Hex Mayfly Hatches in the Pacific Northwest

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Happening soon:
Timothy Lake, Oregon
Lost Lake, Oregon
Clear Lake, Oregon (some evenings)
Merrill, Washington

Patty Barnes observes a freshly hatched Heagenia Dun that emerged from Timothy Lake near Mt. Hood in Oregon, July 2019.

Patty Barnes observes a freshly hatched Heagenia Dun that emerged from Timothy Lake near Mt. Hood in Oregon, July 2019. Some Hex mayflies emerge as early as 7:00 p.m. Most wait until the last light fades. Then duns emerge in mass quickly followed by a spinner fall of Hex that emerged the night before. All this activity can bring the largest trout to the surface.

Hexagenia limbata is one of the most geographically widespread mayflies in North America. It is found from coast to coast as well as from Florida to Canada and often constitutes an important part of the food chain in clear water lakes and slow moving streams. In suitable habitats, Hexagenia nymphs may reach population densities of nearly 500 per square foot of lake bottom. The high densities and productivity rates of these mayflies may constitute a significant component of nutrient and energy cycling within their aquatic habitats and adjacent terrestrial systems. This would be especially true for Florida, where almost one quarter of the state’s land is swamps. In certain Great Lakes and Mississippi River areas Hexagenia hatch in such great number as to become a public nuisance and in some cases have to be bladed off of roads to prevent traffic hazards. In Oregon and Washington Hexagenia hatch in a few small lakes and slower sections of streams. Most notable of these are certain Cascade Lakes and portions of the Williamson River.

The Hexagenia hatch is to some lakes and slower rivers what the Salmonfly hatch is to some larger western rivers. They are both large insects that bring the larges trout to the surface to eat dry flies.

Female Hexagenia and most other mayflies deposit their eggs directly on or in the water. Individual Hexagenia females release as many as 8,000 oval eggs, each less than 1/2 mm long, which sink to the lake bottom. After several days to several months (dependent partly on the water temperature), a tiny nymph hatches from each egg. It immediately burrows into the lake sediment to feed on particulates and to construct a U-shaped burrow, with two openings at the sediment surface. The nymph continually enlarges its burrow as it grows, so that the burrows of mature nymphs can be as much as 5 inches deep.

By undulating its body and moving its feathery abdominal gills in sweeping motions, the nymph keeps the burrow oxygenated. Nymphs are found from just a few inches to fifty feet deep. Depending on the topography of the lake bottom, most Hex nymphs will be found at depths of two to ten feet. Where the water is clear enough to see the lake bottom, it is easy to spot areas where colonies of these nymphs live as their burrows will be easily seen.

 Picture of a Hexagenia mayfly dun from Timothy lake near Mt. Hood in Oregon.

The nymphal life of Hexagenia lasts from about one year in warm climates and usually two years or more in colder regions. It is believed that Hexagenia nymphs remain in Lake Erie over two winters before they mature. This is the common cycle in Pacific Northwest lakes as well. As with all insects, they grow by shedding their exoskeleton or "skin" in a process called molting. Hexagenia nymphs undergo perhaps as many as 20 to 30 molts. When the nymph is ready for its final molt, it leaves the burrow at dusk or soon after and rapidly swims to the lake surface, where its exoskeleton splits lengthwise down its back. From the raft-like exoskeleton emerges a fully winged subimago (or dun), which after only a few minutes takes flight. The winged Hexagenia is the only stage in the insect's life cycle that most people see.

A Hex spinner rests in an anglers hands. You can see that the background is very dark. This spinner has probably already laid its eggs and is about ready to die.

Although the dun may appear at first to be an adult mayfly, it is not fully developed sexually, and its color is usually more opaque than the adult. During the night or the following day, male and female duns molt a final time, leaving behind a subimaginal exoskeleton. They are now sexually mature imagoes or (spinners - adults). Neither the duns nor spinners eat, because their mouth-parts are not completely developed. At dusk or at night, female spinners fly into a large swarm of male spinners. The males and females mate in flight during darkness. Within minutes of mating, the female spinner settles down to the lake surface, extrudes her eggs, and dies. This is called a spinner fall.

In the Oregon Cascades there is strong evidence that more than one subspecies of large burrowing nocturnal mayfly may inhabit some lakes. Several color variations and at least two distinctly different sizes of similar looking mayflies may emerge in a single evening. The large yellow mayfly pictured at the top of this page is common to the three local lakes we fished during the summer of 2000. It ranges up to 1.7" from head to the base of its tails. The two variations pictured here are also present in large numbers in all three lakes but rarely exceeds 1.2". One variation has dark smokey colored wings. On this insect the tails and the under side of the legs are dark brown. The other insect is pale creamy yellow with very light brown markings. The tails and under side of legs are creamy colored. Males and females have been observed in both color phases.

When fishing a Hex hatch, there is no need to get to the water before late afternoon. This is the perfect situation for the angler who wants to do some fishing after the work day is done. The angler will usually see a very few duns around 7:00. This is a false hatch. Unless there is dense cloud cover the real hatch starts about 8:30 or 9:00. The angler should start fishing over Hex beds with a sinking line around 7:00 with a Hex nymph. The sink rate of the line you choose is dependent on the depth of the water. You should let the fly sink to within a foot of the bottom and retrieve fairly rapidly. 3X tippet is advisable. Some of the best fish will be encountered at this level.

As the hatch progresses, both the fish and the nymphs will start to concentrate near the surface. You should carry two rods, one with a floating line and the other with a sinking line. When there is sufficient surface action, switch to the nymph on the floating line.

Find the most useful Hexagenia dry flies here: Hexagenia Dry Flies

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