More than a year and a half after an earthquake and tsunami destroyed the Fukushima Dai-ichi nuclear power plant in Japan, many fish in the area contain levels of radioactive cesium that are just as high as they were soon after the disaster.
The finding suggests that the region's coastal-dwelling fish are still being exposed to new sources of cesium, possibly from the seafloor or from contaminated groundwater that's flowing into the ocean. And even though most fish sampled in the new study had levels of cesium below safe limits for consumption, some fish contained surprisingly large amounts.
Japan has already closed fisheries near Fukushima to reduce human exposure. The new results suggest that it may be a long time before levels of radiation in the ocean decline after nuclear disasters like the Fukushima meltdown.
"If (the cesium) is in the seafloor, it could be many years or even decades for that to go away," said Ken Buesseler, an oceanographer at the Woods Hole Oceanographic Institution in Mass. "That implies we're going to have an issue in coastal fisheries for a long time to come in Japan. We certainly can't say we're out of the woods yet."
"Just because you haven't read about it in the news" lately, he added, "doesn't mean it has gone away."
Because the Japanese rank among the most voracious consumers of seafood in the world, the country's Ministry of Agriculture, Forestry and Fisheries has been closely monitoring radiation levels in coastal fish since the Fukushima disaster in March of 2011.
To play it extra safe, Japan has also tightened restrictions on two radioactive forms of cesium so that fisheries must be closed if levels in fish exceed a limit of 100 units of a measurement called becquerels per kilogram of wet weight. Consuming levels of cesium above that much every day for a year, Buesseler said, would start to constitute a safety risk.
The fisheries agency has been releasing data regularly, including an annual report on more than 8,500 samples of fish, shellfish and seaweed taken from the coastal areas near Fukushima. Even though the data is freely accessible to the public, it can be hard to interpret so much information. So, Buesseler analyzed the numbers to see what kind of patterns they might reveal.
He was immediately struck with the realization that radiation levels in fish had not dropped in the past year. Fish naturally lose a few percent of their concentrations of cesium if they are not re-exposed, Buesseler said, suggesting that the animals are still facing new inputs of the radioactive material.
Cesium levels were highest in fish that live near the seafloor, Buesseler reported today in the journal Science. And bottom-dwellers near Fukushima contained more cesium than did fish that lived further away from the disaster site.
More than 40 percent of fish in the region contained levels greater than the new safe-consumption limit of 100 units per kg. Two greenling fish collected this August contained a surprisingly high level of 25,000 units.
"The most intriguing thing in the study is that one would've expected concentrations to decline significantly if cesium concentrations in the water were declining over the last year," said Nicholas Fisher, an oceanographer at Stony Brook University in New York, whose group detected small amounts of Fukushima-derived cesium in bluefin tuna off the coast of California last year.
"The fact that there's no significant decline in these fish suggests that the fish are being exposed to a constant supply of cesium either from their food or from the water."
Because bottom-dwellers contained more cesium than fish living higher up in the water column, cesium may have accumulated in sediments on the seafloor or in the worms and other invertebrates that live in the sediments. And because it takes 30 years for half of a sample of cesium to break down, Buesseler speculated that decades could pass before exposure levels decline significantly.
It's not clear exactly what the results mean for human health. Fisheries near Fukushima remain closed, so it's not possible to purchase seafood contaminated with cesium from the nuclear meltdown.
Even if people were allowed to eat seafood from the area, the Fukushima fish contain far lower levels of cesium than of naturally occurring radioactive materials, including polonium-210 and postassium-40.
These materials are ubiquitous in marine animals, Fisher said, because the Earth has always been radioactive and 99 percent of radiation in the oceans is natural. Of the remaining one percent, most comes from nuclear weapon testing in the 1960s.
"I don't give advice -- people can make up their own minds about whether they'll eat seafood or not, but they should do so with all available information," Fisher said. "They should know that there's a lot of naturally occurring radioactivity."
For his part, Buesseler ate all of the seafood he was offered on a trip to Japan in July.