It is a sign that the Dai-ichi power plant continues to be a source of pollution more than a year after the nuclear accident.

 

About 40% of fish caught close to Fukushima itself are regarded as unfit for humans under Japanese regulations.

 

The respected US marine¹ chemist Ken² Buesseler has reviewed the data in this week's Science journal.

 

He says there are probably two sources of lingering contamination.

 

"There is the on-going leakage³ into the ocean of polluted ground water from under Fukushima, and there is the contamination that's already in the sediments⁴ just offshore," he told BBC News.

 

"It all points to this issue being long-term and one that will need monitoring for decades into the future."

 

Prof Buesseler is affiliated⁵ to the US Woods Hole Oceanographic Institution (WHOI). 

 

His evaluation⁶ covers a year's worth of data gathered by the Japanese Ministry⁷ of Agriculture, Forestry⁸ and Fisheries (MAFF).

 

Its monthly records detail the levels of radioactive caesium found in fish and other seafood⁹ products from shortly after the March 2011 Tohoku earthquake and tsunami¹⁰ - the double disaster that triggered the Fukushima crisis.

 

The caesium-134 and 137 isotopes¹¹ can be traced directly to releases from the crippled power station. 

 

MAFF uses the information to decide whether certain fisheries along five east-coast prefectures, including Fukushima, should be opened or closed (it is not a measure of contamination in actual market fish).

 

The caesium does not normally stay in the tissues of saltwater fish for very long; a few percent per day on average should flow back into the ocean water. So, the fact that these animals continue to display elevated contamination strongly suggests the pollution source has not yet been completely shut off.

 

He notes that although caesium levels in any fish type and on any day can be highly variable, it is the bottom-dwelling species off Fukushima that consistently show the highest caesium counts. 

 

For the WHOI researcher, this points to the seafloor being a major reservoir for the caesium pollution.

 

"It looks to me like the bottom fish, the fish that are eating, you know, crabs¹² and shellfish, the kinds of things that are particle feeders - they seem to be increasing their accumulation of the caesium isotopes because of their habitat on the seafloor," he explained. 

 

Prof Buesseler stresses however that the vast majority of fish caught off the northeast coast of Japan are fit for human consumption.

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