Yes, it is currently one of the most widely caught commercial fishes.
Whale-small fishies-plankton- algea?
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