Mayflies in Lake Erie are just like any typical insect population. Their population numbers are determined year to year by a number of factors, including weather (breeding success and egg deposit success), predation (fish eating them), bottom current (food supply and water oxygen), and a host of others. They were practically absent back in the late 1960's and early 1970's due to poor water quality but made a grand comeback after the Lake cleaned up. A Midge fly, or "mufflehead", was the big hatch back when the mayflies were very low. I'm sure there is info out there on the Lake mayflies. Start at Ohio Sea Grant.