March Brown Mayfly Hatch on the Deschutes River - Not!
On April 12th, I collected two mayflies on the Deschutes River that I’ve never seen before. That’s right, after 30 years of fishing the Deschutes, and collecting bugs on most of my fishing trips there, two new mayflies show up. You just never know when it comes to bugs!
It all started when Phil Rowley, the lake fishing guru from Edmonton, Alberta, flew into Oregon last week to give a presentation on lake fishing for the Santiam Fly Casters in Salem, Oregon. The next day Phil, myself and Dave Hughes headed over to the Deschutes River at Maupin. This was Phil’s first chance to fish the Deschutes and we were hoping the fish God’s would cooperate with calm weather and enough insects on the water to get some fish looking up, not that I mind using nymphs when needed. The fish God’s cooperated - sort of - plus the bug God’s threw us an interesting curve with a mayfly hatch I’d never seen before.
Day one was just about perfect: The air temp hit 60, light clouds hid the sun, and the infamous Deschutes wind decided to lay low. Like typical day-ones, by the time we drove to the river, stopped at John Smeraglio’s Deschutes Canyon Fly shop, decided where to fish first, and then got rigged up after sorting through a small mountain of fishing rods, vests, and waders, the sun seemed closer to the end of the day than the beginning.
We started out fishing nymphs. A kick sample in a nearby riffle found a rich and abundant supply of food for which the Deschutes is well known, so it didn’t take long to nearly cover the bottom of a white plastic tray and water with hundreds of nymphs. Salmonfly nymphs, from half-inch long babies to three-inch long monsters, lumbered among tiny blue-winged olive nymphs, PMD (pale morning dun) nymphs, net-spinning caddis larva, plus a few oversized green drake nymphs. A closer look revealed even more: yellow sally stones (Isoperla sp), pale evening dun mayflies (Heptagenia sp), Mother’s Day caddis larva (Brachycentrus), tiny midge larvae, aquatic moth larvae (Petrophila sp), and more, all from a couple square feet of stream bottom.
It’s hard to ignore the large salmonfly nymphs when picking a nymph pattern to tie on, but abundance often trumps size, so a caddis larva pattern along with a small (size 16 or 18) mayfly nymph looked like a great combination.
An hour of nymphing produced a few whitefish and one nice trout for Phil. Not exactly red-hot fishing, but not a bad start. All we needed was a good hatch of blue-winged olives to get those stubborn nymph feeding trout to look up. With that in mind we took off to search for quieter water where trout could be found taking little duns off the surface, assuming the little duns showed up.
Again the fish God’s smiled on us. As we walked down to the bank along a large eddy where surface food tends to get trapped, trout were already rising and we could see a dozen or more tiny BWO duns drifting calmly, seemingly unconcerned they were about to become a rainbow’s next meal. A switch in gear and fly patterns was in order, so it took a few minutes before we had our flies drifting down the feeding lanes to rising trout. The trout were a little fussy and the surface currents in the eddy constantly changed making a perfect drift the exception more than the rule, thus when a fish took it felt like it was the result of skill rather than luck, a nice feeling even if it may not be true.
Phil pointed them out first. A larger mayfly floated along next to the sized 18 BWO duns. We had hoped to see a hatch of March browns (Rhithrogena morrisoni), and it sure looked like that was what was on the water. Dave pulled out his trusty aquarium net and scooped several of the size 14 duns off the water for a close look. Surprise! March browns have mottled brown wings and the wings on these guys were a uniform slate gray. Were these some color variation of March brown, or some other species? The fish didn’t care, but it sure bugged the hell out of us, so I plopped a number of them into a vial of alcohol to take home for a closer look and hopefully confirm their identification.
The mystery solved - Almost
One of the problems when trying to identify mayflies to species is that one needs male spinners. That’s because it’s the male genitalia that most species keys use to separate closely related species. All of the specimens we collected in the eddy were duns, and nearly all females to boot. They could be identified to genus, but species identification would be a problem. One male spinner did show up, but not until the next day on a stretch of river miles downstream.
Day two was clear, cold and windy. I mean windy as in hang on to your hat kind of wind that doesn’t let up. And the wind made it possible to collect a male spinner. After fishing nymphs through a nice section of riffles, we went back to the car for a break. That’s when Dave spotted one lone male spinner hiding out of the wind on the back of the car’s lift gate. Plop into the vial he went, with the hope he could solve our mayfly mystery.
It took a close look at home under my microscope with mayfly keys at hand to find out what was what. The duns and spinner were both Rhithrogena, so I was hoping I could make the assumption that even though they were collected at different locations at different times they were sill the same mayfly. A big assumption that may not be true, but better than nothing? A look at the spinner,s genitalia showed clearly it wasn’t R. morrisoni. If not R. morrisoni, then what? Well, I’m stuck with two possibilities, either Rhithrogena undulata or Rhithrogena futilis. With just one male spinner to check, I can’t be sure, so right now the mystery is not completely solved. We can say however, that there’s another March brown like mayfly that produces some good hatch activity on the Deschutes River. After fishing the Deschutes for 30+ years, this is the first time I’ve run into this particular mayfly on the, and once again points out how fickle and fascinating nature can be.
But wait, we collected still another never before seen mayfly (for me at least) on the Deschutes. It was a just one specimen of a mature nymph, so of no consequence for fishing, but its unusual features caught my eye at once. First, it had a very long center tail (four or five times its body’s length), flanked by two very short tails. This quickly identifies it as belonging to the genus Caudatella of the family Ephemerellidae. Severals species of Caudatella call Western streams home. The most common is Caudatella hystrix, and even it is not typically abundant. When I keyed out the little nymph we collected on the Deschutes, however, it turned out to be Caudatella heterocaudata, a less common species. The photos attached here were taken through my microscope and aren’t the best shots. The nymph however, is quite small (only 6 mm long), so even a good macro lens doesn’t quite get close enough. While this little nymph has no fishing importance, it was another unique and interesting mayfly species we found in just one day. There’s always more out there than meets the eye.