There's been a mackerel economy in federal prisons since about 2004, former inmates and some prison consultants say. That's when federal prisons prohibited smoking and, by default, the cigarette pack, which was the earlier gold standard.

Prisoners need a proxy for the dollar because they're not allowed to possess cash.
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Mackerel supplier Global Source Marketing Inc. says demand from prisons has grown since 2004. In recent years, demand has switched from cans -- which wardens don't like because inmates can turn them into makeshift knives -- to plastic-and-foil pouches of mackerel fillets