Awesome. "Unlike those more expensive delicacies [the canned tuna, crab, chicken, and oysters also offered by the commissary], former prisoners say, the mack is a good stand-in for the greenback because each can (or pouch) costs about $1 and few -- other than weight-lifters craving protein -- want to eat it."
Man, I like mackerel. I guess I'd better stay out of prison.