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 National Oceanic and Atmospheric Administration and other institutions plan to analyze hundreds more bluefin tuna, as well as albacore tuna; mahi mahi; ocean sunfish; opa; mako, blue and salmon sharks; loggerhead turtles; and sooty shearwaters, a type of migratory seabird.
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.