Marine biologists from Stanford’s Hopkins Marine Station sampled 15 migratory Pacific bluefin tuna from the shore of San Diego in August 2011. According to their report, the tissue of the recently captured fish had ten times the concentration of the radioactive isotopes cesium-134 and cesium-137 than did the tissue of fish of the same species captured years before the before the devastating earthquake and tsunami in Japan that damaged the nuclear reactors in Fukushima. The U.S. Food and Drug Administration website says these isotopes are strongly linked to nuclear reactor accidents.
The researchers compared cesium levels in their sample of bluefin tuna to that of bluefin tuna sampled near Japan in 2008, to see whether migratory Pacific bluefin tuna contained any cesium before the Fukushima incident. They also compared their sample of tuna to yellowfin tuna sampled near California in August 2011; the yellowfin are a similar species to bluefin but do not migrate outside of the East Pacific Ocean. Neither comparison group showed significant levels of cesium.
David Checkley, a professor at the Scripps Institution of Oceanography who was not involved with the study, said he’s not surprised by the results.
“I think it really shows how connected the oceans are to one another,” said Dr. Checkley, “and it's rather indicative of how fast pollution can get around the oceans.”
The researchers are uncertain what enables bluefins to store the radiation over such long distances, since the fish don’t require cesium for normal biological functioning. According to Stanford's Dan Madigan, the lead researcher on the study, the fish either breathed in the radiation or ingested it by eating smaller radiation-containing fish. The researchers also say that the levels of cesium are low enough that they don’t appear to have any detrimental effect on the tuna.
No threat to public health has yet been detected. The radiation is contained in the muscle tissue, which is the part of the tuna that people eat in both Japan and the United States, most commonly as sushi. But the cesium levels found in the fish were approximately 37 times less than the safe-to-eat limit set by the FDA.
“Just because we can detect radiation doesn’t mean that it’s harmful,” said Nicholas Fisher, a marine researcher from Stony Brook University who participated in the study.
This summer, the researchers will study the effects the radiation in the tuna could have on human health. They also want to extend their research to birds and turtles, and to look at radiation as a potential way to track migration patterns in marine animals.
“This study shows us that stuff can happen in a way we don’t expect and that consequences can go great distances,” said Madigan.