A greenling caught in the Fukushima No. 1 nuclear plant's small harbor contained 510,000 becquerels of radioactive cesium per kilogram, 5,100 times above the state-set safety limit, Tokyo Electric Power Co. said.

If someone were to eat 1 kg of fish with this level, they would be exposed to about 7.7 millisieverts of internal radiation. Also caught during efforts by Tepco to rid the harbor of all fish was a spotbelly rockfish containing 277,000 becquerels of radioactive cesium per kilogram.

At the mouth of the harbor, where the ocean is about 10 meters deep, Tepco has set up a 2-meter-high net at the seafloor to prevent bottom fish from swimming out. The harbor seabed has been seriously contaminated with radioactive substances.

"If we make the net higher, vessels won't be able to go through," said a Tepco official, adding the utility will continue to get rid of fish in the harbor.

The highest level of radioactive cesium found in fish had been 254,000 becquerels per kilogram, also in a spotbelly rockfish caught in the harbor.