May Flies

Thinking out loud and wondering about the May Fly hatch.
Each season the hatch occurs about now and some years they are more numerous and other years, not as many. I live in western Toledo and at times we'd get them "big time" and other years not at all. Any thoughts as to why things happen like this? Is it simply the wind directions that blow them this way? Could it be that there are more fish here in the western basin during certain years and thus getting their fill that helps to keep the fly populations down?
Any thoughts?

http://www.mayflynews.net/news.html
Some good information and years of tracking data

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.