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Mayfly Species Rhithrogena morrisoni (Western March Brown)

Pictures Below
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...

Where & When

Region: West

Time Of Year (?): Late February to mid-June

Preferred Waters: All types of fast water but best in cobble runs of medium depth fast to moderate flow

Hatching Behavior

Time Of Day (?): Late morning on warm days; mid-afternoon on cool days

The duns leave their nymphal shucks (
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Here's an underwater view of the pupal shucks of several already-emerged Brachycentrus numerosus caddisflies.
Shuck: The shed exoskeleton left over when an insect molts into its next stage or instar. Most often it describes the last nymphal or pupal skin exited during emergence into a winged adult.
on the bottom making them the ideal candidate to immitate with subsurface emergers either dead drift or on a controlled swing. With the right conditions, they ride the surface for a long time to dry their wings, often making dun imitations ideal. Regardless, fine dry fly action can usually be found along the margins or in back eddies where large trout often like to leisurely sip their supper.

Spinner Behavior

Time Of Day: Same as emergence

Spinner falls may sometimes be important.

Nymph Biology

Current Speed: All current speeds

Substrate: Gravel

This species is more tolerant of slow water than most in Rhithrogena, but it also does well in the very fast water typical of the genus.

Pictures of 4 Mayfly Specimens in the Species Rhithrogena morrisoni:

Specimen Page:12
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 the Lower Yuba River in California
Added to by Entoman on November 1, 2011
Specimen Page:12

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