I fish the hatch every year in the Gunnison Gorge in Colorado and if I'm lucky up on the Henry's Fork in Idaho as well. Sometimes I feel hitting it right is such a lottery that I feel I've fished it in most situations imaginable. This year I hit it on the tail end of the hatch right as the bugs were moving out of the canyon area. While the fishing was epic, I noticed the following. I got looks on about 70% of all my casts, and strikes on many of those, but the fish after gorging themselves so much were definitely smarter. I sat on one canyon wall and watched the same fish (which I eventually caught
) remain in the exact same location and just waited for the bugs to float overhead. Since the bugs were so prolific, this wasn't a long wait between strikes - but the fish never moved. It hit so many flies that the canyon wall next to it was dripping with water from its splashes. I had it spit my hook several times before I was able to connect.
My thoughts - towards the later end of the hatch the fish get pretty good at distinguishing between a real bug and a fake one, especially when one is presented to them every minute.
I'd also say that once the main hatch passes there are always a few stragglers but it seems to get require alot of them to get the fish really going. On day 3 of our trip this year the fish were very selective. I saw natural flies take a float for 2-3 minutes before any fish would rise to them. There were 1/8th the flies in the air as on day 1 and the fish activity and aggressiveness died with it.