International Atomic Energy Agency inspectors took samples from a fish market near the Fukushima No. 1 nuclear power plant on Thursday following the release of treated wastewater from the wrecked facility in August.
China and Russia have banned Japanese seafood imports since the discharge began but Japan says it is safe, a view backed by the IAEA.
Some 540 Olympic swimming pools' worth of water have been collected since a tsunami sent three reactors at the Fukushima No. 1 plant into a meltdown in 2011 in one of the world's worst nuclear disasters.