Trace amounts of radioactive cesium released from the crippled Fukushima No. 1 power plant have been found in bluefin tuna caught off the U.S. West Coast, according to a study in the Proceedings of the National Academy of Sciences published online Monday.
While the amounts are not considered a health hazard, the finding reflects the magnitude of the disaster, which released massive amounts of radiation into the air and sea.
A Stanford University researcher and two others wrote that they have found "unequivocal evidence" that the tuna transported radioactive cesium released in the disaster across the Pacific Ocean after the powerful earthquake and tsunami crippled the power plant in March last year.
According to the study, the isotopes cesium 134 and 137 were found in 15 bluefin tuna caught off San Diego, California, last August. The cesium 134 reading was 4 bequerels per kilogram and cesium 137 reading was 6.3 bequerels per kg. Cesium 134 has a half-life of two years and cesium 137 a half-life of 30 years.
The tuna are believed to have been near Japan when the meltdowns occurred and migrated to the United States on ocean currents.
In comparison, no cesium 134 was found in tuna caught in 2008, while a tiny amount of cesium 137 that would naturally exist in the environment anyway was found in them.
Tokyo Electric Power Co., which runs the coastal power plant in Fukushima Prefecture, estimates that 900,000 terabequerels of radioactive material were released into the atmosphere and that 18,000 terabequerels of it entered the ocean. A terabequerel is 1 trillion bequerels.