Since May, the environmental watchdog has sent its flagship, Rainbow Warrior II, trolling around the Mediterranean to call attention to this overfishing -- a state of affairs corroborated by scientists unrelated to Greenpeace.

The vessel this week was off Spain, whose Mediterranean coast is one of the critical spawning grounds for this fish that was already a favorite in ancient Rome.

"The facts are sobering," Greenpeace warned in a report at the start of its campaign. "In short, the commercial extinction of bluefin tuna from the Mediterranean Sea is just around the corner."

The organization's website sites three main culprits: "overfishing, tuna ranching and the high market demand for one of the most valuable species of tuna worldwide."

In Paris, Philippe Gros, director of fish research at France's Research Institute for Exploitation of the Sea (IFREMER), conceded that this "infatuation with sushi" -- which has moved way beyond Japan's borders and today is trendy fare in most western capitals -- has led to the market running out of control.

"The species is being exploited in an unsustainable manner," Gros said The base price for bluefin tuna is 30 to 40 dollars (23-31 euros) per kilo, while in Japan the price easily reaches 100 dollars and top quality bluefin can even fetch 500 dollars.

Such high prices have prompted the creation of a massive bluefin ranching industry in the warm waters of the Mediterranean -- the natural breeding ground for this species -- that organizations such as Greenpeace charge are helping to circumvent quotas and deplete rather than manage stocks.

Bluefin tuna reproduce slowly and not in the so-called tuna ranches or farms; the species does not reproduce well in captivity, scientists say.

So the ranches send out ships to three sites in the Mediterranean where bluefins are known to spawn their young. These young are captured and towed gently, to avoid mortality, in giant cages back to the fish farms.

There, they are "fattened" up for seven to eight months to meet the taste and texture standards prized by the Japanese.

The Greenpeace report in May said today "virtually all bluefin tuna from the Mediterranean are exported to Japan".

According to IFREMER the industry has become highly integrated from start to finish, from the huge fishing ships to the fisheries to the distribution of tuna to markets.

"The financial interests are enormous," said Francois Provost, who heads up Greenpeace France's Ocean campaign.

These include not only gains but also funds injected into the sector. Environmentalists, for instance, are quick to point the finger at European Union subsidies to support the bloc's struggling fishing industry as a factor in helping deplete bluefin stocks.

And the high stakes can spark tension -- as the Rainbow Warrior learned on its current tour. This week it was refused a berth in the Spanish port of Cartagena, and earlier this month it found not only its permission revoked by port authorities when it tried to dock at the southern French port of Marseille but also its path there blocked by industrial tuna fishing boats.

The International Commission for the Conservation of Atlantic Tunas (ICCAT) has set an annual fishing quota of 32,000 tonnes, and official statistics are in line with the quota. But experts believe that the actual catch has been more than 50,000 tonnes a year for the last decade, double the 25,000 tonnes that IFREMER estimates to be the sustainable level of fishing. "Countries are competing to exploit stocks in a situation of overcapacity and attractive commercial returns," according to a study by the institute.

In such a situation "the over-exploitation is difficult to counteract by a system based on restricting catches, which in turn creates an incentive to underreport," it concluded.

In addition, the young tuna captured alive and towed for fattening on tuna ranches is not declared as part of a catch, according to IFREMER.

The Worldwide Fund for Nature (WWF) warned recently that bluefin tuna stocks in the East Atlantic and Mediterranean are being stripped bare by "illegal and unscrupulous" practices and called for an immediate halt to fishing.

"Atlantic bluefin tuna stocks risk imminent commercial collapse," said the author of the WWF report, Roberto Mielgo Bregazzi, director of Advanced Tuna Ranching Technologies.

"In the race to catch shrinking tuna stocks, industrial fleets are switching from traditional fishing grounds to the last breeding refuges in the eastern Mediterranean and Libyan waters," he warned.

The WWF report also alleged deliberate misreporting and laundering of bluefin tuna catches, with unreported catches increasingly being slaughtered and processed at sea before being shipped out on board enormous vessels destined for the lucrative Japanese market.

Greenpeace charges that both governments and the ICCAT have failed to enforce protection measures. Like the WWF, it is calling for the immediate closure of Mediterranean bluefin tuna fishery until "new management measures that guarantee the future of the fishery are put in place".

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