Tuna: if it's affordable, it's not bluefin

Here's a handy rule-of-thumb for conservation-minded sushi lovers worried about accidentally eating bluefin tuna: if it's not wildly expensive, its not bluefin.

In Japan, which consumes 80 percent of the Atlantic bluefin catch every year, a single, bite-sized morsel can easily set you back 20 euros (28 dollars).

Five main species of tuna make up the annual worldwide catch of 4.0 to 4.5 million tonnes, and bluefin - Thunnus thynnus - is less than one percent of the total, some 24,000 tonnes in 2008.

Chances are that the raw tuna in your 10-euro (14-dollar) lunch platter, whether in London, Hong Kong, New York or Sydney, is either yellowfin (Thunnus albacares) or bigeye (Thunnus obesus). They make up 24 and 10 percent of the global tuna market respectively.

The most common "chicken of the sea" is not, strictly speaking, even a member of the Thunnus family: skipjack, or Katsuwonus pelamis, accounts for 60 percent of all tuna caught each year, some 2.41 million tonnes.

A lot of it winds up in tins, destined for the US and British markets, along with Europe, Australia and Japan.

Much Thunnus alalunga, better known as albacore, is also destined for supermarket shelves.

Taking all five species together, half the yearly haul is caught in the western Pacific, a quarter in the Indian Ocean, 16 percent in the eastern Pacific and nine percent in the Atlantic.

Japan reels in the biggest catch, more than half-a-million tonnes each year, followed closed by Taiwan.

Indonesia is in third place with nearly 350,000 tonnes, followed by the Philippines, Spain, Korea and Papua New Guinea, which all catch between 200,000 and 300,000 tonnes of tuna annually.

France, with a large fleet in the Indian Ocean, is in eighth place with about 180,000 tonnes.

More than 80 percent of the 500,000-tonne market for fish consumed raw is in Japan, served as is (sashimi) or wrapped in seaweed and with rice (sushi).

Americans have also acquired a taste for uncooked fish, accounting for nine percent, followed by Korea, China, the European Union and Taiwan.

Conservationists caution that the ravenous global appetite for tuna could push other species besides bluefin into dangerous waters, driving up prices and forcing the introduction of quotas to ensure sustainability.

In 1950, the global fishery caught only 700,000 tonnes of the five main species. In 1970 that figure rose to 1.1 million, in 1990 to 2.9 million, and in 2008 to about 4.2 million.

"Scientists estimate that, at the current rate, we will virtually empty the seas of big fish by 2030," said Sue Lieberman, policy director for the Washington-based Pew Environment Group.