ramblings of a catskill fly fisher

The March Brown and Grey Fox Mayflies: confusion and mystery

By TONY BONAVIST
Posted 5/20/20

Soon, if not already—stream flows permitting—Catskill anglers well be treated to hatches of March Brown and Grey Fox mayflies. Both flies are large with the March Brown being the larger …

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ramblings of a catskill fly fisher

The March Brown and Grey Fox Mayflies: confusion and mystery

Posted

Soon, if not already—stream flows permitting—Catskill anglers well be treated to hatches of March Brown and Grey Fox mayflies. Both flies are large with the March Brown being the larger at roughly a size 10; the Grey Fox is a bit smaller. Emergence begins about 11 a.m with both flies hatching concurrently and sporadically throughout the day. These flies do not hatch in large numbers like Hendricksons; instead, they emerge over a period of several hours. Other than Green and Brown Drakes, March Browns are the largest mayflies to appear on Catskill rivers.

Up until recently, both flies were classified in the genus Stenonema (March Brown, S. vicarium and Grey Fox, S. fuscum). Then at some point around 1981, W. Patrick McCafferty, professor of entomology at Purdue University, reclassified several members of the Stenonema group of mayflies into a new genus he named Maccaffertium. Not only that, but he also consolidated March Browns and Grey Foxes into one species, Maccaffertium vicarium. He also changed the name of the Light Cahill from Stenonema canadenses to Maccaffertium ithaca! So, from a taxonomic standpoint, the old names are out and the new are in, perhaps a tribute to McCafferty’s expertise as an entomologist.

What does this all mean from a fly fishing standpoint? Not a thing. Taxonomists fool with scientific names of species all the time and mayflies are no exceptions. And I know that, despite McCafferty’s work, I’ll consider March Browns and Grey Foxes different flies of different species. From a fishing standpoint, the hatch or hatches overlap. So if there are more Grey Foxes on the water, you will need a size 12 dry fly. However, the smaller Grey Fox may be used if March Browns are also on the water. Call them what you will, the duns are different enough to require different sized flies.

Fishing the March Brown/Grey Fox hatch can provide some interesting if difficult challenges, particularly the spinner falls. During emergence, the nymphs take several seconds to leave their shucks, floating along for some distance before hatching is complete. Anglers fishing a soft hackle of wet flies just under the surface may enjoy excellent results using that method. Once on the surface, duns are cumbersome and remain on the water for some time before their wings dry and flight is possible. When trout are actually on the duns, dry-fly fishing can go very well and extend from late morning well into the afternoon.

Then there are the spinner falls or, more accurately, the lack thereof. All mayflies go through three stages of development before reaching adulthood: nymph, dun (subimago), spinner (imago). Of the three stages, I enjoy fishing the spinner falls the most. Hatching is over, so there is no subsurface feeding to be concerned with. March Brown/Grey Fox spinner falls occur at dusk, so shy, large trout are on the prowl. Spinner falls last a relatively short time, sometimes less than half an hour, so feeding may be concentrated and almost frantic. Fishing during spinner falls is the best time to catch really large trout pretty easily on the dry fly.

There are problems with March Brown/Grey Fox spinner falls that do not occur with other species of mayflies. March Brown/Grey Fox spinner falls don’t always take place, even when large numbers of mating adult flies are flitting about overhead, high above the riffles. For some reason, and I’ve seen it more times than not, the spinners just disappear to reappear the next night or another night. This phenomenon is so common with these two mayflies that it is referenced throughout the literature. I find it extremely frustrating, having waited and watched the mating spinners for up to an hour only to have them fly off into the night, leaving a hapless fisherman with just the nightbirds, bats and no rising trout. If upon chance you happen to hit it right and the spinners fall, be prepared for some exceptional fishing opportunity. I’ve had it a few times over the years and, more often than not, a few 20-inch trout were brought to net. Just be patient and hope the spinners fall the night you are on the river.

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