Bighorn River Pale Morning Duns

It’s that time of year again in southern Montana, a time when the flowers bloom and the birds sing, a time when little whitetail fawns leap and cavort in grassy meadows, a time when gentle breezes cause the Cottonwood trees to release their fluffy little seed pods which settle upon the surface of the water and entangle themselves in your leader or fly,

causing you to use language that would make a hard-rock miner blush, a time when ravenous mosquitoes and horse flies descend upon anglers in Amazonian numbers, sucking the lifeblood out of those who have not doused their skin in cancer-causing chemicals, a time when thunder storms develop in the afternoon and bring torrential rains and softball sized hailstones that destroy the roofs of homes and obliterate crops, and … where was I going with all this? Oh yeah, this is also the time of year when Pale Morning Dun mayflies hatch.

Pale Morning Dun is the common name for a species of mayfly. These graceful, light yellow to light olive mayflies bring hungry trout to the surface on many rivers in the northern Rockies. Many anglers have a fondness for Pale Morning Duns because they are larger than those insipid little Baetis mayflies that hatch earlier in the year. Since they are larger, people believe they can see PMD imitations easily. This is not true, however, at least not for many of the people I fish with. I mean, let’s think this through: if you can’t see a size 20 fly, and you also can’t see a size 16 fly, what difference does it really make if one is larger than the other? But I digress.

Anyway, PMDs are a fairly sizeable mayfly, and some people can see them quite well. On the Bighorn River, our PMDs are of the species Ephemerella Infrequens. Actually, this is no longer true. Once upon a time, the mayflies out west that we call PMDs included two species, Ephemerella Inermis and Ephemerella Infrequens, with the latter being a slightly larger insect. Now the more common PMD, Inermis, has been renamed Ephemerella Excrucians, while the Infrequens has been reclassified as a subspecies of Ephemerella Dorothea, the eastern Sulphur. So the Bighorn River PMDs, with the official name being Ephemerella Dorothea Infrequens, are closely related to the eastern Pale Evening Dun. Maybe we should call them Pale Midday Duns. Anyway, I’m not sure of what practical value the preceding information has, but if you encounter a “bug nerd” who wants scientific information, throw all this at him.

PMDs inhabit a variety of habitats in North America, and there is some difference in color and size depending on what river or lake they live in. Ephemerella duns have three tails—that’s a tell-tale sign (I’m so clever) that these insects are not to be confused with Baetis or Calibaetis. PMD emergences usually begin mid to late morning, but depending on the water temperature and location on the river, they can come off in mid-afternoon. PMD nymphs are crawlers, which means that they crawl. Actually, there are four different groups of mayfly nymphs: clingers, burrowers, swimmers and crawlers. Baetis are swimmers, by the way.

PMDs hatch during the months of July and August on the Bighorn, although in low-water years we’ll see them in early September on the upper 3 miles. They’re a delightful hatch in the sense that the duns pop to the surface and march downstream looking like beautiful little yellow sailboats. Or so it seems. In reality, there can be a dark side to the PMD hatch. Almost everybody wants to fish high-floating dun imitations, and they envision big trout confidently sipping these duns. Of course, everybody wants to go to heaven without dying too. But in warm weather emerging PMD duns spend little time on the water’s surface, and the trout tend to key on stillborn or crippled insects. This can be challenging to anglers because trout often ignore their dun patterns, and they are forced into fishing less-visible, transition patterns. To make matters worse, on heavily fished rivers the fish can become very selective during the course of a season.

What do we do about this? There are several patterns you’ll want to fish. My favorite is the PMD Student, Frank Johnson’s soft-hackle, CDC pattern that imitates an insect struggling in the surface film, trying to unfurl it wings. It also looks like a still-born or tipped-over dun. I usually fish this pattern behind a Parachute PMD or other visible pattern. The Student works very well because the fish are looking for defenseless insects in much the same way attorneys search for industrial accident victims. There are a variety of emerger patterns that work, but the Student is at the top of the list.

Another strategy for selective trout is to fish a regular PMD nymph pattern behind your indicator fly. The TS PMD Nymph in size 16 (TS stands for “Trout Shop”) is very effective, as is the Split Case PMD Nymph in the same size. I like to drop these about 20 inches below my visibility fly. Some fish are looking for the darker silhouette of the nymph, as the dun struggles to emerge. PMD nymphs tend to be brown to chocolate brown in color.

As is the case with selective fish everywhere, presentation is paramount during any hatch. There is no magic fly as much as there is the magic cast. And not just one cast, but being able to consistently place the fly over rising fish again and again is the key to success. I won’t even get into the techniques involved in avoiding drag, using down-and-across presentations, or being extra careful in picking your fly off the water, but these practices separate the productive angler from the mediocre. As I’ve learned, and mentioned before in other blogs, telling clients that they are a “crappy caster” is not conducive to receiving large tips. We professionals have to be careful in what we say and how we say it. Sometimes we simply have to take the blame for a lack of fish production. This is emotionally traumatic and we recently hired a psychiatrist to join our staff here at the Bighorn Trout Shop. Yet another dimension of guide mental anguish is due to the fact that even an expert angler who can “get the job done” as far as presentation is concerned, will oftentimes have to work very hard to catch fish. By their very nature, trout feeding on PMDs tend to be tough, so be prepared to make a lot of casts per fish.

There are exceptions to this picky-fish rule, however. If you’re in riffle water in the middle of a heavy emergence, there are times the fish can be relatively easy. You might even begin to think you are some sort of guru who should have his own show on the Outdoor Channel. Then you move downriver a quarter mile to a glassy flat, and reality sets in. So the type of water you choose can be important. By the way, another fly I’ve had good success with on a variety of water types and hatch situations is the Orange Parachute. Actually, it’s kind of a yellowish, orange color with cream or light ginger hackle and tail. I often fish it with the parachute post trimmed down. I’m not sure why the fish like it so much, but the darker profile might suggest a nymph or spinner to the fish. Regardless, it will often out fish traditional dun patterns that are dead-on matches for the insect.

Don’t ignore PMD spinners. They can be a big deal at times. After the duns hatch, they flutter off into the trees and brush to hang out and to enjoy life above water, kind of like teenagers at a beach party. I hope they make the most of their time because they only have a day or two to live. Duns (subimagos) quickly molt into the sexually mature spinners (imagos). PMD spinners, like most mayfly spinners, have almost transparent wings. Also, their color tends to be a shade darker than the duns with a tinge of rust thrown in. I would describe the color of Bighorn PMD spinners as light rusty ginger to rusty-cinnamon olive.

Spinners mate in midair, and then fall lifeless to the water. Their wings are outstretched, and trout become very selective to this spent-wing profile. Because they lay flat against the water, spinners are hard to see and anglers will sometimes scratch their heads as they observe rising fish, but there is no apparent insect hatch. Looking closely at the water’s surface solves the mystery. I’ve had the best spinner fishing in the morning, but depending on weather conditions, you’ll encounter spinner falls in the afternoon and evening as well. A number of patterns can work. Some anglers swear by hen-winged patterns as they offer a crisper silhouette. Others prefer flies with sparkle-poly wings, as they imitate the sparkle and sheen of the natural’s wings quite well. When fishing spinners, don’t be afraid to cheat a little bit and fish a visibility fly above your spinner. I’m fond of fishing a black CDC Caddis pattern, as the dark fly is usually quite visible, and caddis are often on the water during a spinner fall. Some fish are willing to take either fly.

Water flows have come down during the past two weeks, and conditions should be perfect the next several weeks. We’re already seeing PMDs on the water, and we’re hoping it develops into a good hatch. If you’re lucky enough to hit it right, the Bighorn River PMD hatch can provide dry fly fishing you won’t soon forget.