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FILE - This
March 5, 2007 file photo shows workers harvesting bluefin tuna
from
Maricultura's tuna pens near Ensenada, Mexico. New research found
increased
levels of radiation in Pacific bluefin tuna caught off the coast of
Southern
California. Scientists said the radiation found in the fish came from
Japan's
Fukushima nuclear plant that was crippled by the 2011 earthquake
and tsunami. (AP
Photo/Chris Park, File)
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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.
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