The fish, which spawn in Japanese waters, migrate to the Californian coast to feed and have transported the radioactive particles across the ocean. Several thousand tonnes are caught each year for human consumption but marine scientists do not currently think they pose any harm to human health.
The tuna, tested by scientists at Stony Brook University, New York, contain levels of radioactive caesium averaging 5-10 becquerels (Bq), which is below the accepted safe level for human consumption (100Bq/kg). Scientists warn the levels could rise when the fish return to Japanese waters and are once again exposed to the high levels of radionuclides.
Caesium is often present in seawater as a result of atomic weapons testing but Fukushima was the biggest accidental release of radioactive material into the sea in history. Marine biologists, however, have stumbled upon a silver lining because measuring the rate at which the Fukushima caesium decays will enable them to trace migratory patterns in marine wildlife.