Today's Wall Street Journal has a fascinating article about "macks," tins of mackerel that are commonly used for bartering in prison. Macks took over around 2004 when smoking bans in federal prison knocked out the previous currency: packs of cigarettes. From the WSJ:
Unlike… 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.
So inmates stash macks in lockers provided by the prison and use them to buy goods, including illicit ones such as stolen food and home-brewed "prison hooch," as well as services, such as shoeshines and cell cleaning…
There are other threats to the mackerel economy, says (Jon) Linder, of (supplier) Power Commissary. "There are shortages world-wide, in terms of the catch," he says. Combined with the weak dollar, that's led to a surging mack. Now, he says, a pouch of mackerel sells for more than $1 in most commissaries.
Another problem with mackerel is that once a prisoner's sentence is up, there's little to do with it — the fish can't be redeemed for cash, and has little value on the outside. As a result, says Mr. Levine, prisoners approaching their release must either barter or give away their stockpiles.