Now a system of corralling the fish into 'tuna ranches' has combined with a growing tuna fishing fleet to bring stocks dangerously close to collapse, warn scientists from ICCAT -- the body established by bluefin fishing countries to monitor the stock.

"There are plenty of signs that we might be seeing the start of the collapse," said Susana Sainz, a fisheries officer with campaign group WWF.

The environmental impact would be catastrophic, she said: "The bluefin is a top predator so the whole ecology of the Mediterranean would be destabilised."

Tuna has become a big business throughout the Mediterranean, and the lure of up to US$15,000 for the best and biggest fish attracts dozens of new boats to the industry every year -- many controlled by Asian and Italian mafias, sources say.

That in turn depresses prices and compels fishermen to break catch limits.

"IT'S OVER"

Over-exploitation, pollution and climate change have devastated many of the world's commercial fish stocks and campaigners say a UN agreement to restore them by 2015 fails to set strong enough targets.

Some campaigners say it may already be too late to save the bluefin after high-tech fleets -- many guided by illegal spotter planes -- this season converged on an area near Libya that had been considered one of its last refuges.

"It's over, that's my gut feeling from both a stock point of view and a business point of view," said Roberto Mielgo Bregazzi, a fisheries consultant who set up the first 'tuna ranches' 10 years ago.

The ranches -- giant underwater cages where freshly caught tuna are fattened on squid and sardines -- have revolutionised the industry.

The innovation allows fishermen to scoop up shoals of spawning tuna, transfer them to the 50-metre-wide cages and return to fish until the last is caught.

Deep-frozen and shipped to Asia, the bulbuous carcasses are sold in auction rooms like Tokyo's Tsukiji market before the red meat is sold for up to US$75 per 100 grams when served in the city's best restaurants.

Crespo said he soon felt the impact of tuna ranching on his and other fixed trap nets known as 'Almadrabas' -- a labyrinth of nets that fishermen have anchored in the shallows of Spain's south Atlantic coast since pre-Roman times.

"For the last seven or eight years we've seen a drought in the catch," Crespo said as he walked between mountains of net and cable spread about his warehouse in the fishing town of Barbate, on the Costa de la Luz, south of Cadiz.

His employees spend two months setting up the complex system, then wait for the tuna to arrive in April. Every few days fishermen corral the net and hoist it to the surface, while others jump into the thrashing mass of silvery fish to hook and haul them aboard.

Although tourism is developing fast along the coast, Spain's four Almadrabas remain the key employer in towns like Zahara de los Atunes -- or 'Zahara of the Tunas'.

Similar to those in Morocco across the Straits, Spain's Almadrabas once caught up to 2,500 tonnes a year but took just 1,300 tonnes of fish in an abnormally short season this year.

"Hardly any fish over 200 kg showed up this year," he said.

Last year's 1,000-tonne catch plunged Crespo's firm to an 800,000 euro (US$1.13 million) loss and he said some firms may go under.

"It's not right that a resource that has sustained thousands of families for 3,000 years should be finished off by a new technology in 10 years," said Crespo.

DISPUTE

Gerald Scott, the American chairman of ICCAT's scientific committee, said he estimated just 6 percent of the original stock of Mediterranean bluefin remained.

"When you are down at very low biomass levels all it takes is one or two bad years to start the downward spiral from which it would difficult to return," he said.

"We haven't necessarily seen a rapid and drastic decline yet. The point is once you have, it is probably too late."

For almost a decade, fishermen in the Mediterranean have smashed quotas, taking around 50,000 tonnes a year, says Scott.

What worries Roberto Mielgo, who has visited ranches from Croatia to Spain in recent months, is that the industry could barely fill this year's quota and the fish are smaller.

"The Japanese traders are telling me most of the Libyan tuna are less than 100 kg. When I first ranched there 10 years ago, we were pulling out 500 kg monsters," he said.

But there are conflicting views about both the gravity and causes of the problem. David Martinez, a director at the Mediterranean's biggest tuna rancher, Ricardo Fuentes & Sons, said bad weather was the main factor in the smaller catch.

"Everyone has to stick to their quotas, but if they do that there won't be a problem with for the bluefin tuna," he said.

Critics, including the United States, say European countries that control ICCAT (the International Commission for the Conservation of Atlantic Tuna), should take much responsibility for setting quotas at twice the level their own scientists recommend and failing to enforce them.

The Commission did take action last month, banning bluefin fishing for the rest of 2007 and threatening Greece, Malta, Portugal, Spain, Italy, France and Cyprus with court action if they could not prove they were not over-fishing.