The Pacific bluefin tuna (PBFT), Thunnus orientalis, is a highly migratory fish that inhabits the western and eastern North Pacific Ocean at various life stages ( 5) ( Fig. Before the Fukushima discharge, low concentrations (1.5 mBq L в€’1) of the long-lived 137Cs (fallout from weapons testing) were detectable in Japanese waters ( 3), whereas the shorter-lived 134Cs was undetectable in Pacific surface waters and biota. After considerable dilution 2вЂ“3 mo after maximum discharge, surface concentrations still exceeded prior concentrations by up to 10,000-fold in coastal waters ( 3) and up to 1,000-fold over a 150,000 km 2 area of the Pacific up to 600 km east of Japan ( 4). The dominant long-lived Оі-emitting radionuclides 134Cesium ( t 1/2 = 2.1 y) and 137Cs ( t 1/2 = 30 y) were released at a consistent ratio of about 1 (0.99 В± 0.03) ( 3), although the ratio of 134Cs: 137Cs would decline slowly over time because of the differential decay rate constants of the two isotopes ( Fig. The release of radionuclides produced a 1вЂ“2 wk pulse that peaked on Apwith ocean concentrations of 68 MBq m в€’3 ( 3) and an estimated total release of up to 22 Г— 10 15 Bq of 137Cs ( 4) (1 Bq = 1 disintegration s в€’1). Coolant pumps failed to operate and the power plant reactors overheated, leading to a release of radionuclides directly into the ocean exceeding that from any previous accident ( 3).

On March 11, 2011, an earthquake and subsequent tsunami led to flooding of the Fukushima Dai-ichi nuclear power plants in Japan. The infrequency of nuclear accidents coupled with potentially wide-ranging effects on ecosystems and human health make the dynamics and risks of radionuclide discharge into the environment a relatively poorly understood but highly important area of research ( 1 вЂ“ 4). These results reveal tools to trace migration origin (using the presence of 134Cs) and potentially migration timing (using 134Cs: 137Cs ratios) in highly migratory marine species in the Pacific Ocean. Other large, highly migratory marine animals make extensive use of waters around Japan, and these animals may also be transport vectors of Fukushima-derived radionuclides to distant regions of the North and South Pacific Oceans.

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 of radionuclides. We found no 134Cs and background concentrations (в€ј1 Bq kg в€’1) of 137Cs in pre-Fukushima bluefin and post-Fukushima yellowfin tunas, ruling out elevated radiocesium uptake before 2011 or in California waters post-Fukushima.

We measured Оі-emitting radionuclides in California-caught tunas and found 134Cs (4.0 В± 1.4 Bq kg в€’1) and elevated 137Cs (6.3 В± 1.5 Bq kg в€’1) in 15 Pacific bluefin tuna sampled in August 2011. We report unequivocal evidence that Pacific bluefin tuna, Thunnus orientalis, transported Fukushima-derived radionuclides across the entire North Pacific Ocean. The Fukushima Dai-ichi release of radionuclides into ocean waters caused significant local and global concern regarding the spread of radioactive material.

Nicholas scida