KUMANO, Japan » Thousands of tuna, their silver bellies bloated with fat, swim frantically around in netted areas of a small bay, stuffing themselves until they grow twice as heavy as in the wild.

Is this sushi's future? Tuna raised like chickens or cows?

As the world's love affair with raw fish depletes wild tuna populations, long-running efforts to breed the deep-sea fish from egg to adulthood might finally be bearing fruit. Though the challenges are daunting, the potential profits are huge.

By the end of this year, an Australian company says it will begin selling small amounts of southern bluefin tuna hatched in its fishery. A Japanese firm breeding the more prized Pacific bluefin tuna hopes to start sales in 2013.

Whether tuna farming will become viable on a large scale remains an unanswered question. Tuna are much harder to rear than the widely farmed salmon and shrimp. They are large and need room to swim. They spawn only under certain circumstances. In some experiments, fewer than 1 percent of the babies survive. And those that do eat so much that they could wipe out other fish species.

The bulk of the tuna farmed today is not bred from eggs; it is caught in the sea and fattened on farms, which does nothing to save nature's dwindling stock.

But Japan's biggest seafood company, Maruha Nichiro Holdings Inc., is bullish on tuna. Maruha operates several tuna farms, including the one here in Kumano, a small coastal city in western Japan. Here, in a small bay, the fish live in netted sections mostly 160 by 260 feet, smaller than a football field.

“;For years everyone assumed it was impossible to breed tuna on farms,”; says Takashi Kusano, a general manager who has worked for 20 years on cultivating tuna.

Japanese consume 80 percent of the world's Atlantic and Pacific bluefin tuna, the two species most sought after by sushi lovers.

The survival rate for hatched Pacific bluefin is about 0.4 percent of the 28 million eggs collected for tests at Maruha's farms. Another effort, at Japan's Kinki University, has achieved a 6 percent survival rate.

Those numbers sound low, but one tuna lays tens of millions of eggs and the survival rates are improving.

“;I had to solve the puzzle of why our fish kept dying,”; recalls Kusano.

Unlike other fish, which can pump oxygen better through their mouths, tuna must swim continuously at up to 50 mph to absorb oxygen through their gills.

Baby fish, which are not developed enough to brake or steer, often die ramming into the nets that cordon off tuna farms in coastal waters.

Learning about tuna diseases and dietary habits took years of trial and error, Kusano said.

A handful of tuna that Maruha has produced are set to lay eggs next year, a sign that the full life cycle may be finally completed.

Kinki University has already done that, producing 40,000 Pacific bluefin babies this year from eggs laid by tuna on its farms, up from 10,000 last year.

Even if the hurdles to a full life cycle are cleared, other concerns remain, such as the tuna's voracious appetite.

“;Bluefin tuna are like lions and tigers,”; said Mike Hirshfield, chief scientist at Oceana, an advocacy group for the world's oceans. “;They are at the very, very top of the food chain. And they eat other fish. What you are doing is catching wild fish to create bluefin tuna. The anchovies, the sardines and the herrings are already fished to the max.”;

That raises ethical questions about feeding tuna with relatively cheap fish that are needed by people in developing countries, Hirshfield said.

Maruha's answer is a tuna feed, which it patented in 2006, made of fishmeal mixed with oils and nutrients and looking like brown sausages.

The company says its feed is less polluting, fattens tuna three times faster than feeding them small fish, uses fish that are not eaten by people, and can be stored at room temperature, slashing energy needs. Eventually, Maruha hopes to develop a vegetarian tuna feed.

Wild tuna still commands a premium over farmed tuna. Farmed tuna's disadvantage is that “;it doesn't have a fish taste, and its color is almost white,”; said Kazuo Sato, 56, who has run a sushi shop outside of Tokyo for 31 years. But, he added, “;we can't be relying just on natural tuna these days, and there are bound to be improvements in farmed tuna.”;

At Kinki University, Osamu Murata, head of research, says, “;It's our mission to spread to the world our knowledge about producing man-raised tuna that doesn't rely on nature's resources.”;

In Australia, Clean Seas Tuna worked with Kinki to overcome such problems as cannibalism and young tuna crashing into tank walls, the company said.

And Hawaii regulators have approved the world's first commercial farm for bigeye tuna.