By Chris Hogg
BBC News, Tokyo
Environmentalists have welcomed a rare admission from Japan that its fishing vessels have exceeded quotas.
Blue-fin tuna are particularly prized in Japan
The country has also agreed to halve its annual catch of the popular southern blue-fin tuna.
Japan imports about 10,000 tonnes of the fish, which is popular for use in sushi or sashimi dishes.
Despite welcoming the news from Tokyo, scientists say stocks of the fish are shrinking rapidly, and more still needs to be done.
The fatty parts of the tuna fish are known as "toro" in Japan.
They are popular with Japanese diners and command a high price on the open market.
The southern blue-fin tuna have more fatty parts than other species, which is why they are highly prized.
For years Japan has been accused of breaking international agreements and catching more of the species than it was supposed to.
High prices are paid by traders for blue-fin (© WWF-Mediterranean/P Guglielmi)
Now it has acknowledged that its fleet has over-fished, catching a third more of the tuna than it was meant to last year.
It has agreed to cut its quota by 50% for the next five years as a result.
But the environmental group WWF fears that will still not be enough to give the southern blue-fin tuna a chance to recover.
The fund welcomed Japan's decision to accept a reduction in its quota, part of an overall cut of around 20% in the allowed catch in this part of the world.
But it said other countries should have agreed greater cuts in their quotas, in particular Australia, where the quota remains intact.
Almost all the southern blue-fin tuna caught in the world is sent to Japan.
WWF says consumers should think carefully about how the world's stocks of tuna are managed, when they are deciding what to order in a restaurant.