As a result of the Tohoku earthquake and subsequent tsunami there were significant releases of radionuclides from the damaged reactors at the Fukushima Dai-ichi Nuclear Power Plant. Of note were releases of radioactive cesium isotopes (134-Cs with a half-life ~2 years and 137-Cs with a half-life ~30 years) in an approximate ratio of 1:1. Because of its short half-life legacy 134-Cs from atmospheric weapons testing and the Chernobyl nuclear disaster in 1986 is no longer present in the environment so that this isotope serves as a unique tracer of Fukushima derived isotopes in the environment.
Neville and colleagues measured 134-Cs and 137-Cs in albacore tuna (Thunnus alalunga) collected off the coast of Washington and Oregon between 2008-2012.
Locations where albacore tuna were collected 2008-2012 for subsequent Cs isotope analysis.
Of the 26 fish analyzed all collected in 2008-2009 (before the Fukushima disaster) and 7 of 17 albacore collected in 2012 contained no detectable 134-Cs and had between 0.1-0.3 Bq/kg of 137-Cs (for a brief primer on units used to report radionuclide levels in the environment see here
). In 12 of 19 albacore collected in 2011 and 2012, 134-Cs from Fukushima was detected (0.02-0.36 Bq/kg) and 137-Cs was elevated relative to background with activities of 0.23-0.82 Bq/kg. The figure below summarizes the results of the study with panel a) showing 137-Cs present in tuna over all years, b) showing 134-Cs from Fukushima over time in fish and c) showing non-Fukushima 137-Cs in present in fish owing to legacy 137-Cs present in the environment.
137-Cs and 134-Cs in albacore tuna collected off the west coast of North America between 2008 and 2012 showing the contribution of Fukushima Cs to activities before and after the disaster.
The highest total combined activity of cesium in the most contaminated albacore was 1.18 Bq/kg which is 0.1% of the intervention level of 1200 Bq/kg set by the US Food and Drug administration (FDA)
. Consumption of 1 kg of tuna with this level of contamination increases radiation dose to a human consumer by ~20 nanoSievert (10^-9 Sv) or by 0.0006% given average radiation dose from natural sources experienced by the average American
. Fukushima derived radionculides in albacore tuna, therefore, are unlikely to represent a significant radiological health threat. This conclusion is similar to previously published work by others on Pacific bluefin tuna
An interesting conclusion of the study is that Cs isotopes may allow research scientists to answer outstanding questions with respect to the population biology, distribution and migratory behavior of albacore tuna. This is because differences in isotope composition of the fish varied with size and age of the albacore measured in the study.
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