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, almost 10,000 kilometres 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 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 US and Japanese governments.

Previously, smaller fish and plankton were found with elevated levels of radiation in Japanese waters after a magnitude-nine earthquake in March 2011 triggered a tsunami that badly damaged the Fukushima Dai-ichi reactors.

But scientists did not expect the fallout to linger in huge fish that sail the world as they can shed radioactive substances.

One of the largest and speediest fish, Pacific bluefin tuna can grow to three metres and weigh more than 450kg. They spawn off Japan's coast and swim east to school in waters off California and Mexico.

Five months after the Fukushima disaster, Fisher of Stony Brook University in New York and a team tested Pacific bluefin caught off the coast of San Diego.

Tissue samples from all 15 tuna contained levels of radioactive ceisum-134 and cesium-137 higher than in previous catches.

The team also analysed yellowfin tuna, found in the eastern Pacific, and bluefin that migrated to southern California before the 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, which had no role in the research.

Originally published as Radioactive bluefin tuna cross Pacific