Bruce, we have a couple of Trico threads. I'll try to pull the best one up. I believe most say that Tricos typically begin to fall when the air reaches 68-70 degrees, but not all bugs fall at once. The males hatch at night and the females very early in the morning. Often when I arrive very early, long before the cloud begins to fall en masse, there are some spinners on the water--early drops, I assume. If you are sure the bugs were tricos, not blue quills (which I've been told don't fall to the water, but return to streamside vegetation to die) and if you stayed until the cloud was no longer visible in the air and saw bugs on the water, without seeing fish rise, I'd suggest the following possiblities: the fish have been caught out, the fish haven't started keying on tricos yet (unlikely by now), the fish have been put off by very heavy fishing (also unlikely given my experience with tricos and fish), or the stream was not stocked in that section--if it's a stocked stream. Did you watch the clouds fall? You often can literally see the spinners get lower and lower, with some breaking away and descending? Were there lots of spinners on the water? Also, if it warmed up gradually instead of quickly, the spinnerfall may have been gradual, not putting enough spinners on the water at one time to trigger feeding. Was the water low and clear? If it was, if the bugs were tricos, and if you had a concentration of spinners, there probably weren't any fish there. Finally, fish are strange. Just when we think we understand their behavior, they do something very weird. Perhaps they were on strike.
"He spread them a yard and a half. 'And every one that got away is this big.'"
--Fred Chappell