Our Landblog reports:

The fish had migrated from the coast of Japan where the meltdown of nuclear reactors in Fukushima following last year’s tsunami contaminated thousands of square miles of ocean.

The level of radiation is well below that which is considered unsafe for humans. 

From The Guardian: 

Virtually all bluefin tuna on the market in the United States is either farmed or caught far from the Fukushima area, so American consumers should not be affected by radiation contamination in their fish, seafood distributors say. The migratory bluefin studied by the researchers were all caught by sport fishermen and were not headed for the market.