Prison economy

The WSJ writes about currency substitutes for trade among inmates in
American prisons (subscription required):

Mackerel Economics in Prison Leads to Appreciation for Oily Fillets
Packs of Fish Catch On as Currency, Former Inmates Say; Officials Carp

"Unlike those more expensive delicacies, 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."