A. I could not find any information in any of the texts I have on mayflies that substantiate your observation that Hexagenia hatches are stronger every other season. At least this doesn't appear to be a generality pertaining to all Hexagenia populations. However, this doesn't refute your observations and a possible (note "possible") answer may be related to their life history pattern. Hexagenia are predominately semi-voltine, a fancy way of saying that they generally take two years to complete their life cycle. Therefore, if indeed the population you are citing does have a two-year life cycle, it may be that one cohort or age-class (a group of similar-aged nymphs) is larger than the other; thus, every other year the more numerous cohort hatches giving what you observed. Now having said that, I should also point out that some Hexagenia have a one year life cycle, which would negate the above possibility, and they can even be raised to maturity in less than a year under certain conditions. At least you now have a thoroughly "weasel-worded" explanation of what might be happening.

I'm afraid I can't even make a guess as to whether this pattern influences whether larger fish feed more on fry than mayflies during the low hatch years. I've used up my allotment of guesses, and would suggest you pursue this with a local fish biologist.