http://flyfisherman.com/ftb/smcrayfish/index.html
....Early in the summer, the young crayfish are an inch or less long. They are usually light in color and inhabit shallow areas near cover. This time of year, I like to fish areas of current near expansive rock flats with patterns like Theo's Dream, Clouser's Crayfish, or similar small imitations. Because these crayfish are so small and light, they are occasionally swept away in the current, so fishing a #8-10 crayfish imitation in early summer is productive for trout, bass, panfish, and carp. As the summer progresses and the crayfish grow, increase the size of your imitation. Crayfish molt several times a year, and when they do, they are vulnerable. As crayfish grow, they shed their hard exoskeletons that become too small, leaving them defenseless with soft shells for a few days until their new shells harden. Because younger crayfish grow so rapidly their first year, this can happen eight or ten times a season. They also cannot swim well with a soft shell or use their claws to defend themselves against hungry predators. While they progress through this phase they are aggressively eaten by fish simply because it is less work. A crayfish full of fight with a hard shell and nasty claws takes time and energy for a fish to tackle. A molting crayfish is dinner in one gulp. A soft-shell crayfish is lighter in color than one with a hard shell, so fish patterns that are lighter in color than the crayfish you see to imitate this vulnerable stage....
-jamieS