Japan’s Radiation Found in California Bluefin Tuna

David  Perlman

San Francisco  ChronicleMay  29, 2012 04:00 AM

Tuesday, May 29, 2012

Chris Park / Associated Press

Virtually all bluefin tuna on the U.S. market is either  farmed, like these in Mexico, or caught far from the Fukushima area.



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.

David Perlman is The San Francisco Chronicle’s science  editor. E-mail: dperlman@sfchronicle.com

This article appeared on page C – 1 of the  San Francisco Chronicle

 

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