Once, the British were only interested in tuna that came in cans. But no more. The market for the fresh fish in Britain is booming. Justin Webster follows a bluefin tuna from freedom in the ocean through the fattening pens of Spain to a restaurant table near you
He swims through the Straits of Gibraltar in the first days of May. She comes from wintering in the seas near Norway, where she grew fat on the abundant supply of pollack and cod. In her mid-20s, she already weighs more than 300kg.

In the spring, the light changes and she travels south. Alongside her, hundreds of other mothers-to-be form a perfectly ordered school. She bursts into the Mediterranean without knowing the high price on her head. Still staying well below the surface, her school veers north towards the Gulf of Lyon. There, they run into a school of males and rise. Amid the turmoil of water, millions of eggs are released and fertilised. By luck, the fish remain unobserved.

Hungry now, her fat levels dwindling and the water becoming hot for this warm-blooded fish, she begins to head back to the ocean. For the first time, a human eye spots her. It is 3 June, 70km from Cabo de Palos on the southern Spanish coast. Flying slowly at an altitude of 800ft, the pilot of the Cesna can see the school clearly - 100 tonnes of prime-quality bluefin tuna. He radios immediately to the fishermen. Tuna swim 10 times their length in a second when feeding, or fleeing. But these enormous sprinters dawdle unsuspectingly until the boats arrive and quietly encircle them with 1.8km of net, which a winch draws tight underneath them.

Usually at this stage, there is no violence but something goes wrong and the fish panic, feeling the net close, and battle against it. One in five die in the struggle. This is not what the fishermen want. The man employing them has issued precise instructions. He has already sent out a sea-cage from the coast.

Our fish is among the survivors which are transferred to the pen. Later, a wooden tag with the number 22 is tied to her serrated tail. She is nearing the end of her life.

Takao Norita could be mistaken for a lost tourist as he hovers around the loading bay inside a warehouse in San Javier, on the baking hot coast of Spain. It is 21 July. His movements are quick, unlike those of the sunburnt workers he manages to joke with even though he does not speak Spanish.

Norita is a Japanese biologist, an expert on tuna. He was involved in pioneering studies into breeding tuna at Kinki University in Japan. But he left in 1995, after his academic mentor died and there were cuts in his projects, and moved to Spain.

Norita's interest in Mediterranean culture is restricted to the cultivation of Thunnus thynnus, the bluefin tuna, his obsession. "That Japanese," says one of the Spanish workers, "he extracted a fish from a 3mm egg and kept it alive for months." The Mediterranean has long been a source of high- quality tuna for the Japanese market. Bluefin is - along with puffer fish - the piece de resistance of sushi and sashimi. As it is eaten raw, all the subtlety of the dish depends on the selection and preparation of the fish, an art elaborated over centuries. But the best-quality tuna, the flesh that can be sold fresh for astronomical prices, has always been difficult to find.

The Japanese have established links with fishermen in Spain and Sicily, where tuna fishing goes back centuries. The spectacular almadraba, the spring tuna slaughter executed with labyrinthine fixed nets and gaffs at Barbate and Zahara de los Atunes near Gibraltar, is said to date back to the Phoenicians. But Mediterranean methods, designed to supply home markets, have a patchy record in satisfying the exacting Japanese.

In the wholesale fish market in Barcelona, they still talk about the pounds 35,000 paid for the most succulent belly cut of a 500kg fish, reputedly bought for the Emperor of Japan. The official record for a whole bluefin at Tsukiji, the Tokyo fish market, was set on 5 January, 1996, when a 238kg fish, caught in Japanese waters, was sold for pounds 41,635.

On that same day, one of Norita's fish, air-freighted in fresh from Spain, sold for only 30 per cent below that record price. The Japanese are increasingly interested in what is called the "live" market - farmed fish slaughtered for immediate consumption. So if they can get it right, the company Norita works for, Bluefin Tuna SL, could be a goldmine.

At first, Norita's project was financed with Japanese money, but because of technical failures, blamed on the inexperience of the Spanish workers, it failed to farm a single fish worth exporting in 1996. Then Joaquin Albaladejo, an ambitious local fish wholesaler, the son and grandson of fishermen, stepped in, spent $2m on the cages and the warehouse - complete with Japanese refrigerators for lower-grade fish - and is now running a smart operation with 200 tonnes of live tuna.

So far the tuna are "ranched" - fattened up, but not bred, in captivity. In the past, the problem with Mediterranean tuna has been that it is not fatty enough. To counteract this, Norita feeds his 2,000 captive fish eight tonnes of Irish mackerel a day, plus crates of squid and pollack.

In the warehouse, Norita fusses over the first carcass. It is our fish, number 22. She weighs 304.5kg and was slaughtered this morning with 10 other similar-sized fish. They are the first farmed tuna to go to market from Spain this season, and represent a crucial test for the Bluefin company.

The best six will be flown for auction in Tokyo; the other five have been pre-sold to Boston. In the office, there is tension over the shipments. There are delays on the usual flights from Madrid via London to Tokyo. Albaladejo decides to eliminate any risks by starting to prepare the fish at the ungodly hour of 2.30am, after they have spent the minimum five hours chilling in ice tubs, so they can catch any available flight. His employees groan with disbelief when he is gone.

The process is supervised by Takashi Katsuma, a severe 32-year-old Japanese man, who selects the best fish for the Japanese market. For two hours, while the Spanish lift the fish out of their tubs, chainsaw off the heads and fins, and clean up the guts, Katsuma takes meat samples - a semi-circle steak from the tail, and a long section extracted with a steel pole, or sashbo - and agonises over his decision. "It needs a lot of confirmation," he explains, hunched over a Styrofoam board onto which the 11 sample groups are arranged, shining a torch at the dark red slabs of meat and peering at them repeatedly.

A slice of number 22 is in first place on the board. The meat is very red, which is good, but is only average on fat content. Number four is a disaster. An ailment has spoiled its colour and left white stains. More fish like this would spell disaster. Number 10 has yake. This is caused if the tuna is not killed instantaneously. It thrashes around, its blood heats up and the meat turns sour. To avoid this, Bluefin's fish are stunned with a mallet, then their small brain is electrocuted with a rod. But Albaladejo's men are still refining their technique, and the killing is sometimes not as neat as it should be.

Katsuma finds no faults with the rest, and grades them, mostly with Bs and Cs, on a scale of A to E. Cleaned, and as neat as fat torpedoes, they are lifted by slings into the "coffins". These are cardboard boxes panelled with Styrofoam, containing a thick polythene bag, and, finally, a silver thermopolar slip packed with ice to keep the fish cool.

By 5am, number 22 has left by lorry for Madrid. In 16 hours, she will be in Tokyo. The result of the sale will give Albaladejo a sign of whether his investment is going to pay off. He has another 200 tonnes to go in the next six months. In fresh tuna Tokyo sets the trend as surely as Wall Street does in finance. Albaladejo's competitor, Ricardo Fuentes, just down the coast, has 800 tonnes in pools. The bottom end of the Japanese market has been severely hit by the financial crisis in the Far East (there are lorry-loads of unsold octopus in Rotterdam and Barcelona; the price has collapsed). But at the top, prices that have so far remained high. The question is, how will the Japanese dealers rate Spanish-farmed bluefin this year?

Tuna number 607, the second (male) fish in our story, is one of 1,000 small bluefins that Albaladejo caged on 18 June. He is being fattened for London. A Japanese dealer who supplies 80 Japanese restaurants in Britain is interested in these fish. He is importing four to five tonnes of bluefin (up to 1,000 fish) a month. But the non-Japanese market has also taken off. Retail sales of fresh tuna have grown 50-fold, from pounds 44,370 in 1995 to pounds 2.26m last year.

John Shelton, a director of J Bennett in Billingsgate, has worked in the fish market for 35 years and seen the trade in fresh tuna boom in recent months. He now brings in four tonnes of fresh tuna a week. "The Japanese really showed us. They know how to buy, what to look for," he says.

European bluefin arrives in 20-tonne lorries from France, Italy or Spain, unloading at Billingsgate at midnight. With relatively small consignments of tuna, the empty space in the lorries is filled with shellfish and whitefish picked up at Rungis, the Paris fish market, or in Boulogne. The suppliers guarantee delivery in 36 hours, and can charge a lower price if they don't pay for air-freight. Once unloaded, the tuna can be kept for up to five days in cold storage before their value starts to drop.

Oriental habits are being picked up. Shelton is having "tuna-corers" - the Japanese sashbo - made in Hawaii to send to his clients. But he doesn't delude himself about the top end of the market. "I don't think anyone here, including myself, would know how to grade tuna for sashimi," he says.

Shelton is now talking to a Japanese trader, T&S Enterprises, about their buying tuna together and grading it at Billingsgate. The Japanese can take the top-grade stock, and the rest can go to British clients. A deal like this might work because European dealers do not yet compete directly with the Japanese. Yellowfin (Thunnus albacares) not bluefin is the tuna most often sold fresh in Britain. As it loses points on fat, colour and size for the sashimi market, it is not bid for so fiercely by the Japanese. Another species, Big Eye (Thunnus obesus) provides fish big enough (25 to 40kg), for cutting fresh steaks, and sells for the same price as yellowfin. (As a top-quality bluefin, number 607 is likely to bypass Billingsgate and go straight to Japanese clients.)

Trading can be stressful. Importers are currently grinding their teeth over new EU regulations. These have cut off the supply of yellowfin from Oman and Yemen, where some of the best vacuum-packed loins and whole fish are from. "We can't supply the market at the moment," groaned the leading tuna dealer in Rungis. "And there is demand for this fish."

Shelton has travelled widely looking for new suppliers since he started bringing in fresh tuna, as an experiment, from Sri Lanka and Bombay eight years ago. He has even been to Tanzania, where the fish are plentiful, and of excellent quality, but the packaging is still too rudimentary. Most of his supply comes from South America, particularly Ecuador.

In Billingsgate, fresh tuna is trading around pounds 3.80 a pound. The highest it has gone is pounds 5 a pound, or about pounds 11 a kilo, still well below the price for sashimi.

Safeway started selling fresh tuna in two outlets two years ago. Waitrose, too, was in the vanguard. Sainsbury and Tesco have followed suit. Tesco now sells fresh tuna in 550 of its supermarkets. The spiralling demand has some dealers worried. "You're dealing with a wild product," says Shelton. "You have to have at least three suppliers because it's seasonal." It is hard enough satisfying the restaurants. Some are even setting up their own supply lines.

Cutty's, the London fish wholesaler, serves fresh tuna at its own restaurant, Bank, in Holborn, supplies the restaurant "10" in the City, and will be opening a new venue, Fish, in London's Borough Market in December. Richard Ross, the chef at "10", had never seen fresh tuna before he came to London from Edinburgh in 1991; now he offers it almost raw to power-lunching financiers. His pepper-crusted tuna is a regular dish. "We serve it rare, unless anyone asks for something different, and everyone is happy with it," he says.

As farmed bluefin, number 607 is unlikely to end up in a tin. This species has become too valuable. Yet even with the fashion for fresh tuna in full swing, it represents a tiny percentage of the millions of tuna that are turned into tins of flaky meat. Unlike the volatile and risky market for fresh tuna, canning is a vast, mechanised industry, bristling with statistics: 55,800 tonnes of canned tuna were eaten in Britain in the year to June, and 88.3 per cent of British households buy it.

"In terms of the world industry, there are four main catching grounds for tuna," says Carl Baxter, senior tuna buyer for Princes, from his office in Liverpool. His list of countries with the biggest canning factories reads like an exotic travel guide - from Senegal to the Solomon Islands. Papua New Guinea is just starting up. Thailand is at the top of the ranking in tonnage canned and gets its raw material from both the Indian Ocean and the western Pacific. Half of the 3.2 million tonnes of tuna caught each year is the bluefin's small relative skipjack (Euthynnus pelamis). Common in the cannable size of 2 to 3.5kg, it is fished by huge freezer ships. This is the fish in tins in British supermarkets.

Princes relaunched its tuna line last year and is now offering yellowfin in olive oil as a premium product. But although a quarter of the world catch is yellowfin, it accounts for less than 5 per cent of British imports. Yellowfin is more expensive, trading in the global market at $1,500 to $1,800 a tonne, compared to skipjack's $1,200. And one of tinned tuna's vital selling points in Britain is that it is cheap.

The official shelf life for tuna in brine is three years, and five years in oil. The tuna's journey from the sea to the supermarket shelf takes about two months. The fish are frozen at sea, defrosted on land, pre-cooked as part of the sterilising process, boned, cleaned, cut into cross sections, "seamed" in cans at a rate of 200 to 300 cans a minute, which are then cooked again for an hour, for safety. The flavour is said to improve after three months when the meat has "settled down".

British importers submit to an independent body, the Earth Islands Institute, to check their canning operations are dolphin-friendly. But there are concerns about the tuna stocks themselves, especially in the Atlantic. The vast expanse of the western Pacific is thought to be under-exploited, but to set up industrial-size operations in remote parts of the world requires heavy investment. As the demand for canned tuna continues to grow, Baxter predicts the price of the tins will have to rise: "Tuna was as low as $650 a tonne two years ago, but I don't think we will see it coming down to $800 to $900 again," he says.

Before I leave San Javier, Norita comes to find me. I go to his office where his wife is waiting. She translates his plans to breed bluefins and release two-month-old fish into the Mediterranean to replenish the stocks. As I watch Norita waiting restlessly for his wife to finish speaking, I realise why he seems out of place. Business is not his primary concern. He wants the company to succeed so he can carry on with his studies.

Later, I call Bluefin SL to find out how the sale in Tokyo has gone. They are disappointed. No 22, our fish, did not make the grade and was flown on for auction in Boston. The six fish auctioned in Tokyo went for an average of Y4,037 a kilo (pounds 16). One went as high as Y6,695, but another for Y2,400. Still, that made a sale of pounds 3,424 for the average-sized fish, and a second consignment has done better.

Albaladejo's price for number 607 in London is now looking high. But he wants to time the sale for December, when there are fewer fish on the market and his small bluefins will be a fatty 45kg. Number 607 may, finally, be carved by a sushi chef, or be seared in Cutty's new restaurant, but for now, he lives. Whatever happens, as one of the world's most expensive fish, he will travel by plane