Apparently, the Japanese tuna industry has been cheating the system for years - catching well over their scientifically determined quota and pushing the southern bluefin dangerously low levels to dangerously. From the Sydney Morning Herald:

AUSTRALIA'S top fisheries manager has revealed Japan illegally took $2 billion worth of southern bluefin tuna, effectively killing the stock commercially.

An investigation into the imperilled fishery found Japanese fishers and suppliers from other countries caught up to three times the Japanese quota each year for the past 20 years, and hid it.

(snip)

The Bureau of Rural Sciences said the most recent estimate by Australian scientists of southern bluefin's parental biomass - the quantity of adult tuna - was that it stood at as little as 4 per cent of its original size.