California Bluefin Tuna Found Carrying Radiation From Japan's 2011 Fukushima Nuclear Disaster
May 29, 2012

Bluefin Tuna Carry Radiation From Japan’s 2011 Fukushima Nuclear Disaster

Low-level radiation has been found in populations of bluefin tuna off the coast of California, a new study has found, noting that the fish are carrying radioactive material from Japanese waters more than 6,000 miles away.

Researchers have found “modestly elevated levels” of two radioactive isotopes in bluefin tuna caught off the coast of San Diego in August 2011. They said the tuna picked up the radiation by swimming and feeding in the waters off the coast of Japan where the Fukushima nuclear power plant disaster occurred in March 2011.

They were quick to point out, however, that the elevated levels of radiation found in the fish posed no health risk to public health, saying the observed levels were much lower than the Japanese safety limit recommends. They also noted that the radioactive material found would likely decrease over time as the material dilutes in the ocean.

In the study, published online Monday by the Proceedings of the National Academy of Sciences (PNAS), the researchers analyzed 15 bluefin tuna and found radioactive levels of cesium-134 nearly 10 times higher than in fish caught in previous years. Lower levels of cesium-137 were also found in bluefin tuna. However, unlike human-produced cesium-134, cesium-137 occurs naturally in the eastern Pacific.

The team also found no traces of cesium-134 in yellowfin tuna, which are only found in the eastern Pacific.

The analysis provides “unequivocal evidence” that the radiation stems from Fukushima, said study authors Daniel Madigan of California's Stanford University, and Zofia Baumann and Nicholas Fisher, both of Stony Brook University in New York.

“These findings indicate that Pacific bluefin tuna can rapidly transport radionuclides from a point source in Japan to distant ecoregions and demonstrate the importance of migratory animals as transport vectors,” the authors told the AFP news agency.

Radiation from the Fukushima nuclear plant leaked into the air, soil and sea after the March 11, 2011 disaster. The tuna picked up the radiation in the seawater and carried it much faster across the Pacific than wind and water currents were able to carry debris to waters around the Pacific Northwest.

Bluefin tuna spawn only in the western Pacific, typically off the coast of Japan and the Philippines. Some populations of young bluefin migrate east to the California coast, where upwelling ocean water brings lots of food for them and their prey. They arrive as juveniles and spend their lives there.

Judging by their size, the researchers knew they were juveniles and had left Japanese coastal waters about a month after the Fukushima disaster. Still, the finding was puzzling.

“We were frankly kind of startled,” Fisher told The Telegraph. Nuclear fallout typically does not linger in huge fish that sail the world because they have the ability to metabolize and shed radioactive substances, he noted. Bluefin tuna excrete cesium-134 on a daily basis and it also gets diluted in their bodies as they grow.

The authors said the real test will begin this summer when they complete more analysis of bluefin tuna. They will examine tuna that have experienced longer periods in radioactive waters to see if they have been affected by contamination.

“Much will depend on the concentration in the prey fish, which in turn is ultimately dependent on the water concentration,” Fisher told Bloomberg reporter Stuart Biggs. “If concentrations in water will eventually decline, as we would expect, due to dilution and dispersion, then concentrations in living organisms will eventually decline as well.”

Now that they know bluefin tuna can transport radiation, they also want to track other marine life such as turtles, sharks and seabirds to see if they play similar roles in radioactive transportation.

“It is unlikely” that the level will rise in tuna, Madigan told Bloomberg. “However, certain small fish around Japan showed very high levels after the accident. If certain larger predators happen to feed on these prey, higher levels than we observed may be possible.”

Japan´s Chief Cabinet Secretary Osamu Fujimura told reporters that international monitoring of marine life is probably needed. He added that officials in Japan are studying different ways they can collect information on the issue.