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Bluefin tuna carry radiation from Japan

30 May 2012
Bluefin tuna have carried radioactivity to the waters off California. Credit: Marco Care/Marine Photobank

Bluefin tuna have carried radioactivity to the waters off California. Credit: Marco Care/Marine Photobank

Bluefin tuna exposed to radioactivity that leaked into the Pacific Ocean after the earthquake and tsunami damaged Japan’s Fukushima power plants last year have carried that radiation to the waters off California.

This is according to a new study by scientists from Stony Brook University’s School of Marine and Atmospheric Sciences (SoMAS) and Stanford University’s Hopkins Marine Station.

While the radioactivity levels in the Pacific bluefin tuna posed no public health threat, these findings represent the first documented instance of the transport of radioactive materials in the sea through a biological migration.

SoMAS professor Nicholas Fisher, PhD, and postdoctoral scholar Zofia Baumann and Daniel Madigan of Stanford measured the levels of two radioactive isotopes of cesium in bluefin tuna caught in August 2011 off the coast of San Diego, California. Pacific bluefin tuna spawn in the western Pacific and many migrate in their first or second year to the waters of the eastern Pacific. Analysing the radionuclide content in top marine predators such as bluefin tuna should provide unequivocal evidence of migratory routes and timing of these animals, the researchers concluded.

By the time these fish arrived in California, the artificial radioactivity levels in these fish were more than 20 times lower than the Japanese safety limit and over 30 times lower than the naturally occurring potassium-40, another gamma-emitting radioisotope which is present in all marine biota.

"While the radioactivity levels in bluefin tuna caught in California in August 2011 were only about 3% above the natural background radioactivity, levels in this year's bluefin may be higher, given that they would have been exposed to radioactive food and water for about one year prior to migration, unlike in 2011 when they were exposed for only about one month,” Dr Fisher said. “However, radiocesium concentrations have become diluted and dispersed since the disaster occurred, and so the public health aspects will remain unclear until this year's tuna are analysed. We are now preparing to do that."

The study, Pacific Bluefin Tuna Transport Fukushima-Derived Radionuclides from Japan to California has been published in Proceedings of the National Academy of Sciences of the US.

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Bluefin tuna have carried radioactivity to the waters off California. Credit: Marco Care/Marine Photobank

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