The detection of Fukushima-derived radionuclides in Pacific bluefin tuna (PBFT) that crossed the Pacific Ocean to the California Current Large Marine Ecosystem (CCLME) in 2011 presented the potential to use radiocesium as a tracer in highly migratory species. This tracer requires that all western Pacific Ocean emigrants acquire the Cs-134 signal, a radioisotope undetectable in Pacific biota prior to the Fukushima accident in 2011. We tested the efficacy of the radiocesium tracer by measuring Cs-134 and Cs-137 in PBFT (n = 50) caught in the CCLME in 2012, more than a year after the Fukushima accident. All small PBFT (n = 28; recent migrants from Japan) had Cs-134 (0.7 +/- 0.2 Bq kg(-1)) and elevated Cs-137 (2.0 +/- 0.5 Bq kg(-1)) in their white muscle tissue. Most larger, older fish (n = 22) had no Cs-134 and only background levels of Cs-137, showing that one year in the CCLME is sufficient for Cs-134 and Cs-137 values in PBFT to reach pre-Fukushima levels. Radiocesium concentrations in 2012 PBFT were less than half those from 2011 and well below safety guidelines for public health. Detection of Cs-134 in all recent migrant PBFT supports the use of radiocesium as a tracer in migratory animals in 2012.