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Wild Side: Giant mayfly flights fuel fish feeding frenzy

An adult Hexagenia mayfly. Michigan Zoology Department photo

My grandparents had a house in the village of Put-In-Bay on South Bass Island on western Lake Erie. The house was my grandfather's retreat during his long career as a judge in Cleveland, Ohio.

He didn't take a vacation from his work for many years but he went to the island nearly every weekend to go fishing or hunting.

Put-In-Bay in the 1960s was a sleepy old resort town, popular with summer tourists and quiet the rest of the year. Some of my earliest memories are from that island, learning to walk in the park and swimming in the lake. One of my early memories about the island is how the lake smelled fishy, especially in the summer when the "Canadian soldier" mayflies hatched.

People out in the islands and on the Ohio shore of Lake Erie expected the annual invasion of big mayflies in late June to around the Fourth of July. They didn't all come from Canada. The larvae lived in the bottom of the lake. The mayflies hatched out as flying adults and piled up in big fishy-smelling drifts under lights at night.

It was hard to catch a yellow perch, walleye, or smallmouth bass when the mayflies were hatching because the fish were full of them. Carp slurped spent mayflies off the surface in the harbor.

Mayflies are indicators of good water quality. The mayflies on Lake Erie declined due to low dissolved oxygen caused by excessive phosphorus loading to the lake and rotting blue green algae blooms. After the U.S. EPA and Lake Erie basin states reduced sediment and phosphorus loading to the lake, the lake cleared up with green algae and diatoms instead of blue-green algae, the mayflies returned, and the fishery became productive again. That all happened before the zebra mussels invaded and cleared up the lake even more.

The same giant mayflies of the genus Hexagenia that live in the Great Lakes also live in the Mississippi River and in streams around here. Hexagenia limbata is one of the most widely distributed mayfly species in North America, occurring from Florida into Canada. They live most of their lives underwater in the mud at the bottom of lakes and streams as larvae, commonly called "wigglers." The larvae live in U-shaped burrows in the mud and feed on particulate matter by pumping water through their burrows with their feathery external gills. In some places they occur in high densities with as many as 500 larvae per square foot of lake bottom.

Where they occur in abundance, Hexagenia mayflies are an important component in the aquatic ecosystem, converting particulate organic matter into high-protein biomass for fish and birds to eat.

Hexagenia mayflies emerge synchronously, forming large clouds of flying adults. They mate in swarms above the water, usually in late afternoon or evening. The females deposit their egg packets on the water where they sink to the bottom and later hatch into larvae. The adults live out of the water only for a day or two.

Dr. Calvin Fremling, a biology professor emeritus at Winona State University, documented the timing of synchronous Hexagenia emergences on the Upper Mississippi River for many years. This year, the first big mayfly hatches appeared several weeks early, probably due to the warm spring.

So many Hexagenia mayflies hatch off the Mississippi River that their swarms are detected on the National Weather Service's NEXRAD precipitation radar. In late June a swarm of mayflies was detected on radar extending from northern Iowa up the river through La Crosse and Trempealeau Wisconsin. That's a lot of flying biomass!

This year Hexagenia were emerging on the Rush River near Martell in late June in the middle of the afternoon. The mayflies provide big, nutritious and tasty food items for trout and birds. Trout were smacking them at the surface with gusto. Swallows were swooping to gulp them from the air. It's fun to fish a Hexagenia dry fly imitation when the trout are feeding with such enthusiasm. Even after the hatch dies down, a big Hexagenia or grasshopper dry fly will elicit a strike.

We are fortunate to live where the water quality is good enough to support an abundance of elegant Hexagenia mayflies. Swarms of Hexagenia are a sure sign of great water, good fishing and summer.

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