Get yourself a Geiger Counter. I have a pancake style one that is very sensitive. It can detect Alpha, Beta, Gamma, and Xray. Bring it with you when you go food shopping. The Fresh or frozen fish can be tested. The only concern is the packaging that might stop Alpha particles. Before you eat any fish, even if it was caught in a lake, test it for radiation.

Fukushima has been contaminating for awhile now. We have been getting hot rains and those concentrate in ponds and lakes. Radiation tends to settle to the bottom and fish eat the irradiated particles. So I would suspect a lot of bottom feeding fish, like catfish, will have higher concentrations of Alpha particles. You ingest enough Alpha particles and you become toast....