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Western March Browns

Like most common names, "Western March Brown" can refer to more than one taxon. They're previewed below, along with 3 specimens. For more detail click through to the scientific names.

Mayfly Species Rhithrogena morrisoni

These are pretty much always called Western March Browns.
Despite the reputation of Rhithrogena as being the most tenacious of clingers, Rhithrogena morrisoni or Western March Brown has the greatest populations in moderate runs over cobble. It can reach amazing densities in this water type on rivers like the Rogue in Oregon, Lower Yuba In California, and the Henry's Fork in Idaho.

These large flies range between 9mm and 12mm and are the first important hatch of the year. Generally speaking, they have two distinct color phases and hatching periods, with the Coastal states having the lighter version emerging late February through April in lower elevation tailwaters and the darker version in the Rocky mountain states emerging a month or two later. This has led to much confusion as these significant regional differences are seldom pointed out in angling literature.

They offer the western angler some of the finest fishing of the year during moderate precipitation cycles. It's biggest spoiler is high water or early runoff. On many rivers, there is also a fine hatch of a large late Winter stonefly (Skwala spc.) that usually precedes it that helps to get the fish "looking up". It is the West's equivalent of the East's mottle winged Macaffertium vicarium (American March Brown), both in terms of appearance and importance. Primarily due to western fishing seasons of the past that excluded this mayfly's activity, reporting on this species in angling literature is a relatively recent phenomenon. Thus, it lags in tradition compared to the blotchy winged Heptageniidae of the East and Midwest. But given time...
Rhithrogena morrisoni (Western March Brown) Mayfly NymphRhithrogena morrisoni (Western March Brown) Mayfly Nymph View 4 PicturesSize - (excluding tails) - 12 mm
Status at time of photo - preserved, but photographed within days of capture.

W. C. Day, Aquatic Insects of California differentiates morrisoni nymphs by submedian pale dots on tergites (
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
One tergite of this Isonychia bicolor mayfly spinner is highlighted in red.
Tergite: The top (dorsal) part of a single segment on an insect's abdomen when it consists of a single chitinous plate (sclerite), or an individual sclerite if the segment has more than one.
8 & 9, which look to be in evidence. This is one of the "super-hatches" on this river that can be prolific March through April.

Collected March 3, 2011 from in
Added to by on November 1, 2011
Male Rhithrogena morrisoni (Western March Brown) Mayfly AdultMale Rhithrogena morrisoni (Western March Brown) Mayfly Adult View 1 Pictures
Collected May 2, 2011 from in
Added to by on June 27, 2011

Mayfly Species Rhithrogena hageni

These are often called Western March Browns.
According to Fred Arbona in Mayflies, the Angler, and the Trout, this is an excellent hatch and one of the most common fast-water mayflies in the West.
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