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The Hexagenia:
One Big Smorgasbord at the Fish Cafe
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The Hex
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And while a sustained, massive hatch can be over in 10 days or 2 weeks on a particular stretch of stream, Hexagenia hatches are known to wane and then wax again over a several week period. This is particularly evident on the Mississippi River, where a series of 27 Army Corps of Engineers dams have turned the Mighty Mississippi from the Twin Cites to St. Louis into a series of elongated river pools. One researcher, Calvin Fremling, found that mass emergences occurred simultaneously along the entire 1000-kilometer length of the Upper Mississippi at intervals of 6 to 11 days
In late May I went to collect some Hexagenia nymphs for a program on aquatic insects. Collecting at one site on a local river was slow, so I packed up and drove upstream a few miles, parked the car and quickly scanned up and down the river from the car bridge. The river and its muddy shoreline were very little different from the last spot, except ... A tiny tributary formed a quiet backwater leading into the main stream. With almost no current, the bottom had to be fine, soft mud. No gravel, no sand — just mud. And such fine, soft mud is precisely what the nymphs of Hexagenia love. “Jackpot!” I smiled as I knelt at the water’s edge. The clear water was about a foot deep and in a single square foot of oozy bottom mud I could see a dozen or more small holes, looking something like someone had poked a pencil eraser into the mud. I waded into the backwater, placed the bottom of the kick net a couple feet out in front of me and, with one foot, kicked up no more than a square foot of bottom mud against the mesh. Six or eight Hexagenia nymphs immediately appeared at the top of the net and in less than a minute of washing the mud I had at least 18 “wigglers” in the water of my small freezer chest.
A few days later, I returned to the river with my camera and the Hex nymphs. I released the nymphs one at a time beneath my underwater camera housing and tried to photograph them as they burrowed into the mud. I could watch through my macro lens as a nymph would land on the mud, or even slightly arch and dive headlong into it and thrust and then lift its head in the mud to begin its burrow. Then the legs would get into action. The forelegs, with their enlarged tibia (vaguely analogous to your forearm), would dig into the mud, and the middle and hind legs would help toss the mud to the side and rear of the nymph. A dense row of hairs along the front and back of each leg helped move the mud out. The nymph was completely buried in less than a minute. These borrows can extend four or even six inches down into the mud. A burrow isn’t a straight affair — the nymph burrows down and then back up to make a U-shaped burrow, with both an inlet and an outlet. Of course, the burrow must remain open and not collapse. Mud with too much sand (over 50 percent) doesn't work. This limits the distribution of the nymphs to certain areas of a stream or lake, where both mud depth and consistency, plus adequate oxygen, all have to be right.
And if these factors are right? Returning to the 27 Upper Mississippi pools, these pools accumulate vast expanses of muddy bottoms and provide outstanding Hexagenia habitat, assuming that the waste water from cities along the river receives proper sewage treatment and doesn't cause anoxic (no oxygen) conditions in the river. Densities of Hexagenia nymphs in these pools can reach over a hundred per square foot. Even at somewhat lower densities, numbers can reach a million/acre, or three quarters of a billion/square mile. In the 48-mile length of the Mississippi pool above the dam at Keokuk, Iowa, one researcher estimated that there were a total of over 23 billion Hexagenia nymphs. Luckily, they don’t all emerge on the same evening.
This habitat of fine, firm mud, plus adequate oxygen, is found in quiet edges and pools of streams and rivers, shallow water of lakes and, particularly, river reservoirs all across the Midwest. The incredible hatches along the length of the Mississippi from the Twin Cities to St. Louis in June, July and even into August are well known. Here water temperatures can get quite high in mid-summer and I was surprised to learn that Hexagenia nymphs, which normally takes two years to reach maturity in cold Midwest trout streams, can actually complete their complete egg-to-dun stage in about 11 weeks. This means that the offspring of the first Mississippi River hatches of the summer can emerge as adults in August of the same year. These rapidly maturing adults are much smaller than their early-season parents, however.
In our cooler Midwest trout streams and walleye lakes, however, the nymphal stage lasts two years. Analysis of hundreds of larvae from a lake or stream shows two distinct size groups, indicating two age classes or “cohorts.” The maturing nymph grows slowly during its second winter, but as the water warms in the spring, growth rate increases rapidly. When a critical water temperature, which may depend on the individual species involved, is reached, emergence begins. In lakes, the larvae are most common in muddy buttoms nearer to the shore. The lack of oxygen in the cold, stratified water below the thermocline in lakes prevents Hexagenia nymphs from living there. In streams, low oxygen levels would be much less of a problem and Hex nymphs may occur wherever the proper muddy substrate occurs.
Hexagenia nymphs have been found to form an important percentage of the stomach contents of several fish species, including crappies, bluegills, sturgeon, whitefish, tullibee, sauger and rainbow trout in lakes. I know of no experimental work showing that Hex nymphs routinely leave their burrows and are preyed upon by fish; but because they are found regularly in fish stomach analyses, it seems that they must swim about in the water at times and become vulnerable to fish predation. It’s tough to imagine a bluegill digging six inches into the bottom mud for a Hex nymph.
Remember, with a two-year life cycle, sizable Hexagenia nymphs are present year-around, so they are constantly available to fish. Most fly anglers, however, look forward to fishing the major hatches in late June and early July; these can continue, at lower intensities, however, into August, particularly if one seeks cooler and more northern streams.
Dean Hansen’s column appears regularly in Midwest Fly Fishing™ magazine. He has a PhD in aquatic entomology. This article appeared in the summer 2002 issue of MFF.
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By Dean Hansen
Ah, the unmatchable Hexagenia! Now here’s a mayfly that can’t be ignored by stream, river or lake fishers. Known (for some utterly unfathomable reason) as The Michigan Caddis by some anglers or as the Shad Fly, Fish Fly, The Hex or even as those Bleeping Mayflies by others, one of four or five species of this genus inhabits, for example, cool Superior North Shore streams, classic trout streams across Minnesota, Wisconsin, and Michigan, warmer lakes and ponds in Illinois, Indiana, and Ohio, and the entire stretch of the Mississippi from Lake Itasca to St. Louis.
Additionally, the Hexagenia, the fly that drives trout wild in late June and early July, is around longer than most anglers suspect. Consider that Great Lakes steelhead fishers favor imitations of the Hexagenia nymphs well into the fall and smallmouth fishers, too, tell tales of bronzebacks occasionally feeding on them in lake and river during the summer.
To the angler, the exact identification to species is unimportant. Just give nymphs of Hexagenia some fine, soft mud, with certain minimal oxygen levels, and both moving and still waters across the entire Midwest can produce the most stunning hatches of any mayfly of the entire season.
I well remember my first experience fishing a Hex hatch. I met a couple other fishermen by a friend’s cabin on Wisconsin’s Clam River at about 9:15 one evening in late June. Honestly, at first I thought some of the cabin guests were rudely lobbing rocks into the Clam. Schplunk. Schplunk. Schplunk. But the sounds weren’t from rocks — large browns were having a blast taking emerging duns on the water surface.
The next hour was classic Hex fishing: landing (or losing) large browns using a dun imitation on the surface, then switching to a spent spinner as the females which had emerged a day or two before landed on the water to lay their eggs. I even watched, with the help of a flashlight (it was now after 10 p.m. and all but completely dark) as several spent spinners floated by. As luck would have it, one spinner released its burden of eggs as it floated past and I watched as a yellow egg mass sank down out of sight to begin the wonderful two-year-long cycle all over again.
What makes Hexagenia so important and familiar an insect? First, they are, by insect standards, huge — a mature female nymph can reach 1 1/4”, excluding the three long tails. This means that imitations of the nymphs or adults are tied on #4, #6, or #8 hooks. In a single gulp, a fish can down as much insect protein as is available in a hundred Tricos or Blue-winged Olives.
Secondly, because of their incredible numbers and mass emergences, persons living near lakes or rivers, fishermen or
not, are familiar with Hexagenia, even if they don’t know the Latin name. Adult Hex can coat the road surface of a bridge over the Mississippi with such a slippery layer of dead or dying bodies that road crews have to use snow removal equipment to make for safe driving. When walleyes key into feeding on nymphs swimming up to emerge, frustrated spin fishermen can find that their red and white Rapallas are ignored.
The next hour was classic Hex fishing: landing (or losing) large browns using a dun imitation on the surface, then switching to a spent spinner as the females which had emerged a day or two before landed on the water to lay their eggs. I even watched, with the help of a flashlight (it was now after 10 p.m. and all but completely dark) as several spent spinners floated by. As luck would have it, one spinner released its burden of eggs as it floated past and I watched as a yellow egg mass sank down out of sight to begin the wonderful two-year-long cycle all over again.
What makes Hexagenia so important and familiar an insect? First, they are, by insect standards, huge — a mature female nymph can reach 1 1/4”, excluding the three long tails. This means that imitations of the nymphs or adults are tied on #4, #6, or #8 hooks. In a single gulp, a fish can down as much insect protein as is available in a hundred Tricos or Blue-winged Olives.
Secondly, because of their incredible numbers and mass emergences, persons living near lakes or rivers, fishermen or
not, are familiar with Hexagenia, even if they don’t know the Latin name. Adult Hex can coat the road surface of a bridge over the Mississippi with such a slippery layer of dead or dying bodies that road crews have to use snow removal equipment to make for safe driving. When walleyes key into feeding on nymphs swimming up to emerge, frustrated spin fishermen can find that their red and white Rapallas are ignored.
Finally, fly fishermen know that the Hex hatch, be it on a lake or a trout stream, can give some of the most exciting, productive fishing action of the year in June, July and even into August. There’s a reason fishermen travel hundreds of miles to meet the Hex hatch on Michigan’s Au Sable or Pere Marquette, Wisconsin’s Peshtigo or Brule, Minnesota’s Root River or walleye lakes and rivers throughout the Midwest: the fly fishing can be outstanding.
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copyright Midwest Fly Fishing Magazine 1999
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