The Fukushima Daiichi nuclear disaster was a series of equipment failures, nuclear meltdowns and releases of radioactive materials at the Fukushima nuclear power plant in northeastern Japan, following a devastating earthquake and tsunami on 11 March 2011 which claimed nearly 19,000 lives. It is the largest nuclear disaster since Chernobyl in 1986 and only the second disaster to measure Level 7 on the International Nuclear Event Scale.
Japan's fish still radioactive from Fukushima nuclear disaster
A marine chemist's report shows 40pc of bottom-dwelling catch is not safe to eat, with toxicity levels not declining as they should
Julian Ryall in Tokyo
As the sun comes up over Tokyo's Tsukiji fish market, it is business as usual. Cranes are lifting the carcasses of rime-covered tuna from the holds of battered fishing boats.
Traders are using their own sign language to purchase crates of skipjack, shellfish and seaweed. And there is a steady flow of trucks leaving the market, carrying maritime produce to wholesalers, restaurants and supermarkets across Tokyo and the surrounding Kanto region.
The pace is frenetic, and the trading at times frantic.
Only no one is uttering the thoughts that must be on the minds of everyone connected with Japan's fisheries: how much of the catch coming through the world's largest fish market is contaminated with radioactivity from the disaster that befell the Fukushima Dai-ichi nuclear plant on March 11 last year?
Those thoughts will have been brought to the fore - but remained unspoken - after a marine chemist at the Woods Hole Oceanographic Institute in Massachusetts published an article in the journal Science, which showed levels of caesium in fish caught off northeast Japan are not falling more than one year after the disaster.
In the article, Ken Buesseler used Japanese government samplings of caesium-134 and caesium-137 and concluded that 40 per cent of bottom-dwelling fish are still above official safety levels for consumption.
Most worryingly, the levels are showing no signs of declining over time, which means either that more contamination than previously believed has been absorbed into the seabed and continues to affect the entire food chain or, as Buesseler believes, that radioactive water is still seeping out of the plant and into the ocean.
Either scenario is huge cause for concern.
A spokesman for Tsukiji insists it is "business as usual", but then deflects any further inquiries about nuclear contamination to the government.
The radiation from Fukushima is also affecting stocks of freshwater fish, with the Environment Ministry announcing in July that as much as 2,600 becquerels of caesium had been found in 23 varieties of fish caught in rivers and lakes in the prefecture.
The Fisheries Agency under the Ministry of Agriculture, Forestry and Fisheries says it is doing all it can to ensure contaminated fish are not sold - either domestically or for export.
"Since the accident, we have been monitoring how much contamination has occurred in fish, in cooperation with local municipalities and the industry," Haruno Tominaga, a spokesman for the agency, told the South China Morning Post.
"If we find fish that exceed the limits that were set in April, of 100 becquerels per kilogram, then we suspend shipments."
Tominaga said the monitoring that was being carried out "prohibits" contaminated fish being sold.
Unsurprisingly, not everyone is convinced - particularly when news reports suggest there are flaws in the net to catch contaminated fish.
In June, for example, Chinese quarantine authorities detected elevated levels of cadmium in a shipment of saury from Japan that posed a "grave danger to health", according to China National Radio. The shipment was returned to Japan.
"I do not believe the regulations are strict enough, so fish that is contaminated with up to 100 becquerels per kg of radiation is being sold all over Japan," said Kazue Suzuki, a nuclear campaigner with Greenpeace Japan.
"I feel that level is just too high, especially for children," she said, pointing out that the World Health Organisation sets a limit of 1 becquerel per kg of water.
"Personally, I'm not buying as much fish as I used to and I think many people in Japan are doing the same," she said.