Though it may not be evident in the Japanese sashimi and sushi rolls they typically end up in, Pacific bluefin tuna (Thunnus orientalis) are one of the world’s quickest and largest fish, growing up to 10 feet long and weighing nearly 1,000 pounds. From April to August, the predators spawn in the Sea of Japan and swim 6,000 miles east to school in waters off the California coast. But in the wake of the ongoing Fukushima meltdown in Japan, a catastrophe that has forced millions of gallons of nuclear waste to be dumped directly into the Pacific ocean, bluefins are now swimming into the U.S laced with radiation. According to a study published Monday in the Proceedings of the National Academy of Sciences, 15 bluefin tuna caught off the coast of San Diego in August 2011 all had levels of radioactive cesium that were 10 times higher than the amount measured in tuna off the California coast in previous years.
“We were frankly kind of startled,” Nicholas Fisher, one of the study’s authors, said Monday.
The study suggests that the young, 15-pound Japanese bluefins sampled ingested the radiation through their gills while swimming or by eating organisms like krill and squid that had already taken radiation in. During their long journey to California approximately one month after the Fukushima meltdown, the bluefins should have been able to dilute some accumulated radiation through metabolism and growth—however, a surprising quantity of the poison remained in their systems.
“That’s a big ocean. To swim across it and still retain these radionuclides is pretty amazing,” Fisher added.
Also analyzed were closely related but non-migratory yellowfin tuna, found mainly off the California and Mexico coasts, and bluefin that migrated to Southern California before the nuclear crisis. The scientists found no trace of cesium-134—which has a half-life of two years—in either group, and only minute levels of cesium-137, which has a radioactive half-life of about 30 years and still persists from above-ground nuclear bomb tests conducted in the 1950s and ’60s.
“[The cesium-134 increase] is inarguably from Fukushima Daiichi,” Daniel Madigan, a marine biologist at Stanford University and the study’s lead author, told CNN. Madigan was already studying how Pacific bluefin tuna migrate across the Pacific Ocean in early 2011, but when tragedy struck Japan that March, he decided to pursue radiation testing. This summer, he will repeat the study with a “much higher sample size across a greater range of fish, ages and sizes.”
“This year’s fish are going to be really interesting,” Madigan notes. “There were fish born around the time of the accident, and those are the ones showing up in California right now. Those have been, for the most part, swimming around in those contaminated waters their whole lives.”