A study shows that bluefin tuna caught off the western coast of the U.S. are contaminated with radioactive substances leaked from the Fukushima nuclear power plant in Japan.

A research team at the State University of New York at Stony Brook said that the levels of cesium-134 and cesium-137 were ten times higher than previous years in 15 bluefin tunas caught off San Diego, California five months after the Fukushima disaster.

Although the levels of the radioactive materials are far lower than food safety standards set by the U.S. and Japanese governments, the finding will be recorded as the first case of tunas’ role in radioactive substances traveling across the Pacific Ocean.

The researchers assume that the blue fin tunas absorbed radioactive cesium while eating shrimp and squids in waters polluted with radioactive substances.