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
Email: streamdoctor@aol.com
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 streamdoctor@aol.com.
Originally published June 21st, 2004 on Fly Anglers Online by Robert Younghanz.