Radioactivity in fish and shellfish samples from the west coast of Canada after Fukushima (2011-18) 

The purpose of this post is to bring the public up to date on monitoring efforts of a research program into the impact of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on environmental and public health here in North America. This post is part of an ongoing series summarizing work carried out by the Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) project which operated from 2014-2020. Radioactive contamination of the Pacific Ocean following the FDNPP accident raised public concern about seafood safety, particularly in coastal Indigenous communities in British Columbia where I live. To address this, InFORM along with Health Canada,  Department of Fisheries and Oceans Canada and First Nations partners have collected and analyzed a total of 621 samples of commonly consumed salmon, ground fish, and shellfish from the Canadian west coast from 2011 to 2018. We examined the activities of cesium radioisotopes (134Cs half-life ~2 years and 137Cs half-life ~30 years) that were released in relatively large quantities from the Fukushima Dai-ichi Nuclear Power Plant (FDNNP) disaster in 2011 and would be most likely to pose radiological health concerns for human consumers of marine animals. Through careful analysis to determine the amount of radioactive isotopes in the seafood we have been able to carry out a health impact assessment. I wish to thank the following First Nations from British Columbia, Canada, for their generous donation of fish: Tr’ondëk Hwëch’in, Selkirk, Champagne and Aishihik, Taku River Tlingit, Tahltan, Nisga’a, Wet’suwet’en, Wuikinuxv, ‘Namgis, Hupacasath, Syilix, and Vuntut Gwich’in. I also thank Kayla Mohns and Brenna Collicutt of the Hakai Institute for assistance with the collection of shellfish samples. These results have recently been published in the peer-reviewed scientific literature in the Journal of Environmental Radioactivity and can be accessed here.

Highlights of the paper and key findings:

  • the vast majority of fish and shellfish did not have detectable levels of 137Cs or 134Cs where the minimum detectable level was 0.7 — 1.0 Bq kg-1 fresh weight for 6 hours of analysis by counting with a sensitive gamma emission spectrometer
  • 19 fish that had detectable levels of 137Cs were freeze dried and recounted for 336 hours and found to have an average 137Cs content of 0.29±0.02 Bq kg-1 fw
  • 2 of these 19 fish had detectable levels of 134Cs, the short-lived isotope, which showed clearly that fallout from the FDNPP was present in these particular fish. Given that the ratio of 137Cs:134Cs in the releases from the FDNPP was 1:1 we determined that the contribution of contamination in these fish from the nuclear accident was 49% and 24% respectively with the majority of caesium contamination coming from other sources like nuclear weapons testing and the Chernobyl disaster in the 20th century
  • 38 shellfish showed no contamination from FDNPP in either their shells or meat
  • 8 years of measurements showed that radioactivity in fish was dominated by naturally occurring radioisotopes and that levels of human-made radioisotopes remained small in the Pacific off North American following the FDNPP disaster
  • upper bounds for ingested doses of ionizing radiation from 137​​​​​​​Cs was determined to be ~0.26 micro-Sieverts per year and far below the annual effective dose of 2400 micro-Sieverts from exposure to other sources of radiation
  • we conclude that fish and shellfish from the Canadian west coast are not a radiological health concern despite the FDNPP accident of 2011

What we did

Samples were collected with the help of 13 First Nations, the Department of Fisheries and Oceans and the Hakai Institute in coastal waters and rivers of western Canada at sites indicated on the map below.

1-s2.0-S0265931X22001254-gr1_lrg.jpg
Figure 1 (a) Map of fish samples collected from 2014 to 2018 through the InFORM project. (b) Map of shellfish (bivalves) collected in 2016 and 2017.

For fish we removed the skin and bone to measure fillets which are typically consumed. Whole-body tissues of mussels, oysters, and clams were processed as they are generally eaten whole, but for scallops only the muscle itself was processed. In addition, we crushed up the shells of the individual shellfish to determine if radiocesium had accumulated in the shell as they are sometimes used to fertilize garden beds or adjust the hardness of rainwater used for home gardens.  We measured the radiocesium and naturally occurring radioisotopes in the samples using gamma spectrometry which you can learn about here. All samples were analyzed for 6 hours to screen samples for the presence of 137Cs and a subset of 19 were counted for a further 336 hours to determine if any 134Cs was present. This represents 10110 hours (more than 421 days) of counting time.

What we found

Results of the 6 hour counts and the extended counting samples can be found in here and here respectively. Given the extremely low levels of 137Cs present in the fish tissue (almost always below our minimum detectable concentration for individual fish) we averaged the gamma emission spectra of all fish collected in each year to determine the average 137Cs content. 137Cs content of all fish samples in each year fell between 0.18 and 0.25 Bq kg-1 fresh weight with the highest average concentration in 2017 and the lowest 2015. Data are here. The challenge of measuring and quantifying the amount of 137Cs can be understood by looking at the gamma emission spectrum and the averages for each year between 2014 and 2018 in the following figure.

1-s2.0-S0265931X22001254-gr2_lrg.jpg
Figure 2 Spectral summation of fish samples from 2014 to 2018, normalized to sample number. (a) Overlay, 150–2000 keV (b) Overlay with focus on the principal emission line for 137Cs at 661.7 keV

What the figure shows is that even after averaging the results for every fish collected in each year it was difficult for our team to detect human-made caesium isotopes. There was also no clear trend in time with 137Cs neither increasing nor decreasing with time in Pacific fish. In fact, the level of 137Cs found in the Pacific salmon was similar to levels found in Atlantic salmon (Table 4, 0.20 Bq kg-1) we collected in 2017 and analyzed from the Miramichi River on Canada’s east coast in New Brunswick. Careful analysis of 19 fish with longer counting times led us to be able to detect 2 fish with measurable levels of 134Cs which was an unmistakable sign of contamination from the FDNPP. However, given our understanding of the releases of 137Cs and 134Cs from the FDNPP following the disaster, most of the 137Cs present in the fish reflected contamination in the Pacific from nuclear weapons testing and the Chernobyl disaster rather than events following the 2011 meltdowns at FDNPP. For shellfish harvested from Canada’s west coast in 2016 and 2017, spectral summation of fresh weight samples (tissue and shell, respectively) yielded no detectable radiocesium or any other anthropogenic isotopes.

What it means

From 2011 to 2018, radioactivity measurements were made by the Fukushima InFORM project of 621 fish and shellfish samples harvested from Canada’s west coast. To investigate the impact of the oceanic contamination plume of Fukushima radioactivity to coastal waters, we used highly sensitive analyses and data reduction techniques to show that the concentration of 137Cs in the tissue of marine fish has not changed (0.18–0.25 Bq·kg−1 fw) from 2014 to 2018 while that of shellfish was undetectable. Relative to the abundance of naturally occurring isotopes like 210-Polonium in the same fish samples or to the annual dose exposure due to naturally occurring background radiation, it is abundantly clear that, by any metric, the radiocesium content of fish and shellfish from Canada’s west coast does not constitute a health risk, despite the FDNPP accident of 2011. The ecosystem and public health on the west coast of North America was never under threat from the FDNPP accident.

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