The massive radioactive fallout from the Fukushima No. 1 nuclear plant has sparked fear in seafood lovers and commercial fishermen both at home and abroad, and some worry the contamination could pass through and even become more concentrated in the ocean food chain.

But more than 10 months after the three reactor meltdowns, testing of thousands of fish, including tuna, bonito and “sanma” (Pacific saury), caught far from Tohoku’s coast has turned up little contamination.

Nevertheless, experts point out that consumer concern and uncertainty will remain regarding bottom fish from coastal areas near Fukushima Prefecture, including “hirame” (Japanese flounder), as well as freshwater fish from Fukushima and parts of Gunma and Tochigi prefectures.

Radioactive materials tend to accumulate on the seabed near coasts, and they usually remain longer in the closed environment of lakes. Freshwater fish also discharge radioactive cesium slower than sea fish, experts say.

“(Contamination) levels of freshwater fish and seabed fish such as flounder haven’t declined,” says Satoshi Katayama, a professor of fisheries biology and ecology at Tohoku University.

“It’s hard to say the contamination peak among such fish has passed.”

Other than those caught near Fukushima Prefecture, no bottom feeders have been found to exceed the provisional maximum level for radioactive cesium of 500 becquerels per kilogram.

Fishermen have voluntarily stopped all commercial fishing off the Fukushima coast since March, while beach seine fishing off northern Ibaraki Prefecture was halted late last month.

Still, experts stress that close monitoring of bottom fish off Fukushima should continue because there’s no telling when contamination levels will peak.

A few samples of fish such as flounder and “ainame” (rock trout) have been found to contain radioactive cesium in excess of the 500-becquerel threshold since July. One sample of ocean bottom fish off Ibaraki Prefecture was also found to be over the limit in September.

Takashi Ishimaru, a professor of ocean science at Tokyo University of Marine Science and Technology, says contaminated plankton and waste from fish naturally sink to the seabed and are then eaten by miniscule benthos organisms, which bottom fish feed on. As a result, radioactive materials are passed up the food chain.

“Radioactive materials circulate at the bottom of the ocean. . . . It won’t be reduced easily,” he says.

Researchers have yet to obtain data on how contamination from the Fukushima No. 1 plant has spread along the sea floor, so consumers should keep a watchful eye on bottom fish, Ishimaru advises.

“There could be hot spots under the sea,” he warns. “If hot spots exist on the ocean floor, contamination levels of fish that inhabit such areas may spike.”

Meanwhile, samples of freshwater fish caught in Fukushima and Maebashi, Gunma Prefecture, have revealed high levels of contamination exceeding the government limit. Tests conducted between April 19 and Jan. 10 in Gunma on 34 freshwater fish showed that 11 caught in Lake Onuma exceeded the maximum level.

In Tochigi Prefecture, meanwhile, “ayu” (sweet fish) caught in May in the Kinugawa River near the city of Utsunomiya and the Nakagawa River near the town of Motegi contained radioactive cesium above 400 becquerels per kilogram — less than the current provisional limit but four times higher than the new limit the government will introduce in April.

Unlike sea fish that quickly discharge radioactive cesium along with salt, freshwater fish retain salt for a much longer period, says Ishimaru, the ocean science professor.

“Freshwater fish may continue to be contaminated for a longer period than sea fish,” he says.

Meanwhile, regular studies nationwide have found no alarming signs among ocean fish caught outside the coastal areas of Fukushima and Ibaraki prefectures.

According to 5,524 sampling tests conducted by the central and local governments, no ocean fish except those caught off Fukushima and Ibaraki had been found to contain radioactive cesium above 500 becquerels per kilogram as of Jan. 24.

Fish caught in southern waters should be especially safe to consume, because the Black Current (the north-northeast-flowing Kuroshio current along the Pacific coast) is preventing radioactive materials from spreading any farther south than Chiba Prefecture, experts say.

Radioactive materials off the coast of Fukushima are carried southward to Chiba by the Oyashio Current, where it meets the Black Current that pushes the isotopes away from Honshu and out into the Pacific, they say.

“The Black Current flows eastward at a very high speed. So it’s just impossible for radioactive materials to go any farther south beyond that point,” says Tohoku University’s Katayama.

Some migratory tuna and bonito, deep-ocean fish, have tested at around 10 to 20 becquerels of radioactive cesium, but the levels aren’t likely to increase because ocean contamination levels at present are barely detectable and the amount of radiation in the plankton and other materials that fish feed on has declined, according to experts.

“As for migratory fish far off the coast, I’m not too worried,” Katayama says.

Even if fish swim through the polluted waters off Fukushima, “migratory fish only stay there for a few weeks or about a month at the longest. . . . So basically, contamination levels are more likely to decline than increase,” he says.

Ishimaru, the ocean science professor, says that if a contaminated fish is put into clean water, any radioactive cesium it contains will fall by half in about 50 days as fish discharge the isotopes through their gills and in their excreta, he says.

“If it passes through polluted seawater, the fish may become contaminated to a certain level. But the radioactive materials in the seawater has been diluted and diffused,” Ishimaru says.

“As long as contamination levels of their food and seawater don’t increase, (migratory fish contamination) levels will decline.”

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