CdC Biot Dun: The Easy Secret Tie for Finicky Trout

Benchside Sessions

Benchside Sessions is our fly tying series highlighting the patterns that we — as guides and shop staff — actually rely on day in and day out on the water. Every fly featured in this series is tied by someone on our team, chosen because it flat-out works. No fluff, no filler — just the flies we trust when it matters.

Materials

Watch the full tying tutorial on our YouTube channel:

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Step-by-Step Instructions

1. Place the hook in the vise and start the thread, covering the entire shank and stopping even with the hook point.
2. Tie in microfibetts — six for size 16, four for sizes 18–22, two for hooks 24 and smaller.
3. Divide microfibetts with your finger and secure by wrapping between them, creating two distinct tails.
4. Tie in the turkey biot tip-first.
5. Apply a fine layer of superglue to the hook shank.
6. Grasp the turkey biot with hackle pliers and wrap forward about ¾ of the way to the eye. Tie off and trim excess.
7. Tie in CDC with feather tips pointed toward the rear of the fly, making one wrap behind the wing to help prop it upright. Trim CDC butts.
8. Apply a very small amount of Superfine dry fly dubbing to the thread and make 2–3 wraps in front of and behind the CDC wing to build the thorax.
9. Whip finish and trim.

About the CDC Biot Dun

Last week, I took Renee over to the Crooked for Spring Break, with the goal of catching a prime blue-wing olive hatch so she understands why I always want to go there in March and October over almost any other location. She's fished the Crooked many times, but often during the colder winter months or during the spring high flows — both situations that depress hatches and create a dependency on various nymphing tactics if you want to stick some fish.

With low and exceptionally clear flows, we got our wish, hitting one of those classic Crooked River emergences where you don't move more than twenty feet up or downstream over the course of three or four hours. In the clear water, you could spot the fish as they approached the surface and retreated to the bottom, and with regular rise intervals of 5–8 seconds, you could actually pick out the better individual fish to focus on. Literally perfect conditions.

Almost any fly caught some fish. At the peak of the hatch, there were so many fish coming up and so many naturals on the water that Renee turned to a #12 Copper Wulff as a "sighter" — and then when the BWO dropper broke off, she kept catching fish with just the big fly to much success.

I started with my go-to for a Baetis emergence, a CDC Biot Dun. A super simple fly with just a handful of materials — tail, turkey biot, wing, and a tiny amount of dubbing for the thorax — I love this pattern on the Crooked because it works just as well in the barely-there currents of the frogwater as it does in the swirly chutes between the river's innumerable boulders. Despite its neutral color, the puffy wing is highly visible from a variety of directions, matching the little sailboat wings of the natural without a garish blast of white or hot orange.

The CDC Biot Dun works well for any mayfly smaller than size 16. Besides your BWOs, it is suited for Pale Morning Duns, Pale Evening Duns, Mahogany Duns, Sulphers, among others. The virtue of buoyancy is a given — if properly tied, the CDC will float your fly through the roughest water. That is, until you catch two or three fish. After that, the material can become slimy and matted, negatively impacting its floatability. Powdered desiccant can revive the pattern, but only for another fish or two. If you're in a hot and heavy hatch situation like we were this week, make sure your improved clinch knot skills are on point because you'll be subbing in a lot of flies. The ease of tying the CDC Biot Dun makes it easy to have an extended bench to call upon. Fortunately, once this fly completely dries, it fishes like new again.

While a CDC Biot Dun is simple and relatively easy to tie, it offers a case study in choosing materials. Four factors come to mind when deciding how to construct an effective version: the hook, the tails, the body, and the wing.

For the hook, I always opt for a straight-eye hook for dry flies size 18 and smaller. As the hook size decreases, the wire gauge used to construct it also shrinks — but not in a linear relationship. A down-eye on a smaller hook consumes a greater proportion of the hook gap, which gives you less "bite" when you set the hook. The straight eye buys you a precious millimeter that can be the difference between bringing trout to hand rather than stinging the lips of dozens of missed hooksets.

The tail seems like a no-brainer to me, as synthetic microfibetts have dominated the American game when it comes to tailing spinners and flat-water duns. However, in the rest of the fly fishing world, microfibetts are used only sporadically, and the traditional use of stiff cock hackles — especially Coq de Leon — predominates. While Coq de Leon fibers create a beautiful finished product, given the price differential, durability, and floatability of the microfibetts, they are usually my preferred option.

The body for most any mayfly dun or spinner can be constructed with dubbing or turkey biots. Blue Winged Olives exist in a notoriously broad spectrum of colors, from gray to brownish olive to bright, almost-chartreuse green. I have a million shades of dubbing and can only purchase a limited range of biot colors, so I tie most of my patterns with biot and fill the color gaps with dubbed versions. Both are relatively inexpensive materials, and when properly tied — with a thin layer of glue under a biot body — they are quite durable.

When you come into the shop, there are two types of CDC on the racks: normal 1–2" long CDC feathers and CDC "Oiler Puffs." The CDC feathers have multifarious applications — they can be tied in with tips exposed for wings (like with this pattern), wrapped as hackle, or the fibers stripped and tied in for clipped wings (especially spent wings). The Oiler Puffs, well, make wings. Pretty, puffy wings, but they are a one-trick pony. Like Alton Brown recommended in his early cookbooks, unless you're a professional with a specific need you'll use all the time, opt for tools with multiple functions. Oiler Puffs are the watermelon cubing knife of the tying world.

The nice thing about this style of fly is that the wings are almost always best matched with natural dun CDC, though lighter or darker-winged versions can prove useful in terms of their visibility on the water. Simply tie up a half-dozen in about 3–4 shades and three sizes, and you're set for one of the most reliable hatches you'll find on almost any quality trout stream in the country.

About the Fly Tyer: Jeff Morgan

Jeff Morgan, aka "Professor" (reference from A River Runs Through It), is yet another example of that. Jeff, as you will find out, has been passionate about fly fishing, as you can see in the black-and-white photo above, well before color photography.