Assessing Fukushima-Derived Radiocesium in Migratory Pacific Predators.

The 2011 release of Fukushima-derived radionuclides into the Pacific Ocean made migratory sharks, teleosts, and marine mammals a source of speculation and anxiety regarding radiocesium ( ^134+137 Cs) contamination, despite a lack of actual radiocesium measurements for these taxa. We measured radiocesium in a diverse suite of large predators from the North Pacific Ocean and report no detectable (i.e., ≥ 0.1 Bq kg ^-1 dry wt) Fukushima-derived ¹³⁴ Cs in all samples, except in one olive ridley sea turtle (Lepidochelys olivacea) with trace levels (0.1 Bq kg ^-1 ). Levels of ¹³⁷ Cs varied within and across taxa, but were generally consistent with pre-Fukushima levels and were lower than naturally occurring ⁴⁰ K by one to one to two orders of magnitude. Predator size had a weaker effect on ¹³⁷ Cs and ⁴⁰ K levels than tissue lipid content. Predator stable isotope values (δ ¹³ C and δ ¹⁵ N) were used to infer recent migration patterns, and showed that predators in the central, eastern, and western Pacific should not be assumed to accumulate detectable levels of radiocesium a priori. Nondetection of ¹³⁴ Cs and low levels of ¹³⁷ Cs in diverse marine megafauna far from Fukushima confirms negligible increases in radiocesium, with levels comparable to those prior to the release from Fukushima. Reported levels can inform recently developed models of cesium transport and bioaccumulation in marine species.