The big question: How safe is fish to eat?

March 27, 2012


For a fish-eating nation like Japan, research findings on cesium contamination in marine life since the nuclear disaster in Fukushima Prefecture a year ago is quite disconcerting.

Sea creatures dwelling on the ocean floor continue to show high levels of contamination.

Decontamination of the seabed is not feasible, which means it could take years of research to judge the rate of dispersion and whether marine life is safe to eat.

The Fisheries Agency, working with nine prefectures from Hokkaido to Kanagawa, has been studying radioactive substances in the ocean and its effect on marine life.

Their initial efforts found contaminated seawater and detected radioactive substances in sand lance, whitebait and other fish that dwell near the ocean's surface.

Although the concentration of radioactive substances in seawater has gradually dropped to below detectable levels, the contamination has begun to gradually sink to the ocean floor, which is host to much marine life.

Not surprisingly, waters off the coast of Fukushima Prefecture have shown high readings.

Prefectural officials who examined 105 species of fish caught between last June and January said contamination levels exceeding national safety standards (500 becquerels per kilogram) were detected in 13 species, comprising 110 fish.

Except for two specimens, all were bottom-feeders such as flounder, darkbanded rockfish and fat greenling.

Even this year, 12 of 188 fish examined exceeded standards with a "shiromebaru," a type of rockfish, having 1,920 becquerels.

Jun Misonoo, of the Marine Ecology Research Institute, said the food chain is easily compromised.

He explained: Creatures dwelling on the ocean floor, such as ragworms and starfish, feed on cesium-tainted plankton and they, in turn were eaten by creatures living near the sea floor.

Over a prolonged period, higher cesium concentrations are found all the way up in the marine food chain.

"Fish are able to expel cesium from their body at some point, but those living on the ocean floor will continue becoming contaminated so long as they eat contaminated food," says Takashi Ishimaru, a professor at Tokyo University of Marine Science and Technology.

Because the ocean floor is churned slowly by the currents, high concentrations can accumulate--especially in rocky formations. A reading of 1,580 becquerels per kilogram was detected on the ocean floor more than 30 kilometers north of the Fukushima nuclear plant in December. Even by January, a reading of 4,100 becquerels was detected on the ocean floor some 30 kilometers to the south.

Research by Kyoto University and other institutions indicates that cesium in rivers may be seeping into oceans as a result of rainfall and other means.

The only way to reduce cesium levels is to either wait for nature to take its course, or decontaminate the ocean floor. That latter is not even technologically feasible.

The Japanese government passed a resolution accompanying its special measures law in response to radioactive contamination that cited the ocean as an area to consider for decontamination. To date, however, there is no way to accomplish that.

For this reason, researchers must continue surveying marine life dwelling on and near the ocean floor.

Although the number of fish falling within standard levels is rising, Misonoo remains cautious.

"We should investigate the contamination's dispersion in greater detail and observe it for the next 10 years," he says.


At 4 a.m. last Oct. 24, a fishing boat at Ena Port in Iwaki, Fukushima Prefecture, was all lit up prior to its departure.

Masami Yabuki, 48, of the Iwaki city fisheries cooperative, signaled that the boat was about to leave by crying out, "Shukko da!"

The engine of the No. 23 Tsunemasa-Maru rumbled as the boat chugged away from port.

Since early April, the Iwaki association, along with the Soma-Futaba fisheries cooperative and the prefectural marine testing center, has dispatched boats on weekly excursions to assist prefectural authorities in surveying radioactive contamination in the ocean.

On this day, the Tsunemasa-Maru headed northward for some 17 kilometers. On arriving at 7:40 a.m., Yabuki started reeling in the trawl net, hauling up mackerel, conger eel and squid. The haul weighed 400 kilograms.

Not all of the fish were needed for the survey on radioactive substances, so Yabuki threw the mackerel and conger eel back into the sea.

"I could sell them all," he said, with a sigh.

When asked whether he could take the haul back to port, rather than letting it go to waste, he replied, "No, that's prohibited." The crew sorting out the fish forced a smile, but they could not hide their disappointment.

At 9 a.m., the boat finally returned to port with less than half the catch, all packed into 15 plastic containers.

Yabuki's 74-year-old father, Masakazu, is the president of the Iwaki city fisheries cooperative.

"I'm worried about whether decontamination (on the land) will result in more radioactive substances in the ocean," he says. "I want the government to manage this right."

Fishermen in Fukushima Prefecture continue to abstain from commercial fishing.

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Fisheries cooperatives in Fukushima Prefecture are continuing self-imposed restrictions on fishing, limiting their activities to research. (The Asahi Shimbun)

Fisheries cooperatives in Fukushima Prefecture are continuing self-imposed restrictions on fishing, limiting their activities to research. (The Asahi Shimbun)

  • Fisheries cooperatives in Fukushima Prefecture are continuing self-imposed restrictions on fishing, limiting their activities to research. (The Asahi Shimbun)

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