Radioactive cesium levels have steadily declined in fish off Fukushima Prefecture, but samples taken closer to the wrecked nuclear power plant were still showing high readings, according to studies.
And the government, which plans to take charge of the persistent problem of radioactive water leaks at the plant, can only guess at the extent of strontium contamination among marine life in northeastern Japan.
According to the Fisheries Agency, only three, or 0.6 percent, of 472 marine samples tested between Sept. 1 and 19 in Fukushima Prefecture showed radioactive cesium levels exceeding the government safety standard for food of 100 becquerels per kilogram. That compares with 2.7 percent of the samples tested in July and August, and 53 percent during the early stages of the nuclear crisis in the March-June period of 2011.
Of the latest samples, 13.3 percent had levels between 10 and 100 becquerels per kilogram, while 86 percent had concentrations of 10 becquerels or less per kilogram or were undetectable, according to the agency.
In other prefectures, the ratio of samples breaching the safety threshold of 100 becquerels dropped from 6.5 percent in March-June 2011 to 0.4 percent in July-August 2013.
In the September tests, none of the 407 samples outside Fukushima Prefecture contained radioactive cesium exceeding 50 becquerels per kilogram.
In fact, 94.3 percent of the samples fell to 10 becquerels or less or the contamination was undetectable, the agency said.
The targets of the tests were mainly major fish species and those that had previously shown more than 50 becquerels of radioactive cesium per kilogram.
The decreases were particularly sharp among white bait and Japanese sand lance, which showed high concentrations of cesium shortly after the meltdowns at the Fukushima No. 1 nuclear power plant in March 2011.
None of the 215 samples of these species showed a level of even 1 becquerel of cesium in tests across the country since April this year.
In addition, the radioactivity levels in long-distance travelers, such as skipjack tuna, Pacific saury and chum salmon, were all below 5 becquerels in the latest cesium tests, the agency said.
However, Takami Morita, a senior research coordinator at the Fisheries Agency, said some fish living in shallow waters were still showing relatively high levels of cesium.
The highest reading in the September tests was 130 becquerels in rockfish.
“Radioactivity levels have not fallen substantially in fish species in waters along the coast of Fukushima Prefecture as well as those inhabiting the shallow seabed or rocky stretches,” Morita said.
He said these fish are likely ingesting cesium by eating organic matter in the mud.
These species include marbled flounder, stone flounder, olive flounder, sea bass, Japanese rockfish and Japanese black porgy, all living in depths under 100 meters. Cesium levels exceeding 100 becquerels per kilogram have been found in some samples since April.
Although contaminated water continues to leak at the Fukushima nuclear plant, radioactivity levels of seawater outside the harbor of the plant were considered undetectable, according to government studies.
“Despite the fact that contaminated water has been leaking since 2011, not just this summer, the cesium concentration of fisheries products has receded,” Morita said. “The main cause of ongoing radioactive contamination of fisheries products can be said to be the large volumes of radioactive materials that scattered immediately after the accident.”
However, the government’s standard for food is based only on measurements of radioactive cesium.
And the tests are not designed to read levels of other radioactive materials, including strontium, which is believed to accumulate in bones and can cause cancer and leukemia.
Strontium is difficult to detect, and an accurate analysis can take about a week.
The Fisheries Research Agency, commissioned by the Fisheries Agency, has conducted only 40 tests for strontium in marine product samples.
The Ministry of Health, Labor and Welfare said it estimates levels of strontium at about 12 percent of radioactive cesium levels.
“We have to keep carefully studying the situation,” said Jota Kanda, a professor at the Tokyo University of Marine Science and Technology. “We have to increase the number of inspections for strontium levels.”
Eighteen prefectural governments and industry associations in eastern Japan have been testing for radioactive cesium in marine products around once a week since the nuclear crisis started.
In Fukushima Prefecture, 14,070 samples were tested between March 2011 and late August this year, while 23,400 samples were studied in other prefectures over the same period.
The nuclear accident has halted coastal and trawl fishing in Fukushima Prefecture, with the exception of trial fishing operations involving limited species.
Some fishermen in the prefecture resumed trial operations on Sept. 25. But Tokyo Electric Power Co., operator of the stricken plant, belatedly acknowledged that radioactive water at the plant site was spilling into the Pacific Ocean.
Leaks from tanks storing highly radioactive water on the site have also been reported.
The Fukushima fishermen fear that constant problems at the nuclear plant and TEPCO’s struggles to deal with the water crisis will further erode confidence that their catches are safe. South Korea earlier this month banned imports of Japanese marine products from eight prefectures after the leaks at the Fukushima plant were reported.
Shipment restrictions for certain fish have been imposed in other prefectures: sea bass and two other species caught off Miyagi Prefecture; seven species, including rockfish, off Ibaraki Prefecture; and sea bass and Japanese black porgy caught south of Iwate Prefecture’s border with Miyagi Prefecture.
Ibaraki Prefecture has restricted shipments of 13 fish species, including fat greenling, caught in the northern part of the prefecture.
The restrictions are based on the results of the ongoing tests for radioactive cesium.