Large Brook Dun

by Richard Fieldhouse May 7, 2021

The Large Brook Dun (Ecdyonurus torrentis) is a member of the Heptageniidae family. The Heptageniidae are a family of mayflies with over 500 described species. These flies are generally rather small with two long tails. The wings usually have a dark marbling and are sometimes banded with lighter patches. We will learn more about the Heptageniidae family through the season, as they make up the majority of flies the angler is predominantly interested in. The Large Brook Dun is a distinctive mayfly with a brown body and wings which appear slightly marbled brown and white.

Where To Find Them

Large Brook Duns are found in the more upland parts of the UK. The nymphs like rocky, boulder-strewn rivers. Their preferred habitats are acidic rivers flowing over bedrock and boulders because their nymphs live under stones, not in burrows like their more southerly cousins.

As with most of the up-winged family of flies, the Large Brook Dun is only found in running water.

When To Find Them

The Large Brook Dun hatch usually starts in the first two weeks of April and will continue until the end of June.

During these months the hatches usually occur around midday and can last for up to three hours, with major bursts of activity only lasting around 15 to 30 minutes minutes.

What To Look For

Large Brook Duns (LBD) are generally pretty easy to spot (especially in April - as they are generally they are the only up-winged fly you will encounter on the river - they are very similar to the March Brown - as far as we are concerned the imitations are the same). The key distinguishing feature of the LDB is the dark marbling of the wings. Remember these flies only have 2 tails.

Hatches of LBD's can be very localised, one river may be swarming with them - and another close by, be completely devoid of them.

The Large Brook Dun adult does not hatch through the water column (it climbs up stones and backside vegetation), so the emerger is not applicable to the fly angler.

Lifecycle

The life cycle of the Large Brook Dun is exactly the same as the Stonefly (below). There is usually a single generation per year, with the transformation from egg to Dun taking exactly a year.

Large Brook Dun nymphs are small, thin and can be found clinging to stones (one of the many 'stone clinging nymphs') and vegetation around the riverbed where the trout can pick them off, sometimes they get washed off their stones and the trout hoover them up. The best imitation for these is the famous Sawyers Pheasant Tail Nymph (beaded when used in heavier flows). When imitating the LBD nymph, please take note of the body thickness (most commercially bought flies have far too much material in the body), the body should be very thin. LBD nymphs are roughly 12 to 15mm long and only 3 or 4mm wide (just about the same as a size 14 Pheasant Tail Nymph):

Once the nymph decides the time is right to hatch into a Dun (usually around mid-day) the nymph climbs up a rock or bankside vegetation and transforms into the dun (as shown in the stunning photo below - Thanks Dave).

It takes a few moments for its wings to unfold and dry out (it is at this point where it is able to fly and find its mate).

Once mated, the female LBD flies upstream and descends to the surface of the water to release a few eggs by dipping the tip of her abdomen on to the surface at intervals, or by actually settling on the surface for a short period. After several visits to the water, her egg supply of up to 8000 eggs is finished and the spent female falls on to the surface, ready to be eaten by the waiting fish.

From a fly fishing perspective the stages which we must try to imitate are:

Adult/Spent Females; and
Nymphs

Imitations

The most popular imitations for adult Large Brook Duns are exactly the same as you would use to imitate March Browns, don't worry if you don't have these exact patterns, as anything which is in the general colour, profile and size will do. These flies should be fished as you would any other dry fly, the March Brown Upright being especially buoyant. These dry flies should be used as soon as you start to see the hatch appear, this can be anytime from 12:00 until 15:00, depending upon weather conditions.

Adult Females

March Brown Upright

March Brown Jingler

Nymphs

The LBD nymph is generally imitated by the ever-popular Pheasant Tail Nymph (in all its different forms), we especially find the Red-Necked version to be very successful on the tea-stained river of Northern England. Take a look at these for some inspiration (Red-Neck PTN & Copper Head Mary PTN):

Image Sources

Large Brook Dun images: Courtesy of Dave Southall (thanks Dave, you're a star).

Flies

This email is brought to you with the sole intent to spread the information around so we can all maybe learn something. If you would like to buy any of the flies contained within this email, we do hope that you will consider us and see what flies we have to offer. If you would like any further details on any of the above flies, they can be found using these links:

Emerger:
March Brown Upright
March Brown Jingler

Nymphs:
Red-Neck Pheasant Tail Nymph
Pheasant Tail Mary

Previous issues of Hatch Chat can be found on our Hatch Chat blog, here: https://www.barbless-flies.co.uk/blogs/hatch-chat