I have I spot on the rocky by my house in brunswick that is like a crayfish breeding ground. I could probably catch a couple dozen a day in the same spot for the whole summer if I wanted to, which is nice because I have such a hard time finding them anywhere downstream. I actually think the best time to catch them is in full sunlight because they seem to have a tendancy to be out in the open more. Just remember to to work into the current when catching them so you don't have to work in the murked up water (listen to my advice cuz I never do)
a good way to catch them is to simply put out a lantern at night and catch them with a minnow net or your bare hands. for some reason, they are attracted to the light the same as most other fish are. i've noticed them a lot lately while i'm out catfishing. i even caught one last week that was almost 11 inches long!