A short Asahi Shinbun article mentions that in passing.
The August 30 article is for Asahi's Fukushima local version, and it is about the decision by the Fukushima Prefecture's Fishery Cooperatives Association to not do the coastal trawl-net fishing for the month of September.
But at the end of the article there's this:
On the other hand, Onahama Port in Iwaki City had its first landing of 18 tonnes of shipjack tuna caught off the coast of Miyagi Prefecture. A quick testing was done by Iwaki Meisei University didn't find radioactive materials, and the fish were sold in the city and also shipped to the Tokyo region.
I don't know what's involved in the quick testing. So maybe there were no radioactive materials, or maybe there were but below detection limit.
I don't think shipjack tuna know where Fukushima Prefecture ends and where Miyagi Prefecture starts, but the fishery for shipjack tuna is about 200 kilometers off the coast of Tohoku and Kanto. Just because the fish come from Onahama Port in Fukushima Prefecture, it doesn't mean they are potentially radioactive.
(Just remember the radioactive materials that leaked into the ocean is 15,000 terabecquerels, and that number is without calculating the iodine-131 equivalent of cesium, as you would if you were trying to figure out the INES event scale.)