Scientists from plant operator Tokyo Electric Power (TEPCO) say the murasoi fish had 10 times higher radiation than that measured last August in a scorpion fish caught near the crippled reactor complex.
TEPCO says massive nets are being installed about 15 miles offshore to try to prevent contaminated fish from migrating.
The plant was pummeled by a titanic tsunami in 2011 that was generated by a massive undersea earthquake.
It resulted in reactor meltdowns that caused radioactive contamination that will linger for decades to come.
Fishing around Fukushima has since been halted and the government has also banned farming, dairy and livestock production indefinitely.