pattern. Check rocks in moderate flows for clinging nymphs with dark wing pads. The dark wing pads indicate that the wings are fully developed and the nymph is close to emergence. During the migration from faster to moderate water flows, many nymphs are swept from the bottom and end up in the drift. The nymphs are poor swimmers and when set adrift, they often travel long distances before settling to the bottom. To imitate this behavior, fish your nymph pattern drag-free and close to the bottom downstream from riffles or fast runs.
The dun emergence can occur any time from mid-morning to mid-afternoon. At the start of a hatch, when only a few duns are on the water, and no fish are rising, a March Brown nymph or soft-hackle pattern fished along the margins of faster waters or below riffles will catch trout that are feeding on the drifting nymphs. Fish the sunken fly close to the bottom, and at the end of the drift, let it swing up to the surface to imitate an emerging natural.

 As more duns appear on the surface, the trout will start feeding on them. This is the time to switch to a surface pattern. Western March Brown duns often need a little extra time to shed their nymphal shucks, especially during cooler weather. For this reason, emerger patterns are often more effective than the traditional dry flies during the hatch.
The weather often effects where you see trout rise. Trout gather where the greatest numbers of emerging duns are located. On warm sunny days, the duns normally emerge closer to the riffles and margins where the nymphs were holding. On colder spring days, the duns take more time to emerge from their nymphal shucks, and the fish will feed on these helpless emergers in the easier holding water further down the pools.
Western March Brown spinners normally do not offer a fishable spinnerfall; the females lay their eggs sporadically from midday into the evening. If you find fish feeding on the spinners, one of the dun patterns is a close enough match of the natural to fool most trout.