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
It was August 2011, five months since a magnitude 9
The graduate student looked at the tuna and wondered: Could they have transported any of that radiation to California?
For most people, the thought of radioactive sushi tuna is nightmarish, but for Madigan it represented an opportunity.
If radiation from
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.
Madigan began thinking about the globe-spanning migrations of marine animals in 2006, during a fishing trip in the open waters off Costa Rica.
For hours, he and his friends saw nothing — no land, no features in the water, no fish. Then, in a flash, they hooked and released a dozen sailfish, magnificent 150-pound catches known for their spectacular jumping maneuvers and brilliant colors.
"It's black water, and all of a sudden you have a huge animal," said Madigan, a
Intrigued, Madigan enrolled in a doctoral program at
They had also tagged the Pacific bluefin tuna, a creature distinguished by its unusual biology (it's one of the few warm-bodied fish) and its popularity on sushi menus. At a January auction in Tokyo, a buyer paid a record $1.8 million for just one of the fish, which are carved up into toro and other choice bites.
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
"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."
They're digging in to see what secrets the Fukushima cesium might reveal about other animals.
In coming months, the three researchers and colleagues at the
They'll examine samples collected in New Zealand, Hawaii and Alaska as well as in California. They might look through archived specimens for salmon and whales to test. Other research groups may track the contamination to study marine animals too, Madigan said.
If scientists find Fukushima radiation in swordfish, for example, it will be the first evidence that the species migrates across the entire Pacific. The same would be true of opa, sunfish and mako sharks.
In salmon sharks, it's known that males tend to congregate in the western Pacific and females in the east, with both sexes meeting up in Alaska to breed. But biologists don't know whether the segregation is complete. If female salmon sharks test positive for cesium, that might suggest that they occasionally do travel west to Japan, and that Alaska may not be the only place the species goes to reproduce. That could have ramifications for conservation efforts.
Hoyt Peckham, another Stanford-affiliated marine biologist, who is based in Mexico, is sending to Fisher's lab some samples from endangered loggerhead turtles that have died and washed up on the shores of Baja California Sur. Using the tracer to get a better idea of the timing of loggerhead migrations should help with his conservation work too.
And then there's the Pacific bluefin, which isn't endangered but is drastically overfished, with numbers down 96% from unfished levels, according to figures released in December. Knowing how long they spend in different locations could help restore tuna numbers by helping officials control fishing in waters where the fish fatten up before they spawn, Madigan said.
He imagines pulling together a map of the Pacific crisscrossed by the paths of radiation-toting animals — "an amazing image of transport … all from a little dot" in Japan, he said.
The scientists may have to work quickly to capitalize on their opportunity: Radioactive materials decay, and Fukushima's trail will fade.
Like the elusive migrating creatures of the deep, the contaminants will eventually vanish into the vast, seemingly featureless Pacific.