By Chris Hogg

BBC News, Tokyo



The tanks are specially designed to keep the tuna happy

The problem is that, like many other species, stocks of the fish are declining.

The situation is going to get a lot worse as other populous countries such as China are developing a taste for sushi and sashimi, which is what most of the blue-fin are used for.

The species is hard to cultivate because it is difficult to recreate the conditions they are used to in the wild.

The result can be seen at Tokyo's Tsukiji fish market, where the frozen tuna carcasses are laid out on the floor ready for the auction's fierce bidding.

A single giant tuna can cost you more than a new car.

But in the city of Shizoka, in a small shed on a university campus, a businessman is trying to recreate the oceans that the tuna are used to.

Blue-fin tuna have been farmed before, but not indoors, the team behind the project says.

'Magic water'

To get to where these tuna are housed, you have to go through a kind of air lock designed to stop any of the outside light getting in to the dimly-lit interior.

Tuna, it seems, are rather sensitive to daylight, and to pretty much anything else. That is why they are coddled and protected from as much of the outside world as possible.

For Mr Yamamoto. the tuna breeding project is a labour of love

Inside, in the gloom, there are four large circular tanks, each with a diameter of about five metres (16 feet).

The water pumped up from deep down under the surface is just about as clean as you can get - no bacteria, no viruses and no parasites.

Akito Yamamoto, the man behind the project, calls it "magic water" - not too hot and not too cold; a constant 21C (70F) which is just right for vigorous tuna to swim around in.

The water flows in circles in each tank - creating an effect like a treadmill for the 15 fish in each tank.

They need to keep moving to keep breathing. Normally well-travelled fish, they are capable of crossing the Atlantic in less than 50 days.

Here, though, in the confines of the tank, every detail has been designed to keep them happy and healthy.

The tuna have to be shown where to swim, so there are streams of bubbles flowing away from the edges of the tanks which guide the tuna away from the sides.

All this equipment has cost thousands of dollars, and has been developed specially, with no guarantee that it will actually work and produce full-size healthy tuna.

The blue-fin will not be big enough to breed or be eaten for at least three years.

"I know some people are puzzled about why I am spending so much on this," said Mr Yamamoto. "But I am trying to make a facility that could be used for 10 or 20 years."

"However much we spend it's worth it if we can provide safe food for consumers."

Wild vs farmed fish

This is clearly a labour of love, but how will he feel when the time comes to send his fish to the market to be slaughtered for the first time?

"It will be like sending my daughters off to get married," he says with a grin. "Joy and sadness." But will he be eating them? "Definitely!"

The risk, of course, is that the farmed blue-fin tuna will not be as tasty as the wild ones.

At a nearby sushi restaurant the chef Yutaka Kuroda skilfully fillets large pieces of tuna, cutting the delicate slices for the lunchtime orders.

He says it will be hard to persuade his customers to make the switch.

"The quality of farmed tuna is improving, mainly because they feed them better," he said. "I don't think it tastes all that bad, but still most Japanese people believe a wild tuna tastes better than a farmed one."

The reality is of course that in the end the Japanese may have no choice. Sushi from farmed fish could one day be the only option on the menu.

The huge appetite for fresh wild fish today may mean that tomorrow there is none left in the oceans, so farmed blue-fin tuna may be a taste they have to get used to.