Some of the best political decisions happen for the worst reasons. No clearer case can be found than the announcement by President Nicolas Sarkozy of France that he will support a ban on the international trade in bluefin tuna, a fish as endangered as the giant panda but far more palatable. The bluefin has been over-fished to near extinction within the borders of the European Union under the indifferent eyes of its fisheries inspectors.

The great irony underlying Sarkozy’s conversion to conservation is that it is France’s technically advanced but politically uncontrollable fishing fleet that is principally responsible for over-fishing the giant bluefin.

What Sarkozy said is extraordinary. It amounts to an admission by a European government that one of the world’s great environmental disasters has been going on in EU waters as a result of illegal and uncontrolled fishing by the Mediterranean nations and Japan. If a ban on international trade is all that will save the bluefin, we have on our hands a disaster comparable to the destruction of the blue whale or the northern cod.

Sarkozy is right when he says: “Ours is the last generation with the ability to take action before it is too late — we must protect marine resources now in order to fish better in future. We owe this to fishermen and we owe it to future generations.” This is the most obvious home-grown fisheries disaster since the foundation of the common fisheries policy. Brussels will now have to admit it — which can only spur reform.

Another reason why Sarkozy’s elegant speech is significant is that it amounts to the first shot in what is likely to be one of the great set-piece battles for conservation, as resonant as that to save the blue whale in the 1960s or the African elephant in the 1980s. For months scientists have been saying there are no more mature spawning adult bluefin left in the Mediterranean; a third of those on the Japanese market are below legal landing size; the population appears to have collapsed in 2007. Now the conservationists’ battle is likely to be taken up by whole nations in the run-up to the meeting in March of the Convention on International Trade in Endangered Species (Cites).

It is far too early to predict the outcome. A ban on international trade in bluefin under Cites is likely to be opposed by Japan as well as by the other big culprits in over-fishing. All sorts of dirty tricks may be expected. Success in reaching the necessary two-thirds majority of votes at Cites before the bluefin season opens again next year is likely to depend largely on alliances built in Europe and the United States before the formal listing application goes in this autumn.

Until Sarkozy stood up to speak on Thursday, no one knew whether Monaco’s proposal to list the bluefin under appendix 1 of the convention — the vehicle used successfully to stop the epidemic of ivory poaching that threatened the African elephant in 1989 — had a prayer. People were waiting for America or the UK to decide whether they were brave enough to support Monaco. Sarkozy was assumed to be opposed to a listing application. Huw Irranca-Davies, Britain’s fisheries minister, was reduced to rushing out a feeble statement supporting Sarkozy. In fact, our policy has been to support a less rigorous position that Canada and the United States, which have sport fisheries for bluefin, might back. Not much leadership there.

What impelled Sarkozy to take the lead in an area of policy in which France has long lagged behind? I suspect it was a battle of wills and egos. Sarkozy hates the fact that the fishing industry in the south of France — which was politically allied to Jacques Chirac, his predecessor and enemy — is out of control. He clashed with it over unpaid tax as a finance minister. This year he arranged a deal under which fishermen could apply for generous terms if they scrapped their vessels and gave up fishing. By February, the deadline, only one vessel, owned by a Spanish tuna firm, had applied. An international trade ban was the last weapon available against the industry that defied him.

The significance of France, as Europe’s sinner that repenteth, goes far beyond the bluefin. It will impact on other countries, including Britain, which participate in the disgraceful annual horse-trading round for quotas on European fish species in December. Sarkozy declared that every management decision for the seas and fish stocks in French waters would in future be based on scientific advice. He also said France would place 20% of its waters under special protection by 2020 to build up fish stocks (in half that area there will be a total ban on fishing). This is better than Britain’s proposals for marine reserves.

It remains to be seen how far Sarkozy’s conversion to conservation will play in the deliberations of the European Council or the International Commission for the Conservation of Atlantic Tunas — particularly if French fishermen decide to blockade ports and hold their government to ransom. What is certain is that he has raised the bar for other heads of state. What is Gordon Brown’s contribution going to be to restoring Europe’s seas other than a marine bill that is more about access to the coast than conserving fish? I am looking forward to finding out.

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