Article: Neville, D.R.; Phillips, A.J.; Brodeur, R.D.; Higley, K.A. (2014) Trace levels of Fukushima disaster radionuclides in East Pacific albacore. Environmental Science & Technology, 48:4739-4743. DOI:10.1021/es500129b
As you all might recall, there was a wee bit of an earthquake (magnitude 9.0) in the western Pacific that occurred on March 11, 2011, resulting in a tsunami that caused the destruction of the Fukushima Daiichi nuclear plant in Japan (Figure 1). The explosions from three reactor buildings released a substantial amount of radioactive particles into the atmosphere and ocean, including the fission products Cesium-134 and Cesium-137 (134Cs and 137Cs). These cesium particles subsequently made their way into the food chain and were found in biota up to 600 meters offshore of the nuclear plant within one month of the accident. However, it has been estimated that surface ocean currents won’t transport the liquid plume with radionuclides from Fukushima to United States waters until 2014-2016.
But, ocean currents aren’t the only mechanism of radionuclide transport to the eastern Pacific. Some fish species are known to make trans-Pacific migrations. One such species is the Pacific albacore (Figure 2), which, between the ages of 2 and 5 years old, migrates annually between Japan and the United States. A survey of Pacific albacore conducted off the coast of Oregon and Washington every summer from 2008 to 2012 was able to show that not only was radiocesium present in Pacific Albacore, but that there were significant differences between pre-Fukushima fish and post-Fukushima fish.
A total of 7 albacore from 2008, 2 from 2011, and 17 from 2012 were ashed and counted on a high purity germanium gamma spectrometer. Although 137Cs was present in all fish sampled, 134Cs was only detected in fish collected after March 2011. 134Cs is primarily produced in nuclear fuel, but it is not released by U.S and Canadian reactors; therefore, any presence in albacore samples is assumed to be solely derived from Fukushima contamination. 134Cs has a half-life of only 2 years and the ratio of 134Cs/137Cs released from Fukushima is believed to be 1:1 (one-to-one), so, any 137Cs unaccounted for in the albacore samples after ratio corrections was attributed to pre-Fukushima levels (Figure 3).
These findings are great for providing insight into the mechanisms of radionuclide transport and show that fish can introduce nuclear contamination to different regions of the ocean well before ocean currents can. As a bonus, this research is also useful for inferring migration patterns of Pacific albacore caught along the U.S. Pacific Northwest coast. Not all of the fish caught after March 2011 contained 134Cs, indicating that some fish had not been exposed to Japanese waters. This could be due to migration pattern and age. Juvenile albacore departing the U.S. coast have been observed to undergo five different migration patterns, only one of which brings them to Japanese waters. Additionally, the data from this study show a strong relationship between age and inferred migration near Japan. Since Pacific albacore migrate across the Pacific between the ages 2 and 5 years, fish that are only 2 years old are less likely to have been exposed to Japanese waters than those that are 3 or 4 years old. While both 3 and 4 year old fish caught in the summer of 2012 would have had two opportunities to migrate to Japan and back since the 2011 earthquake, it is possible only the 4 year old fish had actually made the trip twice since 3 year old fish were found to have lower 134Cs concentrations (Figure 4). However, it must be clarified that this study does not determine whether these concentration differences between ages are due to differences in migration patterns (i.e. exposure) or simply age-based ability to accumulate heavy metal contaminants (metabolism).
This study found that the radiocesium in exposed Pacific albacore was only about 0.1% of the U.S. Food and Drug Administration level of concern, and therefore does not appear to be significant to food safety. Such fish monitoring can help us track radionuclide transport from the Fukushima-Daiichi disaster and provide insight into Pacific albacore migration routes by using radionuclides as tracers. Future work will include sampling of both northern and southern U.S. albacore fisheries to see if radionuclides in the fish can help establish migration patterns. It is currently hypothesized that the north and the south are actually two different stocks of fish, with only the northern substock migrating to Japan.