December 07, 2017
By Matthew Green
Every year in late March and early April, Theo Copeland and I search the lower reaches of the Tailwater in Tennessee, looking for evidence of the Brachycentrus caddis emergence. We're often alone in our habitual evening search in a section of river nicknamed The Country Mile, which extends from the CSX railroad bridge near the town of Watauga to the river mouth at Boone Reservoir.
The river here is home to a wild rainbow trout population reminiscent of the Missouri River below Holter Dam in Craig, Montana with some of the best dry-fly fishing east of the Mississippi.
The caddis hatches on the Watauga are more intense than most other Southeastern tailwaters, and the event draws fly fishers from all over the East Coast. The Watauga's neighbor river, the South Holston, is one of the best trout streams in the region, but it doesn't have a Brachycentrus population. These two nearby tailwaters have different aquatic insect communities that are driven by minute changes in annual temperature range, discharge, and flow rate, with the Watauga being the more diverse of the two.
Beginning as the outflow of Wilbur Reservoir near the small town of Siam, Tennessee, the 16-mile Watauga tailwater meanders through the city of Elizabethton before reaching Boone Reservoir downstream. Like its neighbor, the South Holston, the Watauga runs over dolomite ridges and uplands (Knox-dolomite formation) of the lower Great Valley and is influenced by many dolomite springs, which increase the river's alkalinity and pH level. The resulting increase in alkalinity produces abundant plant growth and increased aquatic insect biomass.
The flow in the tailwater varies from base flow of 220 cubic feet per second (cfs) to power generation flows of 1,500 to 3,000 cfs. The Watauga has a higher average stream gradient than the South Holston, which creates a much greater flow diversity. The increased dam discharge relative to the South Holston also keeps the entire Watauga colder on average than the South Holston.
These two variables increased discharge and lower annual temperatures produce one of the best springtime hatches in the country, and fertile feeding grounds for wild tailwater trout.
There are many important Brachycentrus species in the United States. In the West, Brachycentrus americanus (American Grannom) and B. occidentalis (Western Mother's Day Caddis) are of the greatest interest to fly fishers and are incredibly prolific. Their early May emergence prior to or during runoff also typifies their common name, with the emergence of these caddisflies occurring close to Mother's Day.
In the East, fly fishers should be mostly concerned with B. lateralis (Striped Grannom), B. spinae (Smoky Mountain Grannom), B. numerosus (Dark Black Grannom), and B. appalachia (Apple Grannom Caddis or Eastern Mother's Day Caddis), all of which are regionally referred to as the Mother's Day Caddis, grannom caddis, or in Pennsylvania and the Catskill Mountains of New York, shadflies.
The heads of Brachycentrus caddisflies range from dark brown with pale orange, brown, and yellow vertical stripes to solid black with brown sclerotized plates directly behind the head with light olive breaks in between. For most Brachycentrus, late instar larvae are light olive, with black legs, white epithelial gills, and they live in a 16 mm long case.
There are three different pupal stages, each with a different morphology and coloration. Stage 1 pupae are similar to late instar larvae, but are slightly fatter. Stage 2 pupae are light olive, with clear wings, black dorsal and ventral medial spots, and light brown, transparent legs. During the final pupal stage (Stage 3), pupae develop more pronounced adult coloration, including darkening of the body, wing pads, legs, and head, and the development of a medial olive abdominal stripe. Stage 2 and Stage 3 pupae average 12 to 14 mm in length.
Adult Brachycentrus have mottled dark gray, brown, or light black wings with light tan and brown medial and apical markings. In bright light, the wings of females appear gray.
Both adults have black or dark brown bodies with a pale green olive stripe that runs along the center of the abdomen. Females (14 mm) are longer than males (12 mm) and normally have a large, ovoid 5 mm to 6 mm light olive egg sac under their wings that fits snug against the terminus of their abdomen. Male abdomens are not truncated to accommodate an egg sac, and are tapered evenly, ending in prominent genitalia.
Eastern Grannoms range from northern Georgia to Maine and Nova Scotia, and the larvae build rectangular cases using fine plant tissue and detritus as building material. As the larva grows, it adds new layers of plant tissue to the case using fine silk. These caddisflies most often attach their cases to cobbles in riffle habitats but occasionally live in slower water attached to woody debris.
I have watched Brachycentrus caddis suspend themselves outside their cases with a fine piece of silk in order to collect and eat fine particulates and algae, but mostly they use fine, stiff hairs on their legs called setae to rake in food that floats by in the current. The latter is actually quite important for fly fishers who spend considerable time on the Watauga.
Predicting the Brachycentrus Emergence
For about two weeks prior to the Brachycentrus caddis emergence, dry-fly fishing on the Watauga is almost nonexistent. After two important spring mayfly hatches (Baetis sp. and Acentrella/Plauditus sp.) the trout focus almost exclusively on subsurface feeding and caddisfly larvae, cases, pupae, and various mayfly nymphs and midges. Many fly fishers fail to capitalize on this opportunity to use caddis larval and cased patterns, which should be fished in fast riffles close to the bottom to imitate the suspending behavior of larvae.
One of the most challenging and rewarding moments in fly fishing is predicting a hatch. With Brachycentrus, this prediction helps you beat the crowds, improve your catch rate (at the beginning of the emergence fish tend to be easier to catch), and experience the greatest intensity of insects.
There are a number of different biological indicators I use to predict the emergence accurately on the Watauga, and many of these indicators will work for the Brachycentrus hatch on your home rivers. These caddis hatch "bio-indicators" are diverse and include caddisflies that emerge prior to Brachycentrus "the pre-caddis fauna," larval pupation timing, dissecting and studying pupae, and coastal river shad spawning runs.
The pre-Brachycentrus fauna that I use to predict the hatch are fairly reliable from year to year. Glossosoma nigrior and Micrasema sp. (Tiny Black Caddisflies), Hydropsyche sp. (Spotted Sedges), Cheumatopsyche sp. (Little Sister Sedges), and a small Ephemerella invaria (true Eastern Sulphur) hatch at the end of March, and precede Brachycentrus by about two or three weeks.
Hendricksons (Ephemerella subvaria), are also an excellent predictor of potential grannom emergence on most Northeast rivers, but they are not abundant on the Watauga and are easily overlooked. On the nearby Holston River, a spring emergence of Tiny Black Caddisflies typically occurs two weeks before the Brachycentrus on the Watauga, so there are regional indicators as well.
Fly fishers can also predict the timing of the caddis emergence by watching the progression of larval pupation in the river. This happens when cased larvae seal off their case with a thick layer of silk. Typically, pupation lasts for approximately two to three weeks depending on water temperature (warm water temperatures lead to faster pupal development) and is characterized by the three different morphological stages discussed previously.
You can predict when the hatch will occur by monitoring and assessing this caddis pupa development. To do this, you must remove individual caddis pupae from their cases by taking a small twig and pushing the pupa out from the back of the case. Larvae normally remain as Stage 1 pupae for approximately four to five days, develop into Stage 2 pupae after ten days, and transition to Stage 3 pupae after 16 to 18 days. Pupae remain in Stage 3-development until they emerge.
In the Watauga tailwater, caddis closer to Boone Reservoir (in The Country Mile) pupate before the caddis farther upstream in middle and upper river sections due to warmer water temperatures. Caddis in the middle river (near Elizabethton) pupate approximately a week after the caddis in The Country Mile, and the Brachycentrus in the upper river (upstream of Elizabethton) follow a week and a half after.
The caddis emergence on the Watauga moves upstream at a rate consistent with prior pupation timing. About the time caddis begin to emerge, significant leafing occurs in riparian vegetation and increasing air temperatures produce spring-like conditions.
Rapidly increasing air temperatures (mid-upper 70s) can accelerate pupae development upstream of The Country Mile. During a "normal" spring, the emergence can take more than a week to move upstream, but if it's exceptionally warm, the emergence can take as little as a few days to move upstream or it can happen all at once a fast and furious effect.
Occasionally, cooler morning air temperatures during the first few days of the Brachycentrus hatch can reduce the visible insect activity. If you arrive to the river one morning expecting caddis to be flying around in the trees and don't see any, just shake or inspect the riverside vegetation to see if the hatch is in progress.
Regional Timing for the Brachycentrus Hatch
The timing of hickory and American shad migratory spawning runs closely mirrors Brachycentrus emergences. In fact, Brachycentrus emergence occurs at a similar rate from east to west (longitudinally) as it does south to north (latitudinally) with emergence beginning in the coastal plain of the Mid-Atlantic states and in the southern latitudes of the Deep South while working west and north.
At about the time shad show themselves in major Mid-Atlantic river basins such as the Roanoke, Potomac, Susquehanna, and Delaware, Brachycentrus species emerge in these same river systems. Brachycentrus in mountain streams emerge approximately 3 to 4 weeks after coastal plain species. If you're a mountain trout fisher, simply keep up with shad reports on the coast, and you'll get an idea of when to expect the hatch to arrive on your home water.
Brachycentrus Caddis Daily Cycle
When the Brachycentrus emergence begins, the river erupts with some of the most exciting fishing of the entire spring season. The fishing starts early in the morning at around 7 A.M., and the emergence can last until noon, often peaking midmorning. During the morning hatch, there is often significant egg-laying activity as well. When the hatch stops around noon, the interest of the trout wanes for a few hours.
Many fly fishers fail to take advantage of morning emergence, but fishing this hatch is a full-day excursion. To imitate emerging pupae, I typically fish two traditional wet flies plus a pupa imitation using the Leisenring lift technique. My usual choices for the wet flies are a Royal Coachman, Leadwing Coachman, or Hare's Ear, together with my own custom pupa pattern with a Zelon trailing shuck to imitate exuviae.
Caddis mating occurs in stream riparian vegetation and trees. Females make morning and afternoon egg-laying flights in groups of a few thousand individuals while flying up and down shallow riffle areas. Later in the evening, egg-laying flights are more concentrated and synchronous where hundreds of thousands of caddis females congregate and oviposit by dipping their abdomens on the water while sometimes skating and scurrying on the surface. Typical evening egg-laying flights occur after 6 P.M. and peak between 7 and 8 P.M. When it happens, you may have to use windshield wipers while driving, as riverside roads become potential egg-laying sites for confused females.
Wet-fly techniques still catch fish in the evenings, but these are the glory hours for dry-fly fishers. During the Brachycentrus emergence, all you need is a dark brown or black Marjan Fratnik F Fly with a body of dark brown or black dubbing, and a wing of two to three brown CDC feathers. To imitate the egg-laying females later in the evening, I tie a female version with a shortened abdomen and use different dubbing to imitate the light olive egg sacs of the ovipositing females.
A variety of techniques work during the egg-laying evening event. Early on, when I see a lot of slashing, chasing rises I skate and twitch the F Fly at the surface. Later, when most of the caddis are dead on the water and the trout settle down into normal surface-sipping rises, I use a 10- or 12-foot leader with 5X or 6X tippet and focus on making long, drag-free drifts to specific trout.
During the first few days of the Brachycentrus hatch the insects seem larger, some as large as #12, and after just a few days the insects are noticeably smaller with #16 caddis becoming the most common.
If the Brachycentrus emergence occurs late enough in the spring, it can overlap with the beginning of the Sulphur hatch in the lower river. This makes for some interesting and difficult fishing where the peak of evening caddis egg-laying coincides with the evening Sulphur hatch, creating epic rises from some of the largest trout in the entire tailwater. Even the river's massive brown trout which normally don't bother much with surface insects can't resist the blanket hatches that occur when both Brachycentrus and Sulphurs are on the water at the same time.
The best fishing and surface feeding during the caddis hatch often occurs in low-water conditions when there is no power generation. During low water, the trout occupy shallower, more traditional feeding lanes, and are more confident to seek adults, emerging pupae, and flies close to the surface. Early April thunderstorms can bring high-water conditions and increased spring power generation that can lower water temperatures and delay larval development. This power generation delays pupation induction and termination, and eventually influences the timing of the emergence.
During the caddis emergence, I also pay close attention to the generation schedule for Wilbur Reservoir on the Tennessee Valley Authority (TVA) website. If power generation ceases, get out and fish as soon as you can. Once the TVA resumes power generation, the fishing (especially on the surface) declines greatly.
As the years have gone by, I've had amazing fishing experiences, been washed out by rain, and had everything in between. And I've also discovered that finding and timing the Brachycentrus hatch is more than a reward, it fills the soul. It's one of nature's great rituals of spring, and for those of us who wait for it along The Country Mile, it's like seeing an old fishing friend after months of winter drudgery.
While this so-called Mother's Day Hatch is phenomenal on trout streams throughout the Northeast, on Southern TVA trout tailwaters, it is our Hexagenia, our Green Drake, and our Salmonfly hatch put together.
Matthew Green has a master's degree in biology from Appalachian State University in Boone, North Carolina. He currently lives in central Pennsylvania.