Q.
I am a UK trout fisher. My river beat had
masses of mayfly in May then finished, as
expected. I then fished Normandy rivers
(near to Dieppe) through July. Mayfly were
hatching steadily and I took brown trout on
mayfly patterns. I asked the locals why, this
far south of the English waters, mayfly were
hatching in significant numbers when they had
ceased further north in UK. Answer was “comme-ci,
comme-ca”. Does not explain it. Any ideas?
A.
Well, I have a couple of ideas on the subject.
If you’ve read some of my earlier responses (you
can retrieve earlier ones in the archives) you’ll
remember that I’ve explained that hatching of
aquatic insects is based on degree-days. Each
species must undergo a certain thermal history
for maturation from egg to adult. So, one of
the explanations might be that the mayflies in
the Normandy area did not experience a sufficient
number of degree-days until July. The water might
be colder or not warm up as early. Another possible
answer is that you may have been seeing different
species in Normandy than you did in UK, and these
species require a greater number of degree-days
than the UK mayflies. Obviously, you can see
that any number of explanations, or even combinations
of explanations, could explain your quandary. Without
knowing what species you observed in both places and
something about the thermal regimes of the waters,
I can’t do much better than offer the above.
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 August 23rd, 2004 on Fly Anglers Online by Robert Younghanz.