Levels of radioactive contamination in fish caught off the east coast of Japan remain raised, official data shows.
It is a sign that the Fukushima power plant continues to be a source of pollution more than 18 months after the nuclear accident caused by the March 2011 earthquake and tsunami.
About 40% of fish caught close to Fukushima itself are regarded as unfit for humans under Japanese regulations.
American marine chemist Ken Buesseler, whose review of a year's worth of data gathered by the Japanese ministry of agriculture, forestry and fisheries is published in the latest issue of Science, 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 the BBC.
"With these results it's hard to predict for how long some fisheries might have to be closed. It all points to this issue being long-term and one that will need monitoring for decades into the future."
Figure is slightly misleading
Professor Buesseler notes that although caesium levels in any fish type and on any day can be highly variable, the bottom-dwelling species off Fukushima consistently show the highest counts.
He says that points to the seafloor being a major reservoir for the caesium pollution.
He stresses, however, that the vast majority of fish caught off the north-east coast of Japan are fit for human consumption.
And while the 40% 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.
The tightening of the threshold immediately reclassified fish previously deemed fit as unfit, even though their actual contamination count had not changed.
Less lenient standard than in the US
It is also worth comparing the Japanese limit with international standards. In the United States, for example, the threshold is set at 1200 - significantly more lenient than even the pre-April Japanese requirement.
Nonetheless, the BBC reports, the contamination question is a pertinent one in Japan simply because its people consume far more fish per head than in most other countries.
With Japanese colleagues, Professor Buesseler is organising a scientific symposium in Tokyo on 12-13 November 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 14 November.
