Although radioactive leaks in the Fukushima plant are now plugged , Japan's coastal waters have taken a beating. Authorities dumped some of the waste seawater used to cool the reactors back into the sea, where it joined contaminated water leaking from cracks in the plant and nuclear fallout from the air. Almost as soon as the reactor broke, contamination of Japanese food exports became a concern . Now the Japanese are screening all fish products , some of which do indeed have high levels of radioactivity, and health officials there have set a radiation safety limit on seafood consumption. It's unclear whether the contaminated fish were caught in or out of the 20-kilometer no-go zone around Fukushima, water is reported to have spiked at 7.5 million times the legal limit at its highest  .
Still, analysts don't believe that any of the fish have high enough levels of radioactivity to harm human health . For instance, because it takes only 8 days for the radioactive isotope iodine-131 to decay away to half its original level of radioactivity, simply waiting should be a fix. Cesium-137, however, can accumulate in muscles and is a bigger concern with a half-life of 30 years; scientists plan to track it more carefully .
In the meantime, some marine ecologists are curious about how these levels of radiation could be harming marine life itself , but most aren't terribly worried; the vastness of the ocean dilutes radiation quickly .
Additionally, crustaceans and insects that live in the ocean are extremely hardy and resistant to radiation, according to marine radioecologist Bruno Fievet of IRSN, the French Institute for Radiological Protection and Nuclear Safety. The level of radiation necessary to kill marine species—or even damage them—is orders of magnitude higher than the level that would harm a human, says Fievet, who studies the effects of nuclear plants on developing shellfish larvae. When researchers bring samples of marine species into the lab to study whether radiation can cause mutations, they expose them to gamma rays, the same kind of radiation used to sterilize equipment and food. The level of radiation Fievet uses in his lab is many times higher than the radiation in the water around Fukushima.
But marine life in the coastal area, however, is being exposed to radiation through a more direct route. Rather than just external gamma rays, plants and animals are absorbing and ingesting particles of radioactive cesium and iodine, putting these isotopes directly into their tissues. With irradiation from the inside as well as out, it may take much less to cause genetic changes and impair growth and reproduction, even if it's still not enough to kill the species. But scientists just don't know what the effects will be, Fievet says, because handling these radionuclides is just too hard to do safely and efficiently in the lab.
Radioecologists at IRSN, in U.S. national labs , at the International Atomic Energy Agency, and at universities around the world are beginning to plan long-term studies-once the situation stabilizes-on how radiation moves through food chains and builds up in organisms: a process called bioaccumulation. If marine species take up these radioactive particles faster than they break them down, the radiation can concentrate and affect larger animals that eat them. Some species, such as brown seaweed are extremely efficient at bioaccumulation. Dominique Boust, another marine radioecologist at IRSN, says that the seaweed can store iodine-137 in its tissues at 100,000 times the concentration that it exists in the water. (In fact, planting brown seaweed off the coast of Fukushima has even been proposed as a way to clean up the water, although most scientists don't believe this is a good fix.) The best cleaner, Fievet says, is dilution in the massive Pacific Ocean.
*This item has been corrected. The article originally stated that radiation in the water surrounding Fukushima was 3555 times the legal limit. It has been amended to reflect the fact that levels of radioactivity in the water vary across the area and over time. One sample found levels of iodine-131 to be as high as 7.5 million times the legal limit at one point on Tuesday.