TONY EASTLEY: Japan's government is disputing a study by an international research group which suggests radiation from the crippled Fukushima nuclear plant is still entering the food chain.

Last month the US-based Woods Hole Institution revealed that about 40 per cent of the fish caught off Fukushima is contaminated with radioactive caesium above the government's limit.

Japan's Fisheries Agency insists the contamination is sinking deeper into the seabed and is not entering the food chain.

The nuclear plant operator TEPCO denies any tainted water is leaking from the facility.

With more here's North Asia correspondent Mark Willacy.

(Sounds of a fishermen and boats)

MARK WILLACY: It's an unfortunate name for a fishing boat from Fukushima, and the irony isn't lost on the skipper of the Lucky Treasure.

(Kozo Endo speaking)

"We can't sell any of these fish, it's such a waste," says Kozo Endo. "We can only catch them for radiation sampling. Those that are left over - well, all of us working on the boat take them home to eat," says the 52-year old fisherman.

Once Kozo Endo's haul of halibut, cod, and greenling would have been bound for market and the crew of the Lucky Treasure would be living up to their boat's name.

But a recent study by the Woods Hole Oceanographic Institute in the United States suggested that in the 12 months since the Fukushima nuclear meltdowns about 40 per cent of fish caught off the coast there contained radioactive caesium above the government's safe limit.

This would in turn suggest that radioactive particles have accumulated on the sea floor and are still entering the food chain.

The report's author is Ken Buesseler.

KEN BUESSELER: There has to be a source, and they're cooling those reactors quite extensively. Some of that water is getting back into the ocean, either actively being pumped out after some decontamination or through leaks in the building, so not able to contain all of the water that they use to cool.

MARK WILLACY: There is no doubt that TEPCO, the operator of the Fukushima nuclear plant, has used millions of litres of water to keep the overheating reactors under control and some of that water, especially in the early days of the crisis, leaked into the sea.

(Yoshimi Hitosugi speaking)

"We're doing surveys of the seabed and looking at the things fish eat, such as small organisms like shrimp, but so far we have not found very high levels in either," says TEPCO spokesman Yoshimi Hitosugi. "So at the present time we are not in a position to clearly understand why there is still radiation being found in fish off Fukushima," he says.

One theory is that radioactive particles deposited on hills and mountains during the meltdowns are slowly washing into rivers and estuaries, and are then being flushed out into the Pacific before settling on the seabed, where it is absorbed by bottom-feeding fish like greenling.

But Japan's Fisheries agency says it is not enough to contaminate large numbers of marine species.

(Koichi Tahara speaking)

"We think the caesium is gradually absorbed into the seabed, and once it's absorbed it becomes more difficult for it to enter back into living organisms," says fisheries agency researcher Koichi Tahara.

But if you're a fisherman like Lucky Treasure skipper Kozo Endo, the damage has already been done and with 36 species of fish still banned from sale, that means his boat is hardly living up to its name.

This is Mark Willacy in Tokyo for AM.