LOS ANGELES — Mighty bluefin tuna retained radioactive contamination in their flesh from Japan’s crippled Fukushima nuclear plant while crossing the vast Pacific Ocean to U.S. shores 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. 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 the magnitude-9 earthquake in March 2011 triggered a tsunami that badly damaged the Fukushima 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 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 nuclear-plant disaster, Fisher of New York’s Stony Brook University 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 into 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.
Researchers will repeat the study with a larger sample this summer.
