Levels of radioactive contamination in fish caught off the east coast of Japan remain raised, official data shows.

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 percent of fish caught close to Fukushima itself are regarded as unfit for humans under Japanese regulations. The respected U.S. marine chemist Ken Buesseler hasreviewed the data in this week’s Science journal.

He says there are probably two sources of lingering contamination. “There is the ongoing leakage into the ocean of polluted ground water from under Fukushima, and there is the contamination that’s already in the sediments 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.”

Buesseler is affiliated to the U.S. Woods Hole Oceanographic Institution. His evaluation covers a year’s worth of data gathered by the Japanese Ministry of Agriculture, Forestry and Fisheries.

Its monthly records detail the levels of radioactive cesium found in fish and other seafood products from shortly after the March 2011 Tohoku earthquake and tsunami – the double disaster that triggered the Fukushima crisis.

The cesium-134 and cesium-137 isotopes 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 cesium 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 cesium 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 cesium counts.

For the Woods Hole Oceanographic Institution researcher, this points to the seafloor being a major reservoir for the cesium pollution.

“It looks to me like the bottom fish, the fish that are eating, you know, crabs and shellfish, the kinds of things that are particle feeders – they seem to be increasing their accumulation of the cesium isotopes because of their habitat on the seafloor,” he explained.

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

And while the 40 percent figure for unsafe catch in the Fukushima prefecture may sound alarming, the bald number is slightly misleading.

Last April, the Japanese authorities tried to instil greater market confidence by lowering the maximum permitted concentration of radioactivity in fish and fish products from 500 becquerels per kilogram of wet weight to 100 becquerels per kilogram.

This tightening of the threshold immediately reclassified fish previously deemed fit as unfit, even though their actual contamination count had not changed.

It is also worth comparing the Japanese limit with international standards. In the U.S., for example, the threshold is set at 1,200 becquerels per kilogram – significantly more lenient than even the pre-April Japanese requirement.

And Buesseler makes the point that some naturally occurring radionuclides, such as potassium-40, appear in fish at similar or even higher levels than the radioactive cesium.

Nonetheless, the contamination question is a pertinent one in the Asian nation simply because its people consume far more fish per capita than in most other countries.

“At one level, there shouldn’t be any surprises here but on another, people need to come to grips with the fact that for some species and for some areas this is going to be a long-term issue; and with these results it’s hard to predict for how long some fisheries might have to be closed,” said the Woods Hole Oceanographic Institution scientist.

Buesseler, with Japanese colleagues, is organizing a scientific symposium in Tokyo on Nov. 12 and 13 to present the latest thinking on Fukushima and its impacts on the ocean. The information will then be shared with the public in a free colloquium on Nov. 14.