Everybody in the business knows that the Atlantic population of bluefin tuna is in worse trouble than the Pacific population, but how much worse? Well, here’s one measure: Stanford University’s Tag-a-Giant program is now paying $1,000 per tag to fishermen in the Atlantic and Mediterranean who return the tags after they have caught the tuna, whereas fishermen in the Pacific only get $500 for a tag. Trust the market to tell you the truth.

Another measure of the bluefin’s scarcity value is the fact that two months ago, the owners of two sushi restaurants in Japan and one in Hong Kong banded together to pay $175,000 for a 513-pound (233-kg) bluefin tuna at Tokyo’s Tsukiji fish market. The primary market for bluefin tuna is sushi, and the demand is so great that the fish are disappearing fast in both oceans.

That’s why the first order of business at the CITES conference that opens in Doha, Qatar on March 13 is a complete ban on the international trade in bluefin tuna. CITES (the Convention on International Trade in Endangered Species) is the only port of call, because no other international organization can intervene in defence of a fish species. Whales have the International Whaling Commission, but for tuna, CITES is all there is.

So are the bluefin near the edge? Probably yes. When they tagged 600 of them in the North Pacific, they got three hundred tags back: half the tuna that Tag-a-Giant caught were caught again by the commercial fishery.

In the eastern Atlantic and Mediterranean, scientific data suggest that the species has dwindled by 60 percent in the past six years.

It would help a lot if the European Union were solidly behind a ban, for CITES needs a two-thirds majority of the 175 member states to put a species on the endangered list or take it off again. Most Atlantic bluefin tuna are caught in the Mediterranean, where they migrate to breed, but even within the EU there is not unanimous support for a ban.

France and Italy have recently come around to a total ban, but Spain, Greece, Cyprus and Malta still oppose it. Even France and Italy want to exempt the so-called “artisanal” fishery, in which local boats from the Mediterranean countries would continue to fish for local consumption only.

In practice that would mean exactly the same boats as before, catching the same fish, but with a legal requirement not to sell any of their catch internationally (i.e. to Japan).

According to a 2006 report in the scientific journal Nature, 90 per cent of the really big fish — tuna, marlin, swordfish and the like — are already gone. The middle-sized fish are following, and the solution does not lie in last-minute bans on fishing for the next species to reach the brink of extinction. These fish are all part of a food chain, and the whole ecosystem must be given a chance to recover.

Short-term pain for long-term gain.

We are going to lose the principal source of protein for one-fifth of the human race in the next few decades unless drastic measures are taken. The world’s fishing fleet needs to be reduced by at least two-thirds, bottom-trawling must be banned outright, and fishing moratoriums for large areas of the oceans need to be imposed for a decade or even longer.

Fish breed fast. Let them breed back up to their historic levels, and we could then sustainably take a catch that is three or four times greater than the current, unsustainable level. Or we can go on squabbling about the last few fish until they are all gone.

Gwynne Dyer is a London-based independent journalist.