Much of the radioactive material released is still
circulating in the form of radioactive Cesium-134.
In March 2011, a massive earthquake and tsunami triggered multiple meltdowns at Japan’s Fukushima Daiichi nuclear power plant. Explosions launched enormous amounts of radioactive material into the air, most of which settled into the Pacific Ocean. Since then, radioactive material has continued to leak into the ocean from the plant.
Much of the radioactive material released is still circulating in the form of radioactive cesium. Cesium-134 has a half-life of two years, and essentially all of it in the Pacific Ocean comes from the Fukushima disaster. Cesium-137 has a half-life of 30 years. Much of the Cesium-137 in the Pacific dates to nuclear tests as far back as the 1950s.
Since radioactive cesium floats, it is easily swallowed by fish, and steadily moves up the aquatic food chain. As top predators, tuna tend to accumulate this and other toxins in their bodies.
In a 2012 study, researchers from Stanford University and Stony Brook University tested the radioactive cesium levels in Pacific bluefin tuna caught off the coast of California. They found the radioactive element in every single fish tested.