Japan's Environment Ministry carried out a study that found fish caught near the plant had more radiation than fish caught elsewhere, RIA Novosti reported.
The levels found ranged from 4,400 becquerels per kilogram to 11,400 becquerels per kilogram, against the maximum "safe" level of 100 becquerels per kilogram.
The findings have prompted a government investigation into the physiological basis for contamination and why radiation readings in some specimens remain hundreds of times over the official safe limit, The Asahi Shimbun reported.
The Council for Science and Technology Policy, a government panel, is to study why radiation levels in fish near the plant have not decreased since March.
Researchers will analyze cesium levels in the fish's otolith, a part of the inner ear.
Researchers will measure cesium levels in contaminated seabed soil near the nuclear plant, in an effort to determine when the fish were contaminated.