Here’s a great classic video from John Arnold of Scumliner Media. It features Mark Raisler, of Headhunters Fly Shop, talking about the massive Trico hatches on the Missouri River and what it takes to catch fish then. Lots of trout coming to the surface for millions of spinners does not equal easy fishing, that’s for sure.

It’s enough to make you salivate.
Cool video … always wanted to get to the Mo for this. Nitpicking but I have caught enough trout on tricos to know that fish WILL take your fish as it is dragged through their feeding lane (when the fly is submerged). Whether it is because there are still duns emerging or some other hatch is mixing in there is no question that it works often enough to make it worth trying.
Doug, you are right that there are times when trout will take a swinging underwater Trico. I have also experience strikes with a swinging SJ worm, and I KNOW they don’t rise. I think trout are not as wise as we fishermen would like to attribute to them. When they are in a feeding mood, they eat anything that moves. Especially underwater when their caution is low, and flies move in 3D across changing, sometimes swirling currents. But on the surface, the currents are relatively unchanged, and they move essentially in 1D feeding lane (yes, a snake-like 1D), not even true 2D (where you go outside the lane). I find that 90% of the time after you catch the first trout or even a second trout, you have no chance within the same pod with an imperfect fly and/or imperfect presentation. They just stop rising even to the naturals. Especially in anything but the shallowest water, where they have less time to inspect. Different pod, maybe. Unpressured trout, maybe. But not the original pod without some time for them to forget. Just my $0.02 worth.
There are many interesting things in the world that do not depend on the will of people. It’s just that we think everyone’s strong.
But we’re weak.