If you're fishing in the Potomac, you have two types of catfish to choose from, the Channel Cat and the Mudcat (0r Yellow Cat). The Channel Cats are longer thinner and are a dark brown with a bluish tinge to it, have a forked tail and are considered the more edible of the two. The Mudcats are thicker and squatter, a lighter brown color with a yellowish tinge to it and have a flat tail.

Actually, I guess you don't choose them, so much as they choose you
(or your worm, chicken liver, or whatever).