Researchers say that giant bluefin tuna captured off the coast of the U.S. last fall contained trace amounts of cesium-134, a radioactive element released into the ocean by the quake-damaged Fukushima nuclear power plant. According to the National Academy of Sciences, the radiation levels were ten times the normal levels of that measured in previous fish, but were still below the levels that are deemed safe for humans to eat. However, because cesium-134 is only produced through human activity, scientists concluded that it must have come from Fukushima shortly after the plant was damaged by a massive earthquake and tsunami last March. 

The tuna, which are born near Japan and migrate to the eastern Pacific, were able to carry the radiation from Japan to the U.S. much faster than tides and wind could have. The radioactive tuna were caught just four months after the disaster struck, giving scientists a unique way to track the spread of radiation across the ocean. Also, because fish can metabolize and shed radioactive materials as they age, their radiation levels may have been higher before they made it all the way to the West Coast. That's also why the radiation levels will not linger in fish over the coming years, and will eventually return to normal. Cesium-134 has a half-life of just two years.