LOS ANGELES, Calif. - Across the vast Pacific, the mighty 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 tuna off the California coast in previous years. But even so, that's still far below safe-to-eat limits set by the 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 Dai-ichi reactors.

But scientists did not expect the nuclear fallout to linger in huge fish that sail the world 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 the Japan coast and swim east at breakneck speed to school in waters off California and the tip of Baja California, Mexico.

Five months after the Fukushima disaster, Fisher of Stony Brook University in New York and a team decided to test Pacific bluefin that were caught off the coast of San Diego. To their surprise, tissue samples from all 15 tuna captured contained levels of two radioactive substances — ceisum-134 and cesium-137 — that were higher than in previous catches.

To rule out the possibility that the radiation was carried by ocean currents or deposited in the sea through the atmosphere, the team also analyzed yellowfin tuna, found in the eastern Pacific, and bluefin that migrated to Southern California before the nuclear crisis. They found no trace of cesium-134 and only background levels of cesium-137 left over from nuclear weapons testing in the 1960s.

The results "are unequivocal. Fukushima was the source," said Ken Buesseler of the Woods Hole Oceanographic Institution, who had no role in the research.

Bluefin tuna absorbed radioactive cesium from swimming in contaminated waters and feeding on contaminated prey such as krill and squid, the scientists said. As the predators made the journey east, they shed some of the radiation through metabolism and as they grew larger. Even so, they weren't able to completely flush out all the contamination from their system.

"That's a big ocean. To swim across it and still retain these radionuclides is pretty amazing," Fisher said.

Pacific bluefin tuna are prized in Japan where a thin slice of the tender red meat prepared as sushi can fetch $24 per piece at top Tokyo restaurants. Japanese consume 80 per cent of the world's Pacific and Atlantic bluefin tuna.

The real test of how radioactivity affects tuna populations comes this summer when researchers planned to repeat the study with a larger number of samples. Bluefin tuna that journeyed last year were exposed to radiation for about a month. The upcoming travellers have been swimming in radioactive waters for a longer period. How this will affect concentrations of contamination remains to be seen.

Now that scientists know that bluefin tuna can transport radiation, they also want to track the movements of other migratory species including sea turtles, sharks and seabirds.

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Online:

Journal: http://www.pnas.org

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Follow Alicia Chang at http://www.twitter.com/SciWriAlicia

View photos from before/after the Japan tsunami:
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  • In this combination photo, Tayo Kitamura, 40, kneels in the street to caress and talk to the wrapped body of her mother Kuniko Kitamura, 69, after Japanese firemen discovered the dead body in the ruins of her home in Onagawa, Japan, on March 19, 2011, top, and a newly built home sits at the site of the now-cleared but destroyed area on Wednesday, Feb. 22, 2012. (AP Photo/David Guttenfelder)

  • A few homes have been rebuilt in the year since an earthquake and tsunami roared across Japan's coastline, killing 19,000 people. But most communities remain unrecognizable, and their residents' futures uncertain. (AP Photo/David Guttenfelder)

  • The tsunami that slammed into Japan's coastline one year ago was merciless, sparing little in its path. Homes were reduced to rubble, cars tossed about like toys, and boats -- such as this one photographed in Kesennuma, Japan, on March 28, 2011 -- flung from the sea into streets and onto roofs. The ocean's fury, and the earthquake that preceded it, left around 19,000 people dead, hundreds of thousands homeless, and sparked the worst nuclear crisis the world had seen in a quarter century. (AP Photo/David Guttenfelder)

  • In this combination photo, Japanese vehicles pass through the ruins of the leveled city of Minamisanriku, Japan, on March 15, 2011, top, four days after the tsunami, and vehicles pass through the same area on Thursday, Feb. 23, 2012. (AP Photo/David Guttenfelder)

  • The earthquake and tsunami, which killed around 19,000 people, delivered one of their worst hits to the once-scenic, blue-collar fishing town of Minamisanriku, Japan, photographed here on March 15, 2011. The wall of water spared little in its path, sweeping away nearly every business and every job, and leaving more than half the town's residents dead or homeless. (AP Photo/David Guttenfelder)

  • A year after the earthquake and tsunami people across Japan and leveled this town, there are hints of progress _ the main roads are free of debris, and some temporary houses have been built. But many in Minamisanriku, and elsewhere across Japan's battered coastline, remain in a hellish state of limbo. (AP Photo/David Guttenfelder)

  • In this combination photo, a ship washed away by the tsunami sits in a destroyed residential neighborhood in Kesennuma, northeastern Japan, on March 28, 2011, top, and the same ship sits on the same spot on Thursday, Feb. 23, 2012. (AP Photo/David Guttenfelder)

  • A year after an earthquake and tsunami ravaged the country's coastline and killed around 19,000 people, many of the boats carried inland by the wall of water have been removed. But some, like this one, remain _ providing a stark reminder of nature's fearsome power. (AP Photo/David Guttenfelder File)

  • One year later, more than 3,200 people presumed killed in the earthquake and tsunami have yet to be found. They are among the 19,000 people who lost their lives on March 11, 2011. (AP Photo/David Guttenfelder)

  • In this combination photo, Japanese residents of Kesennuma, northeastern Japan, pass through a road that was cleared by bulldozer through the ruins of the city on March 17, 2011, six days after the tsunami, top, and people cross the same street on Thursday, Feb. 23, 2012. (AP Photo/David Guttenfelder)

  • In the days after the earthquake and tsunami ravaged Japan's coastal towns, the bulldozers began to arrive, clearing away the rubble that littered the roads, such as this street in Kesennuma, Japan, photographed on March 17, 2011. Those tasked with clearing away the wreckage faced a monstrous task: towering piles of twisted metal and wood, boats perched atop roofs, mountains of family heirlooms, sodden furniture and children's toys. They also faced the grim reality that many of the 19,000 people killed lay entombed in the rubble, waiting to be discovered. (AP Photo/David Guttenfelder)

  • One year after a powerful tsunami battered Japan and killed around 19,000 people, the streets have been cleared and the wreckage removed from town centers. But the process of destroying all that debris has been slow, with much of it still sitting in huge mountains in temporary holding areas. (AP Photo/David Guttenfelder)

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