Even more than a year after the devastating earthquake and tsunami that led to meltdowns at the Fukushima Daiichi nuclear plant in northern Japan, fish in the region are still carrying some of the fallout. One scientist thinks it’s possible that radioactive elements may be lingering on the ocean floor.

In a paper published Friday in the journal Science, Woods Hole Oceanographic Institution researcher Ken Buesseler examined data released by the Japanese Ministry of Agriculture, Forestry and Fisheries, or MAFF, which has been monitoring radioactive material in fish and seafood since the accident.

In April, Japanese regulators tightened regulations on the amount of radioactive cesium isotopes that could be found in seafood, lowering the limit from 500 becquerels per kilogram in wet weight to 100.

 “Cesium levels have not decreased 1 year after the accident… and as of August 2012, fish are still being found with cesium levels above” the new regulatory level, Buesseler wrote.

Cesium accumulates in fish muscle tissues, typically at about 100 times the concentration levels in the surrounding seawater. The levels increase slightly as you go up the food chain, with bigger fish eating smaller fish.  But the fish also expel cesium at a rate of a few percent per day on average, which leads Buesseler to think there must be a remaining source of radioactive material contributing to the problem.

MAFF data shows that bottom-dwelling fish -- a group that includes commercially important species -- caught near Fukushima and four surrounding prefectures contain high levels of radioactive cesium. This suggests that the cesium contamination is likely in seafloor sediment, according to Buesseler.

“Given the 30-year half-life of 137-Cs, this means that even if these sources were to be shut off completely, the sediments would remain contaminated for decades to come,” Buesseler says.

Studies of cesium in fish are not enough, Buesseler argues – to make truly informed decisions about potential contamination of seafood, we need to better understand the dynamics of how radioactive bits of material accumulate in the environment.

“Such knowledge would support smarter and better targeted decision-making, reduce public concern about seafood, and potentially help to revive local fisheries safely, with confidence, and in a timely manner,” Buesseler says.

In April, Buesseler and colleagues published a paper describing how they detected radioactive elements in the ocean up to 600 kilometers (373 miles) off the Japanese coast. While the levels they found were not immediately threatening, there could be problems down the line as fish eat contaminated plankton, or when radioactive elements settle onto the ocean floor.

SOURCE: Buesseler, Ken. “Fishing for Answers off Fukushima.” Science 338: 480-482, 26 October 2012.