The findings suggest that contaminated water is still leaking from the stricken power plant, the sea bottom itself is now laced with radionuclides, or both. Concentrations in the ocean water itself remain below any human health concern but they do pass into fish that swim through those waters.
"When fish 'drink' they take [cesium] and other salts up from the water they are swimming in, that accumulates in the muscle tissue," explains marine chemist Ken Buessler of the Woods Hole Oceanographic Institution, who compiled the analysis of publicly released Japanese fisheries data and published it in Science on October 26. But the fish also shed that cesium if they swim in uncontaminated waters, as has been seen in tuna that migrated from near Japan to near San Diego, suggesting that levels in fish should decrease over time. For this reason, most of the fish caught off Japan's northeastern coast are not radioactive. But roughly 40 percent of bottom-dwelling fish, such as flatfish or halibut, caught off the coast adjacent to Fukushima bear radionuclides above the Japanese food safety standard of 100 becquerels per kilogram.*
According to a response to questions from Scientific American that was prepared by staff at the U.S. Nuclear Regulatory Commission, ingesting fish at that level "would only produce a dose that is a small fraction of the dose that people receive from natural levels." For example, as Buessler notes, fish caught off Japan in June 2011 boasted levels of potassium-40—a naturally occurring radionuclide—10 times higher than those of radioactive cesium from Fukushima. More >>