Every summer from late June to early July we have a hatch of the biggest mayflies, Hexagenia Limbata, aka Hex flies, on Elk Lake, Skegemog Lake, Rapid River, and most Michigan bodies of water. They also are called giant mayfly, Michigan mayfly or simply fish fly. Their adult life cycle occurs over just a couple days from emergence, mating and egg release, to death. In that short adult stage, they cling to just about anything; boats, docks, hoists, houses, trees, etc. They die over the water and form the big mats of floating corpses we observe.
It was observed by many on Elk and Skegemog that there was an unusually small or non-existent hatch in 2020. The MiDNR has received reports of similar occurrences around the state. They report that normally there are a few lakes each year in the northern Lower Peninsula where this occurs, but it was more widespread in 2020. Typically, the hatch returns to normal the next year. Additionally, many hatches of Hex flies and other mayflies that emerge in spring and fall were later than normal in 2020. Cause is unknown.
The primary habitat requirement for these bugs is good water quality, followed by the bottom substrate along the shoreline shallows. The best nearshore habitat has silt, organic matter, woody debris; a natural shoreline. Armoring shorelines with metal or rock walls degrades their habitat. Mayflies are near the bottom of the food chain, especially their underwater nymph stage. Their populations are a significant food source for fish like minnows, perch, bass, trout, and whitefish. Mayfly nymphs feed on microorganisms that are also fed upon by invasives like zebra mussels.
At this time there is no known cause among the list of potentials for the unusually small hatch. ESLA has monitored the water quality of Elk and Skegemog for many years and no degradation has been identified leading to this occurrence. There has been some degradation of the natural shoreline habitat over recent years due to development. However, that change has been continuous and is not presumed to have caused the sudden decrease we experienced in 2020. ESLA will continue to monitor the mayfly hatches on our waters, and around Michigan in future years.