Meanwhile, studies of animals on land around Fukushima and around the world's only other comparable nuclear disaster -- Chernobyl -- suggest that it may take years for the effects of the radiation to play out on animals.
"The mean the level of radiation was higher and less variable at Fukushima than at Chernobyl, implying that we should expect more negative effects on the abundance of animals at Fukushima if immediate effects of radiation were important," wrote Anders Pape Møller and colleagues in a paper in the January 2013 issue of the journal Ecological Indicators.
They counted spiders, grasshoppers, dragonflies, butterflies, bumblebees, cicadas and birds at 1,198 sites in Chernobyl and Fukushima. Surprisingly, Fukushima didn't follow the expected pattern.
"While (groups of animals) showed significant declines in abundance with increasing levels of background radiation in Chernobyl, only three out of seven taxa showed such an effect at Fukushima," they wrote.
The difference, say the researchers, is that drops in any animals populations in Fukushima are probably due to sudden, lethal doses of radiation, while those at Chernobyl are probably due to years of accumulated genetic mutations that cause a drop in the reproduction levels of animals.