A pair of greenlings have shown the highest level of radioactive caesium detected in fish and shellfish caught in waters off Japan's crippled Fukushima nuclear power plant, its operator said.

The fishes, captured 20 kilometres (12.5 miles) off the plant on August 1, registered 25,800 becquerels of caesium per kilo, Tokyo Electric Power Co. (TEPCO) said -- 258 times the level the government deems safe for consumption.

The previous record in fish and shellfish off Fukushima was 18,700 becquerels per kilo detected in cherry salmons, according to the government's Fisheries Agency.

TEPCO said the greenlings might have fed in radioactive hotspots and that it would sample more of the fish, their feed and the seabed soil in the area in the coming weeks to determine the cause of the high radiation.

Fishermen have been allowed since June to catch -- on an experimental basis -- several kinds of fish and shellfish, but only in areas more than 50 kilometres off the plant.

Those catches have shown only small amounts of radioactivity.

Greenlings have not been caught by fishermen off Fukushima since the massive earthquake and tsunami of March 2011 triggered meltdowns in reactors at the plant.