Fish auction prices at a port south of the Fukushima Daiichi nuclear power plant have fallen amid uncertainty over how seafood consumers will respond to the release of treated and diluted radioactive wastewater into the ocean.

The plant, which was damaged in the 2011 earthquake and tsunami, began sending the treated water into the Pacific on Thursday amid protests at home and in nearby countries that are adding political and diplomatic pressures to the economic worries.

Hideaki Igari, a middleman at the Numanouchi fishing port, said prices of flounder, Fukushima’s signature fish known as Joban-mono, were more than 10 per cent lower at the Friday morning auction, the first since the water release began.

The decades-long release has been strongly opposed by fishing groups and criticised by neighbouring countries.

China immediately banned imports of seafood from Japan in response, adding to worries in the fisheries community and related businesses.

A citizens’ radiation testing centre said it was fielding inquiries and expected more people might bring in food, water and other samples with radiation data now a key barometer in deciding what to eat.

‘Prices may crash again’

Japanese fishing groups fear the release will do more harm to the reputation of seafood from the Fukushima area. They are still striving to repair the damage to their business from the meltdown at the power plant after the earthquake and tsunami.

“We now have this water after all these years of struggle when the fish market price is finally becoming stable,” Igari said after Friday’s auction.

“Fisheries people fear that prices of the fish they catch for their living may crash again, and worry about their future living.”

The Japanese government and the plant’s operator, Tokyo Electric Power Company Holdings, say the water must be released to make way for the facility’s decommissioning and to prevent accidental leaks of insufficiently treated water.

Much of the tank-held water still contains radioactive materials that exceed release levels.

Some wastewater at the plant is recycled as coolant after treatment, and the rest is stored in about a thousand filled to 98 per cent of their 1.37 million-ton capacity.

The tanks cover much of the complex and must be cleared out to make room for new facilities needed for the decommissioning process, officials say.

Authorities say the wastewater after treatment and dilution is safer than international standards require and its environmental impact will be negligible.

On Friday, the first seawater samples collected after the release were significantly below the legal release levels, the power company said.

Having suffered a series of accidental and intended releases of contaminated water from the plant early in the disaster, hard feelings and distrust of the government and TEPCO run deep in Fukushima, especially in the fishing community.

30-year window

There are worries that the release, which TEPCO says will take 30 years or until the end of the plant decommissioning, could mean a tough future for younger people in the fishing town where many businesses are family-run.

Fukushima’s current catch already is only about one-fifth of its pre-disaster level due to a decline in the number of fishermen and decreases in catch sizes.

The government has allocated 80 billion yen ($A859 million) to support fisheries and seafood processing and combat potential reputation damage by sponsoring campaigns to promote Fukushima’s Joban-mono and processed seafood.

TEPCO has promised to “appropriately” deal with reputational damage claims and those hurt by China’s export ban.

Tetsu Nozaki, head of the Fukushima prefectural fisheries co-operatives, said in a statement on Thursday the fishing community’s worries would continue for as long as the water was released.

“Our only wish is to continue fishing for generations in our home town, like we used to before the accident,” Nozaki said.

Fish prices depend on the sentiment of wholesalers and consumers in the Tokyo region, where large portions of Fukushima’s catch go.

At the Friday auction at the Numanouchi port, the price for flounder was down from its usual level of about 3500 yen ($A37) per kilogram to about 3000 yen ($A31), said Igari, the middleman.

“I suspect the result is because of the start of the treated water release from the Fukushima Daiichi and fear about its impact,” he said.

Igari said the discharge was discouraging but hoped careful testing could prove the safety of their fish.

“From the consumers’ point of view about food safety at home, I think the best barometer is data,” he said.

At Mother’s Radiation Lab Fukushima in Iwaki, a citizens’ testing centre known as Tarachine, tests were being conducted on water samples, including on tritium levels for seawater the lab collected from just off the Fukushima Daiichi plant before the release.

Lab director Ai Kimura said anyone could bring in food, water or even soil, although the lab had big backlogs because testing took time.

-AAP