Since they are a small fish (max size 8 inches) they are eaten by many of the larger predatory species like salmon and trout.
Because large fish have to eat whatever they can get.
Yes, they also eat other dead crayfish, dead plant matter, live plant matter, and even smaller live crayfish
Most predatory fish like stone fish or lion fish eat damselfish. These fish are great replacements for feeder fish.
bigger fish probably eat smaller fishes eggs.