Radiation levels in fish caught near the Fukushima Daiichi nuclear power plant remain high long after the 2011 meltdowns there, suggesting contamination from the site might still be seeping into Pacific waters, a U.S. researcher said, according to CNN.

The "vast majority" of fish caught off Japan show no sign of radioactive contamination at levels dangerous to humans, even at the tighter limits Japan imposed after the Fukushima Daiichi accident, Ken Buesseler, a marine radiochemist at the Woods Hole Oceanographic Institution in Massachusetts, wrote in this week's edition of the journal Science.

But close to the plant, an ongoing high level of the reactor byproducts cesium-134 and cesium-137 "implies that cesium is still being released to the food chain," he wrote.

The sources are likely to be from radioactive particles released by the plant that settled into sediment on the sea floor or from groundwater seeping into the ocean from the plant, as operators continue to pour tons of water a day into the reactors to keep them cool. Researchers can't say definitely which is the more likely source, "but we know both are happening," Buesseler said.

The plant is no longer venting radioactive steam into the atmosphere, as it was at the height of the disaster. And the plant's operator, the Tokyo Electric Power Company, has set up a system to absorb cesium from reactor coolant water. But not all that water is being recovered, Buesseler said.