Across the vast Pacific, the mighty bluefin tuna carried radioactive contamination that leaked from Japan’s crippled nuclear plant to the shores of the US 9,650km 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 on Monday in the US’ 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. However, even so, that is 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 9 earthquake in March last year triggered a tsunami that badly damaged reactors at the Fukushima Dai-ichi nuclear power plant.
However, 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 spawn off the Japanese 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 Dai-ichi 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 — cesium-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.