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Radioactive Bluefin Tuna: Japan Nuclear Plant Contaminated Fish Found Off California Coast


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#1 williamsr40

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Posted 31 May 2012 - 10:02 AM

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LOS ANGELES -- 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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#2 williamsr40

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Posted 01 June 2012 - 09:47 AM

Japan's radiation found in California bluefin tuna

For the first time, scientists have detected radioactivity in fish that have migrated into California waters from the ocean off Japan, where radiation contaminated the sea after explosions tore through the Fukushima nuclear reactors last year.

Radioactive cesium was detected in samples of highly prized Pacific bluefin tuna, but it is well below levels considered unsafe for humans, the scientists say.


The evidence is "unequivocal" that the tuna - caught off San Diego a year ago - were contaminated with radiation from Japan's nuclear disaster, the researchers said.

Virtually all bluefin tuna on the market in the United States is either farmed or caught far from the Fukushima area, so American consumers should not be affected by radiation contamination in their fish, seafood distributors say. The migratory bluefin studied by the researchers were all caught by sport fishermen and were not headed for the market.

Daniel J. Madigan, a marine ecologist at Stanford's Hopkins Marine Station in Pacific Grove (Monterey County), Nicholas Fisher, a marine scientist internationally known as a specialist in radiation hazards at Stony Brook University on Long Island, and Zophia Baumann, a staff scientist in Fisher's laboratory, reported their discovery Monday in the early online edition of the Proceedings of the National Academy of Sciences.


Unexpected discovery

The finding was wholly unexpected, Madigan said. It came about when he was researching the migratory patterns of bluefin tuna as part of a broader study of Pacific fish migration.

Madigan had collected samples of muscle tissue from 15 2-year-old tuna given to him by San Diego fishermen in August, and when tests detected radioactivity in one sample he sent all 15 samples to Fisher in Long Island, he said.

The young tuna, averaging about 13 pounds apiece, were found to be contaminated with two radioactive forms of the element cesium. Isotopes called cesium-134 and cesium-137 do not exist in nature but are produced only in nuclear explosions such as the weapons tests of the Cold War era.

Before the Fukushima earthquake and tsunami, low levels of the radioactive cesium-137 , which decays to harmlessness only over thousands of years, had been measured in Japanese waters, while the shorter-lived cesium-134 was undetectable, the scientists said. That difference, they said, was crucial in concluding that the radioactive contamination was linked to the Fukushima disaster.

Increased concentrations of radioactivity contaminated nearly 60,000 square miles of the ocean off Japan after workers at Fukushima pumped thousands of tons of seawater over reactors last year to prevent a complete meltdown of the reactor cores.

Fisher said there is one unanticipated benefit from Madigan's discovery of radioactivity in the bluefin tuna. If the cesium isotopes are also detected in other migratory ocean species like turtles, sharks, seals and seabirds, that information should prove valuable as "tracers" that would add fresh details of migratory patterns to what is now gathered by widespread tagging programs, he said.

A new study planned


Meanwhile, Madigan said, he is preparing to collect samples from a new group of bluefin tuna that have recently migrated to the waters off San Diego in order to determine their levels of radioactive cesium.

They will have lived in Fukushima's contaminated ocean for a full year longer than the first fish he collected, and the scientists will seek to know whether radiation levels in the tunas' bodies have increased or decreased, he said.

"We don't think there will be any public health concern from the results of the new tests," Fisher said, "but if we do see any higher concentrations of cesium, we will certainly alert public health agencies again."

In Japan the fatty muscle in the tuna is particularly prized as a delicacy, sliced and eaten raw as sushi. It is very pricey, and early this year a nearly 600-pound Pacific bluefin sold in a Japanese wholesale market for the equivalent of $736,000 - $1,238 a pound.



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