LOS ANGELES (AP) — 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 percent 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 travelers 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.
