Radioactive cesium from Fukushima Japan detected in bluefin tuna off California: Implications for public health and for tracking migration

Bluefin tuna are highly migratory pelagic fish. Pacific bluefin tuna are spawned in waters off Japan and some juveniles migrate across the Pacific to waters off California. Bluefin that had recently migrated and caught in waters off San Diego, California in August 2011 were found to have ¹³⁴Cs and ¹³⁷Cs in their muscle tissue, at concentrations that could only have come from the Fukushima nuclear reactor in Japan. Bluefin tuna caught 3 years earlier off San Diego and yellowfin tuna that are resident to the eastern Pacific had no ¹³⁴Cs and only background levels of ¹³⁷Cs. The radioactivity in tuna attributable to these Cs isotopes was only 2.8% of that from the naturally occurring ⁴⁰K, suggesting that consumption of these fish poses little risk to public health. The presence of these isotopes in animal tissues can be used to discern migratory routes and timing for those animals that use the western Pacific.