Pacific bluefin tuna that swam off the Japanese coast after the 2011 nuclear meltdown carry radioactive memorabilia of the Fukushima disaster. That’s the splashy news out of Stanford University’s Hopkins Marine Station in Pacific Grove.


But fear of nuclear seafood is not the point, according to doctoral student and study lead Daniel Madigan; the radiation level is so low it’s not really a health concern. More importantly, the glimmers of cesium-137 and cesium-134 in bluefin flesh help scientists map their movement. And that could be key to more sustainable management.


Pacific bluefin are down about 96 percent from their historic population levels, according to a December stock assessment. “It’s still on our ‘avoid’ list,” says Alison Barratt, spokeswoman for the Monterey Bay Aquarium’s Seafood Watch program. “The stock is doing really badly.” 


Madigan sampled young bluefin bycatch a few months after the meltdowns of the Fukushima Daiichi nuclear reactors. Because Pacific bluefin only spawn near Japan, he wondered if these 1 – to 2-year-old fish carried contaminants from the accident. Madigan sent 15 samples to a marine radioactivity expert at Stony Brook University.


“He called me after the first [sample] and was like, ‘You’re not going to believe it, but they’re carrying the cesium from Fukushima,’” Madigan recalls.


That initial finding, published in 2012, set off a wave of media attention and a fair bit of public hysteria. “Since it was such an important and highly consumed species, it created kind of a panic,” Madigan says.


But the tuna radiation levels were “super super low,” he adds, hundreds of times lower than the background radiation levels. All food is radioactive to some degree – like the potassium-40 that gives us the “banana equivalent dose” as a unit of exposure.


Madigan saw a more important scientific application for the radioactivity: as a tracer of migration. In 2012, with federal and private funding, he had samples from 50 bluefin (caught off Southern California by sports fishermen) tested for the isotopes. The results: All the younger fish carried them, validating the use of radioactive cesium as a tracer. Madigan’s report was accepted by Environmental Science & Technology in February.


Scientists know Pacific bluefin are born in Japan, migrate to the California current and return to Japan for spawning. But they don’t know how long the young fish stay in Japan, how often they cross the sea or what prompts them to migrate. By validating the Fukushima radiation as a tracer, Madigan’s research sets the stage for a larger study of bluefin migration. 


“There’s this whole dynamic of migration patterns across the Pacific that’s been poorly understood,” says Heidi Dewar, a research biologist for NOAA Fisheries and a coauthor of Madigan’s latest paper. 


The tracer is not just applicable to tuna. It can be used to track other animals – like sea turtles, whales, sharks and seabirds – whose migration routes include the waters off Japan. If scientists can figure out which animals cross the Pacific and when, Dewar says, they can begin to investigate why. And that can help them project, for example, how climate change could impact the migrations of sensitive species.


Even though Americans don’t do much catching or eating of bluefin, she adds, the U.S. can use migration data to influence international fisheries management. “We have a big place at the table,” she says.