The work Madigan and his colleagues did prove that the cesium isotopes work as a tracer. So far the technique confirms what scientists know about bluefin tuna: They found that all fish younger than 1.6 years old were migrants. Only five of the 22 fish older than 1.7 years were migrants. The larger fish had left Japan earlier in the year, but the smaller fish hung around their birthplace until early- or mid-June.
The transpacific journey took an average of approximately two months for the fish Madigan sampled. One bluefin may have managed to make the trip in just 30 days—a figure that jibes with the known daily swimming speed of approximately 172.3 kilometers per day. The team reported their results in the March issue of Environmental Science & Technology.
Using Fukushima-derived radiocesium is a novel way of tracking the movements of oceangoing animals, wrote Texas A&M University at Galveston marine biologist Jay Rooker in an e-mail. The approach "shows promise for tracking the movement of other highly migratory species in the Pacific Ocean—whales, turtles and sharks," he added. Because the mixing of populations and transoceanic migrations can affect scientists' ability to estimate population size and fishing mortality, these kinds of studies are vital to informing management strategies Rooker wrote.
The short half-life of the cesium 134 means that soon the levels will be too low to be useful, but Madigan explains that there are other chemical techniques that researchers use to track migrating marine animals. The cesium isotopes provide unequivocal evidence that the tuna came from the waters near Japan. By matching the isotope signature with other methods—such as stable isotopes of carbon and nitrogen, which vary from region to region—researchers can use the longer-lasting isotopes as a proxy for the same information. "One method is finite and one is infinite," Madigan says. "Once you've hammered down the relationship you can just use the infinite one in the future."
The next steps for Madigan and the team are to look at other species. Those ocean-dwelling animals include albacore tuna, blue sharks, Pacific loggerhead sea turtles, salmon sharks, common minke whales and even birds such as sooty shearwaters. If any of those animals carry cesium isotopes from Fukushima, they can be classified as Japanese migrants. The fallout from the disaster could unlock secrets of ocean life.