Traces of radiation from Japan's earthquake-damaged Fukushima Daiichi nuclear power plant are showing up in the muscles of bluefin tuna off California. Although this sounds like bad news, the levels are too low to harm humans or fish but just high enough to help conservation scientists track and protect the overfished species.

Last spring Dan Madigan, a doctoral student at Stanford University, and his colleagues found traces of cesium 137 and cesium 134 in the flesh of bluefin tuna caught off the San Diego coast, which the fish probably picked up by feeding on contaminated plankton and small fish near Japan. They then devised a way of using the half-life of radioactive isotopes to study the fish. Cesium 134 has a half-life of 2.1 years; cesium 137 has a half-life of 30.1 years. The scientists calculated the ratio of the two isotopes in each fish to see if it would reflect how recently the migrants had arrived. (A higher ratio of 134 to 137 would indicate a more recent immigrant.)

The results jibed with what scientists knew about the fish: Pacific bluefin spawn in waters surrounding Japan and spend the first year of their lives foraging there before either staying put or migrating to the California coast to fatten up for mating. Madigan's team found that all fish aged 1.6 years or younger were migrants and that the trip from Japan had taken around two months, which validated their approach. They reported their results in Environmental Science & Technology in March.

The new technique shows promise for tracking the movement of other Pacific migratory species, including whales, turtles and sharks. Although cesium 134 levels will soon be too low to be useful, Madigan's team has correlated cesium levels with those of longer-lived stable isotopes such as carbon and nitrogen so that researchers will be able to use them as proxies. “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.”