Like minnows, crayfish are an extremely important food source for large trout. Many feed on nothing else for weeks at a time. Anglers are perpetually on the search for the perfect crayfish imitation, and so far no fly tyer outside the realm of rumor and legend has devised one.
A crayfish chews on a Hexagenia limbata nymph shortly after a small Hex emergence. I didn't catch any fish, but playing around with my flashlight and camera in the rocks proved productive.