The Stream Doctor

August 23rd, 2004

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

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

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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