Those looking for evidence of the March 2011 Fukushima Daiichi nuclear disaster in Japan may need search no further than their next plate of sushi, Stanford University researchers report.
The researchers tested 15 Pacific bluefin tuna that had migrated from Japan to the California coast and found that the levels of radioactive cesium in these fish were 10 times higher than those found in bluefin tuna from the years before the disaster.
Before you swear off your maguro nigiri, it's important to realize that the levels of radiation the researchers found from the cesium in the tuna were exceedingly low - about 30 times less than the amount of radiation given off by other common, naturally occurring elements in the tuna we eat.
The findings appeared Monday in the Proceedings of the National Academy of Sciences.
"The finding should be reassuring to the public," said Timothy J. Jorgensen, associate professor of radiation medicine at Georgetown University, who was not involved with the study. "As anticipated, the tuna contained only trace levels of radioactivity that originated from Japan. These levels amounted to only a small fraction of the naturally occurring radioactivity in the tuna, and were much too small to have any impact on public health.
"Thus, there is no human health threat posed by consuming migratory tuna caught off the west coast of the United States."
Still, the fact that the researchers could trace this radioactive material back to its source in Japan could have implications for seafood monitoring methods in the future. Dr. Michael Harbut, director of the Environmental Cancer Program at Wayne State University's Karmanos Cancer Institute in Detroit, agreed that the findings are no cause for panic. But he said that the finding that tuna and migratory food animals could carry this radioactive material so far across the ocean deserves consideration.
"In general, when you hear the word 'radiation' at all, it's cause for some alarm, and I agree always a cause for significant attention."
Harbut, who described himself as a physician, scientist and "guy who likes sushi," added that while the levels of radiation found should not be seen as a direct threat to human health, scientists should focus their efforts on how this extra little bit of contamination fits into the bigger picture of food safety.
"For somebody to say this is an immediate threat to large numbers of humans and their health is irresponsible," Harbut said. "We don't see people dying left and right all over the West Coast from radiation poisoning. But to say this is nothing to worry about is equally irresponsible, because you have radioactive material ingested by fish, which is in turn being eaten by people."
For now, the findings may be most important as a demonstration of how migratory food animals connect different areas of the globe - and how an event in one part of the world can affect food animals in an entirely different region.
"[The findings] should be of value to both environmental studies of the marine ecosystem and to ensure that the public is not exposed to seafood contaminated with significant levels of marine radioactivity," Jorgensen said.
Harbut said that the next step is for governments to learn more about this issue and act appropriately to ensure the seafood safety.
"I think that the appropriate government agencies have to appoint appropriately trained people to give the public an honest assessment," Harbut said. "Not something tailor made for ignorance, like 'This will definitely kill you,' or 'This poses absolutely no risk to human health.'
"We've gone too far in poisoning the world to settle for simple 'yes'es and 'no's like that."