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 programme is now paying US$1000 ($1425) a tag to fishermen in the Atlantic and Mediterranean who return the tags after they have caught the tuna, whereas fishermen in the Pacific get only US$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 US$175,000 for a 233kg 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 opened in Doha, Qatar at the weekend was 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 organisation can intervene in defence of a fish species.

So are the bluefin near the edge? Probably yes. When they tagged 600 of them in the North Pacific, they got 300 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 per cent 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 "artesanal" fishery, in which local boats from the Mediterranean countries would continue to fish for local consumption only.

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

If you think that would work, when prime bluefin tuna is already selling wholesale in Japan at $770kg, you are very trusting indeed.

As for Japan, which consumes around 80 per cent of the world's bluefin tuna catch, it does not just oppose the ban. Its chief delegate to the Cites conference, Masanori Miyahara, says it will "take a reservation" to any ban: that is, ignore it.

It is good that Cites, which used to devote most of its effort to protecting more visible land animals and plants, is now paying attention to endangered fish species as well.

But the pattern is always the same: the species only gets protection when its numbers are already so low that it is at risk of extinction - and even with protection, it may never regain its former numbers.

The bluefin tuna population of the western Atlantic (which spawns in the Caribbean) was over-fished in the 1970s and 1980s.

The "spawning stock biomass" fell to 15 per cent of its former level before that population got protection - and while its numbers then stabilised, despite the passage of several decades they have never recovered.

Good luck to Cites on the tuna issue - and in the equally important business of stopping over-fishing of several shark species (mainly for their fins) whose populations have dropped by up to 90 per cent already. But we are systematically emptying the seas and we need a system-wide solution to the problem.

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.

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 present, 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 whose articles are published in 45 countries.