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RENEE MONTAGNE, HOST:

In the next few minutes, we're going to hear about some of the world's more elegant seafood. That would be: caviar, coming from and unexpected place, and bluefin tuna from and unfortunate place.

DAVID GREENE, HOST:

And that would be Japan. Last years' nuclear meltdown at the Fukushima power station contaminated marine life. And it turns out some of those fish have made their way across the Pacific Ocean into American waters.

MONTAGNE: Still, experts say the amount of radioactivity found in the fish is far too small to pose a health risk. NPR's Richard Harris has more.

RICHARD HARRIS, BYLINE: Pacific bluefin tuna are magnificent creatures. When they are not being overfished for sushi - as they are around the world - they are making incredible ocean voyages.

NICHOLAS FISHER: Bluefin tuna spawn in waters in the western Pacific, including waters around Japan. And they actually swim across the entire Pacific Ocean and make it to waters off California and Mexico.

HARRIS: Nicholas Fisher at Stony Brook University studies the contaminants that end up in Pacific bluefin tuna. Methyl mercury is the main concern. It can damage brain tissue. That's why pregnant women in particular are urged to limit their consumption of tuna.

Last summer, Fisher and his colleagues collected 15 bluefin tuna from fishermen in San Diego to look for contaminants. Since the Fukushima meltdowns happened at about the time those fish were swimming off Japan, the researchers decided to see if they could find any radioactive material in the flesh.

FISHER: It was mostly just to see if we could detect it. And we were quite surprised.

HARRIS: They did find traces of radioactive cesium, as they report in the Proceedings of the National Academy of Sciences. And they could tell by the mix of two types of cesium that the material came from Fukushima. The amounts were tiny. Fish already contain traces of naturally occurring radioactive elements such as potassium-40. And that natural radiation in the fish flesh was far greater than the added radiation from Fukushima by a factor of 30.

Bob Emery, at the University of Texas Health Science Center at Houston, says radiation may be spooky, but in this case there's no health risk.

BOB EMERY: In order for a member of the general public to reach the annual public dose limit, they'd have to eat between 5,000 and 8,000 pounds of tuna.

(SOUNDBITE OF LAUGHTER)

HARRIS: In a year. And by the way, much of the bluefin tuna caught of the West Coast is actually shipped back to Japan to be sold in its lucrative sushi market. But the idea of radioactive tuna sparked the imagination of editors around the world, and sent marine biologist Nick Fisher scrambling.

FISHER: Literally every 30 seconds, there's another request for an interview from as far a field as Korea, Al Jazeera television. I mean, I don't commonly get, you know, inquiries from Al Jazeera.

HARRIS: The cool part of his research is that this otherwise miserable tsunami and nuclear meltdown has created a natural tracer for biologists. By tracking the cesium, Fisher says they can study the movements of migratory sea life.

FISHER: And this would include other fish. It could include birds, mammals, turtles. So we think this could be actually a very useful tool.

HARRIS: That is, once the hot tuna story dies down a bit.

Richard Harris, NPR News.

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