The Stream Doctor

June 21st, 2004

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. Why does the HEX hatch come so strong only every other season? Also, does this affect fish by the way of every other year when the hatch is not strong, the larger fish eat the fry?

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.

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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