Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California

Abstract

The Fukushima Dai-ichi release of radionuclides into ocean waters caused significant local and global concern regarding the spread of radioactive material. We report unequivocal evidence that Pacific bluefin tuna, Thunnus orientalis, transported Fukushima-derived radionuclides across the entire North Pacific Ocean. We measured γ-emitting radionuclides in California-caught tunas and found (134)Cs (4.0 ± 1.4 Bq kg(-1)) and elevated (137)Cs (6.3 ± 1.5 Bq kg(-1)) in 15 Pacific bluefin tuna sampled in August 2011. We found no (134)Cs and background concentrations (~1 Bq kg(-1)) of (137)Cs in pre-Fukushima bluefin and post-Fukushima yellowfin tunas, ruling out elevated radiocesium uptake before 2011 or in California waters post-Fukushima. 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. 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 results reveal tools to trace migration origin (using the presence of (134)Cs) and potentially migration timing (using (134)Cs:(137)Cs ratios) in highly migratory marine species in the Pacific Ocean.

Fig. 1.

(A) Map of the northern Pacific ocean showing simplified movement patterns for juvenile PBFT (blue arrows) from Japan to the CCLME and juvenile YFT (yellow arrows) in the CCLME. Kuroshio Current (gray arrow east of Japan) and CCLME (gray region west of N. America) are shown. Pie charts show mean concentrations of ¹³⁴Cs (red) and ¹³⁷Cs (dark gray) in seawater (Bq m³) (3), and muscle tissue (Bq kg⁻¹) in PBFT after Fukushima, PBFT before Fukushima, and YFT after Fukushima. Pie charts for fish are sized to scale of total radiocesium concentrations. Black arrows show uptake (solid arrows) and efflux (dotted arrows) of radiocesium in different ocean regions (arrow thickness scaled for relative efflux and uptake rates). (B) Simplified migration patterns of some highly migratory species in the Pacific that inhabit waters around Japan and make subsequent long distance migrations to distant ecoregions including Kamchatka, the Aleutian Islands, North America, South America, and New Zealand. Migration patterns are shown for salmon sharks (17) (short dashed line), sooty shearwaters (18) (long dashed line), Pacific bluefin tuna (6) (solid lines), and loggerhead turtles (19) (dotted line).

Fig. 2.

Measured (corrected for background levels of ¹³⁷Cs) and back-calculated estimates of radiocesium concentrations in muscle of post-Fukushima Pacific bluefin tuna Thunnus orientalis (n = 15). Estimated values of ¹³⁴Cs and ¹³⁷Cs concentrations in 2011 PBFT for various times before capture in California (30, 60, 90, 120 d) account for background levels of ¹³⁷Cs, decreases in tissue concentrations because of growth dilution and efflux rates of radiocesium out of the fish during their migration across the Pacific (detailed in SI Methods). Mean concentrations of ¹³⁴Cs (red triangles) or ¹³⁷Cs (gray squares) shown on the left y axis. Error bars represent 1 SD. Ratios of ¹³⁴Cs:¹³⁷Cs for each individual fish (empty circles) shown with scale on the right y axis. Dotted line represents 1:1 ratio of ¹³⁴Cs:¹³⁷Cs, the ratio expected in tuna while in waters off Japan contaminated with radiocesium at a ¹³⁴Cs:¹³⁷Cs ratio of 1.0.