Two years after the Fukushima nuclear disaster, most marine products from the disaster-affected areas fall beneath national radiation limits, data from the Fisheries Agency shows -- though contamination appears to persist in some areas.
The agency said 24,848 seafood products from ocean waters and rivers in eastern Japan had been tested for radiation from the disaster as of Jan. 31 this year. Of these 90.1 percent were at or below the national radiation limit of 100 becquerels per kilogram.
Restricted to Fukushima Prefecture, the corresponding figure fell to 77.9 percent, while the average for prefectures other than Fukushima rose to 97.3 percent.
In the weeks after the March 2011 quake and tsunami that triggered the nuclear disaster and brought fishing to a halt, the government adopted a system to bolster radiation testing of seafood products before shipment. Thanks to this system, there has only been one time, last summer, when seafood products exceeding the national limit were released to the market and couldn't all be recalled.
Japanese fish farmers have refrained from using feed produced after the nuclear disaster, and besides a few exceptions within Fukushima Prefecture, radiation levels in farmed fish have remained under the national limit. Migratory fish like Pacific saury have also stayed below the limit.
Due to the influence of the Kuroshio Current there has been little contamination of fish south of the waters off Choshi, Chiba Prefecture, but there have been high contamination levels for bottom-dwelling fish like fat greenlings, olive flounders, and black rockfish taken from waters between the south side of the nuclear plant and northern Ibaraki Prefecture.
Jota Kanda, a professor of ocean chemistry at the Tokyo University of Marine Science and Technology who continues to test fish, says, "Radioactive cesium gets into fish through water or their feed. When you move them to clean water, their concentrations of radioactive cesium fall by a half or more after a month."
In July 2011, Kanda's team captured plankton within three kilometers of the coastline of Iwaki, Fukushima Prefecture. Plankton taken from near the shore was contaminated with radiation reaching hundreds of becquerels per kilogram, while plankton collected further out to sea had lower radiation levels. The radiation in seawater was too low to register a reading.
"It is important to discover how the feed of fish gets contaminated," Kanda said.
At the same time, fat greenlings caught between August last year and February this year in waters some 20 kilometers north of the Fukushima nuclear plant and from the plant's bay itself were contaminated with levels of radioactive cesium ranging between 20,000 and 510,000 becquerels per kilogram. Experts strongly suspect that some fish were contaminated in the bay soon after the nuclear disaster began, then swam to other areas. Tokyo Electric Power Co. aims to replace fishing nets separating the bay from outside waters with metal blocks around the middle of this year to stop contaminated fish from escaping.
Freshwater fish take three to four times as long as saltwater fish to see the amount of radioactive material in their bodies drop by half. However, according to research by the Fisheries Research Agency, radioactivity in the Japanese samelt, a freshwater fish, fell greatly a year after the nuclear disaster. These fish live for about a year, so it is thought that those born after the disaster have low levels of contamination. In future research, the Fisheries Agency plans to mark fish that are not contaminated, release them into rivers or oceans in contaminated areas, and investigate the radiation levels of new generations of fish.