Detectable amounts of cesium-137 and cesium-134 were found in bluefin tuna caught off the coast of California about four months after the Fukushima nuclear disaster in Japan, US scientists reported on Monday.
The timing is important because it shows that migrating fish carried radiation much further and faster than either wind or ocean currents. In addition, of course, the bluefin are a highly prized edible fish found in sushi restaurants all over the world.
Without making a definitive judgment on the safety of the fish, lead author Daniel Madigan of Stanford University's Hopkins Marine Station told Reuters that the amount of radioactive material detected was far less than the Japanese safety limit.
"I wouldn't tell anyone what's safe to eat or what's not safe to eat," she said. "It's become clear that some people feel that any amount of radioactivity, in their minds, is bad and they'd like to avoid it."
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There was about five times the background amount of cesium 137 in the bluefin tuna they tested, but that is still a tiny quantity, Madigan said: 5 becquerels instead of 1 becquerel. (It takes 37 billion becquerels to equal 1 curie; for context, a pound of uranium-238 has 0.00015 curies of radioactivity, so one becquerel would be a truly miniscule proportion.)
The researchers figured that the elevated levels of cesium 137 and all of the cesium 134 they detected came from Fukushima because of the way bluefin tuna migrate across the Pacific.
Bluefin tuna spawn only in the western Pacific, off the coasts of Japan and the Philippines. As young fish, some migrate east to the California coast, where upwelling ocean water brings lots of food for them and their prey. They get to these waters as juveniles or adolescents, and remain there, fattening up.
Judging by the size of the bluefin tuna they sampled - they averaged about 15 pounds (6 kg) - the researchers knew these were young fish that had left Japanese water about a month after the accident.
Most of the radiation was released over a few days in April 2011, and unlike some other compounds, radioactive cesium does not quickly sink to the sea bottom but remains dispersed in the water column, from the surface to the ocean floor.
Fish can swim right through it, ingesting it through their gills, by taking in seawater or by eating organisms that have already taken it in, Madigan said.
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The Guardian adds:
The results "are unequivocal. Fukushima was the source", said Ken Buesseler of the Woods Hole Oceanographic Institution, who played no part in the research.
Japan's chief cabinet secretary, Osamu Fujimura, conceded that the findings suggested the monitoring of radiation levels in fish outside Japanese waters may have to be stepped up. [...]
"We were frankly kind of startled," said Nicholas Fisher, an expert at Stony Brook University in New York who took part in the study. "That's a big ocean. To swim across it and still retain these radionuclides is pretty amazing."
The operator of the Fukushima Daiichi plant, Tokyo Electric Power, estimates that 18,000 terabecquerels of radioactive materials flowed into the Pacific after the accident, either in the form of fallout, or through mixing with water that leaked from the facility. A terabecquerel is equal to 1tn becquerels.
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