I have stolen this WSJ story from Paul Krugman...

Inmates of America's vast prison system are not allowed to possess cash. Traditionally this meant that cigarettes were used as a medium of exchange - but then smoking was banned. The replacement is, for some reason, mackerel fillets.

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.

Question, though: surely the reason that cigarettes were used in the first place is that lots of people did want them? Why aren't the prisoners using some more desirable substitute for cigarettes? Stamps are mentioned, which seem ideal. After all, they're durable, compact, have an obvious face value, and easily convertible into cash outside the prison, unlike mackerel - the non-convertibility of the "mack" is mentioned as a problem. Why has mackerel outdone stamps - or the other alternatives such as PowerBars? The article suggests that it's because of mackerel's undesirability, but that just seems wrong.

Maybe what we have is a QWERTY situation; there were so many good alternatives springing up after the cigarette ban that none of them were able to dominate the market. Mackerel, purely by chance - perhaps some large criminal group decided to start accepting the mackerel as settlement for all debts public and private - became the leading contender, and after that network effects took over; mackerel became widely used because it was widely used, and everyone knew that other traders would accept the mackerel, despite its apparent disadvantages. A bit like the Maria Theresa Thaler.

The next step is to watch for the emergence of notes of hand backed by large reserves of mackerel held by a reliable (in prison terms) central authority...