It is the highest contamination recorded in marine life so far.
The operator TEPCO recently measured radiation in 20 kinds of fish and shellfish caught along the coast.
It says it detected 38,000 becquerels per kilogram of radioactive caesium in a rock trout caught about a kilometre from Minamisoma City, north of the shattered plant.
TEPCO says it also found high levels of caesium in nine other fish species.
In March 2011 an earthquake and tsunami triggered meltdowns in reactors at the plant. ABC
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Bluefin tuna contaminated from the nuclear catastrophe have been found off the California coast.
Octopus and snail fished from waters near Fukushima showed little cesium contamination and was put out for public sale, but the samples were boiled before testing, potentially compromising the data. Cesium is volatile and boiling could have removed it from the fish flesh. Reports vary on whether all fish was boiled before being put out for sale or whether just the samples were boiled. Of course, the cesium would have to go somewhere, like the boiling water or the steam. Because the samples were boiled, it is unclear how contaminated the fish might have been right out of the ocean. Cesium is only one radionuclide of concern in fish. Strontium-90, which can concentrate in fish bones, is a danger in the waters off Fukushima particularly because a lot if washed into the ocean during the ongoing catastrophe.