For most people, the thought of radioactive sushi tuna is nightmarish, but for one scientist it represented an opportunity: If radiation from Fukushima was found in the fish, it could provide clues about animal behavior.
LOS ANGELES — Marine biologist Dan Madigan stood on a dock in San Diego and considered some freshly caught Pacific bluefin tuna. The fish had managed to swim 5,000 miles from their spawning grounds near Japan to California’s shores, only to end up as the catch of local fishermen.
It was August 2011, five months after a magnitude-9 earthquake and tsunami had struck in Japan, crippling the Fukushima Dai-ichi nuclear plant. Madigan couldn’t stop thinking about pictures he’d seen on TV of Japanese emergency crews dumping radioactive water from the failing reactors into the Pacific Ocean.
The graduate student looked at the tuna and wondered: Could they have transported any of that radiation to California?
If radiation from Fukushima was detectable, scientists might look for traces of the contamination in all sorts of amazing creatures that make epic journeys across the open seas, from tuna to sharks to turtles to birds. They might learn more about where the animals came from, when they made their journeys, and why.
They might learn how a single, man-made event — the plant failure in Fukushima — could be linked to the lives and fates of animals making homes over half the globe.
Madigan bagged some tuna steaks he had collected from the fishermen, threw them in a cooler and made a mental note to call Nicholas Fisher, a scientist he knew who would be able to tell him whether the tuna had carried radiation from the disaster.
Maybe the fish could still tell their story.
Pacific bluefin tuna migration is mysterious. Only some of the tuna born each year leave the Western Pacific around Japan for California, swimming for two months or more to reach their destination. They stay here for a few years, and then they swim back to the waters where they were born so that they can reproduce. Some tuna are thought to cross the ocean multiple times.
Researchers don’t really understand why. It may have to do with food availability, ocean temperatures or other factors.
Madigan’s doctoral research tries to fill in some of the blanks by looking for nitrogen and carbon isotopes in tissue that serve as signatures of where the fish have lived and for how long. But interpreting the chemical signatures can be tricky. If Madigan could use the radioactive signal from Fukushima to confirm the results of the chemical analysis, he realized, it might bolster his work.
Or that was the theory, anyway.
Madigan, a 30-year-old whose casual demeanor can mask the intensity he brings to his research, called Fisher about the San Diego steaks.
The marine radioactivity expert, who works as a professor at Stony Brook University in New York, doubted they’d detect any radiation in the bluefins. Surely, he thought, any radiation the fish might have picked up would have dissipated over the months it took for them to cross the Pacific. What’s more, it was hardly certain that the animals ever got close enough to Fukushima to encounter its plume in the first place.
“I thought, OK, I can do this, but I wasn’t expecting anything," Fisher said.
Still, he forged ahead, analyzing quarter-cup-sized piles of freeze-dried, powdered muscle from some of the younger tuna Madigan had seen that day on the dock.
After examining a sample from the first fish, Fisher called Madigan.
“He was like, ‘You’re not going to believe it, but here it is,’" Madigan said.
The tuna had tested positive for cesium-134 and cesium-37, both known waste products from Fukushima. For Madigan, it was “a real discovery moment, like in the movies," he said.
A second fish also tested positive for the isotopes. So did a third. And a fourth.
In the end, every single one of the 15 fish they examined carried radiation from the power plant.
In May, Fisher and Stony Brook postdoctoral researcher Zofia Baumann published a paper in the journal Proceedings of the National Academies of Sciences detailing their findings. The team believed it was the first time that anyone had demonstrated that migratory animals could transport radioactive contaminants across the Pacific.
The amounts the fish carried were minuscule — far less, ounce for ounce, than the amount of naturally occurring radiation in a banana — but possibly enough for scientists to gain insight into animal migration, the team wrote in their report.
Madigan collected additional tuna samples in 2012, testing 50 to see whether the cesium signal was still detectable more than a year after the accident. In a study published online this month by the journal Environmental Science & Technology, he and his colleagues reported that it was.
They concluded that their tracking method worked, and that Fukushima provided “an unprecedented opportunity" for scientists to use radioactive tracers to follow animal movement.
“This was just nature being amazing," Fisher said. “Now, potentially, we have a very useful tool for understanding these animals."