Hi' Richard (SBF) A point I should have mentioned before, the emergence of aquatic flies can be dependent upon relative air/water temperatures.
I would not say that it was a hard and fast rule, but if the air temp is lower than that of the water, in my experience, an emergence is less likely than it would be if the air temp was higher than that of the water.
In fact, on a number of occasions in late winter/early spring, I have watched the air temp creep up as the morning wore on until, say, a few large dark olives appeared. When they did, the air temp had just exceeded that of the water by a degree or two. My most recent check was done this last spring on the Eden, when neither LDOs or grannom sedges came out until air was warmer than water temp.
However, do not be too deterred by a cool morning, as trout can be brought up without naturals on the water. My old pal and I caught trout regularly on dry fly on opening days on which we never saw an LDO on the water, nor a rise from a trout -- except to our flies. Learn the hot spots, places where trout are always in residence, near a feeding lane, and just fish the water, down the seams, alongside branches, rocks and ledges etc. It works, honest!!


Cheers, hope that helps, Terry C