A Japanese fishmonger checks large bluefin tuna at Tokyo's Tsukiji fish market. (File photo)
A new study has found "modestly elevated levels" of radioactive material from Japan's crippled Fukushima nuclear plant in tuna fish caught off the US coast.
The study, published Monday by the Proceedings of the National Academy of Sciences (PNAS), examined 15 bluefin tuna caught off the coast of San Diego, California in August 2011, AFP reported.
Researchers found slightly higher levels of radioactive cesium-134 and cesium-137 in the 15 tuna, compared to bluefin tuna caught in the same area before the Fukushima nuclear disaster.
The study also compared the radioactivity with the levels found in yellowfin tuna -- which keep to the eastern Pacific -- caught before or after the accident.
In March 2011, a 9.0-magnitude earthquake triggered a massive tsunami that killed thousands of people and crippled the cooling systems at the Fukushima plant.
The incident led to meltdowns in several of the plant’s reactors and thus leakage of radioactive material into the air, soil and sea around the plant.
It said the results provide "unequivocal evidence" that the fish had carried the radioactive material leaked from the Fukushima plant across the ocean in their migration eastward.
"These findings indicate that Pacific bluefin tuna can rapidly transport radionuclides from a point source in Japan to distant ecoregions and demonstrate the importance of migratory animals as transport vectors," it said.
The researchers, however, assured the elevated radioactivity posed no threat to public health, putting the observed levels as more than an order of magnitude lower than the Japanese safety limit.
They also noted that the cesium isotopes detected give off far less radiation than potassium-40, a naturally-present isotope found in all the fish.