LOS ANGELES Across the vast Pacific, the bluefin tuna carried radioactive contamination that leaked from Japan's crippled nuclear plant to the shores of the United States 6,000 miles away – the first time a huge migrating fish has been shown to carry radioactivity such a distance.
“We were frankly kind of startled,” said Nicholas Fisher, one of the researchers reporting the findings online Monday in the Proceedings of the National Academy of Sciences.
The levels of radioactive cesium were 10 times higher than the amount measured in previous years. Even so, that's far below safety limits set by U.S. and Japanese governments.
Previously, smaller fish and plankton were found with elevated levels of radiation in Japanese waters after a magnitude 9 earthquake in March 2011 triggered a tsunami that badly damaged the Fukushima Daiichi reactors.
But the fallout was not expected to linger in huge fish because such fish can metabolize and shed radioactive substances.
One of the largest and speediest fish, Pacific bluefin tuna can grow to 10 feet and weigh more than 1,000 pounds. They spawn off Japan and swim east to school in waters off California and the tip of Baja California.
Five months after the Fukushima disaster, Fisher, of Stony Brook University in New York, and a team tested bluefin caught off San Diego. Tissue samples from all 15 tuna contained levels of two radioactive substances – cesium-134 and cesium-137 – higher than in previous catches.
To rule out the possibility that the radiation was carried by ocean currents or dropped through the atmosphere, the team also tested yellowfin tuna in the eastern Pacific and bluefin found off Southern California before the crisis. They found no trace of cesium-134 and background levels of cesium-137, from 1960s-era weapons testing.
The results “are unequivocal. Fukushima was the source,” said Ken Buesseler of the Woods Hole Oceanographic Institution, who had no role in the research.
The radioactive cesium came from swimming in contaminated waters and feeding on contaminated prey such as krill and squid, the scientists said. As the tuna moved east, they shed some radiation through metabolism and as they grew. “That's a big ocean. To swim across it and still retain these radionuclides is pretty amazing,” Fisher said.
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