By Jay T. Cullen

Introduction

As part of an ongoing series documenting the impacts of the Fukushima disaster on the North Pacific and west coast, this post summarizes a recently published study by Delvan Neville and colleagues in the peer-reviewed journal Environmental Science and Technology. The paper reports measurements of Cesium-134 and Cesium-137 in 26 albacore tuna caught off the west coast of North America between 2008 and summer 2012. Because of its relatively short half-life (~2 years) 134-Cs is an unambiguous tracer of radionuclides released from the Fukushima Dai-ichi disaster which began in March 2011. Fish collected in 2011 and 2012 had higher 134-Cs and 137-Cs that was due to Fukushima sourced cesium in the Pacific. Fish collected in 2008-2009 had lower 137-Cs activities that largely reflected historic releases of the isotope from atmospheric nuclear weapons testing in the 20th century. The authors conclude that given the highest levels of Cs isotopes measured in albacore tuna, human consumption of the fish would not not represent a significant increase in annual radiation dose. The corresponding radiological health risk due to Fukushima derived radiocesium in these tuna is, therefore, very small.

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.

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. By late 2012 Fukushima derived 134-Cs was not detected in the tuna.

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.