米国沖のマグロから微量セシウム / 原発事故の影響と発表
U.S. Tuna Has Fukushima Taint
Wall Street Journal, Updated May 29, 2012, 12:52 p.m.ET
Pacific bluefin tuna migrating last year from coastal Japan to the waters off Southern California contained radioactive cesium isotopes from the Fukushima nuclear disaster, scientists reported Monday.
The amount of radioactivity in the fish was one-tenth the level the U.S. and Japan consider dangerous, and likely posed no public-health hazard or risk to people who ate the seafood, the scientists said. But the study showed for the first time that migrating sea life rapidly brought traces of radioactive elements from the damaged Fukushima Daiichi nuclear reactors across vast distances.
Mildly radioactive Pacific bluefin tuna migrated from Japan to the coastal waters of Southern California last year, carrying in their tissue cesium isotopes released by the Fukushima nuclear disaster
“The tuna packaged it up and brought it across the world’s largest ocean,” said marine ecologist Daniel Madigan at Stanford University, who led the study team. “We were definitely surprised to see it at all and even more surprised to see it in every one we measured.”
Their findings raise the possibility that other wide-ranging sea life that foraged near Japan, such as turtles, sharks and seabirds, may also have carried low levels of radioactive cesium from the accident around the Pacific basin. The scientists expect to conduct more tests on migrating bluefin as well as albacore tuna, sea turtles, and several shark species this summer.
Their research was published Monday in the Proceedings of the National Academy of Sciences.
Prized as a sushi delicacy in Japan and around the world, Pacific bluefin tuna spawn in the Sea of Japan, among other locales. As they grow, the fish usually travel around the southern tip of Japan and follow the Kuroshio Current up the country’s east coast, past the scene of the nuclear accident, before migrating more than 6,000 nautical miles to the eastern Pacific. The fish eventually return to their birth waters to spawn.
In their study, Mr. Madigan and his colleagues tested tissue from 15 young Pacific bluefin tuna caught by recreational fisherman off the coast of San Diego in August 2011, about five months after an earthquake and a tsunami severely damaged the Fukushima reactors, triggering the largest known accidental release of radioactivity into the ocean.
For weeks after the accident, levels of radioactivity were up to 10,000 times normal in the coastal waters off eastern Japan, where the bluefin tuna spend their early life before migrating across the ocean.
In the young bluefin tuna that reached California, the researchers found slightly elevated levels of cesium-137 and cesium-134, two primary products of nuclear fission that tend to concentrate in muscle tissue. The amount of cesium 137 was five times as much as the background level, leftover from nuclear-weapons testing decades ago. Prior to the Fukushima accident, cesium-134, which has a half-life of about two years, was undetectable in seawater or marine life.
Overall, the levels were just enough to raise the naturally occurring radioactivity of the fish by about 3%, the scientists said.
“We found that absolutely every one of them had comparable concentrations of cesium-134 and cesium-137,” said marine biologist Nicholas Fisher at Stony Brook University in New York state, who was part of the study group. “It is crystal-clear data.”
The researchers also tested tissue from yellowfin tuna caught at the same time in August 2011 and tissue preserved from bluefin tuna caught in 2008, three years before the nuclear accident. Yellowfin tuna typically spend their entire lives off the coast of California and Mexico’s Baja Peninsula.
In both the yellowfin and the tissue of the 2008 bluefin, the scientists didn’t find any cesium-134 and detected only the expected background levels of cesium-137.
Write to Robert Lee Hotz at email@example.com
A version of this article appeared May 29, 2012, on page A6 in the U.S. edition of The Wall Street Journal, with the headline: Tuna Carry Fukushima Taint.