It is really all just one big ocean. Low levels of nuclear radiation from the tsunami-damaged Fukushima power plant have turned up in bluefin tuna off the California coast, 6,000 miles from the damaged Japanese nuclear reactors. Small amounts of cesium-137 and cesium-134 were detected in 15 tuna caught near San Diego in August 2011. The radiation levels were 10 times higher than those found in tuna in the same area in previous years, but still low enough so that the tuna are considered to be safe for human consumption. The tuna, which spawn off the coast of Japan and the Philippines often migrate to the coast of California.
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 scientists did not expect the nuclear fallout to linger in huge fish that sail the world because such fish can metabolise and shed radioactive substances.
One of the largest and speediest fish, Pacific bluefin tuna can grow to 10 feet (3 meters) and weigh more than 1,000 pounds (450 kilograms). 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 caesium-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 analysed yellowfin tuna, found in the eastern Pacific, and bluefin that migrated to Southern California before the nuclear crisis. They found no trace of caesium-134 and only background levels of caesium-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.