According to their findings in the Proceedings of the National Academy of Sciences, they radiation levels are 10 times higher than the amount usually found in those fish. They've carried the radioactive contamination 6,000 miles from the waters near Japan's crippled nuclear plant.
Scientists sampled 15 tuna caught off San Diego. All of them contained two radioactive substances, caesium-134 and caesium-137.
"I think people imagine those surprising moments happen all the time when you do research, but they haven't for me, until this one. We really didn't expect to see it," said Dan Madigan, a co-author of the study.
Previously, smaller fish and plankton were found with high-than-average levels of radiation in Japanese waters after the 9.0-magnitude earthquake of 2011. The temblor triggered a tsunami that badly damaged nuclear reactors.
Researchers said it's the first time a huge migrating fish has been shown to carry radioactivity that far.
According to a variety of scientists polled on the matter, the leves are still well within the safe-to-eat limits.
The Associated Press contributed to this report.