By Jay T. Cullen

The purpose of this post is to present measurements of artificial radionuclides in wild Atlantic salmon (Salmo salar) made in 1990’s and reported in a peer reviewed paper published in the Canadian Journal of Fisheries and Aquatic Sciences by Tucker and colleagues in 1999. This post is part of an ongoing series dedicated to communicating the results of scientific research aimed at The paper combines an understanding of bioconcentration of the artificial radionuclide 137-cesium (¹³⁷Cs half life ~30 years) in marine food webs with quality measurements of the contaminant in salmon that spent their lives in the North Atlantic Ocean.  The North Atlantic at the time had a strong east to west gradient in ¹³⁷Cs concentrations in seawater with >10 Bq m^-3 in the east owing to spent nuclear fuel reprocessing in Europe and the recent impact of the Chernobyl disaster and <1.5 Bq m^-3 in the west near to Canada.  Salmon returning to the Ste. Marguerite River in Canada had a wide range of radiocesium in their bodies which reflected the entire range of values seen in fish harvested across the Atlantic Ocean.  The results indicate that the migration routes of these salmon extended all the way across the Atlantic to the Irish and Norwegian seas.  The study is relevant to understanding the impact of the Fukushima disaster on radiocesium levels in Pacific salmon as the maximum levels of contamination of seawater we see in the central and eastern North Pacific is lower than the maximum levels studies by Tucker and colleagues.  Given this fact we would predict that levels of Fukushima derived contaminants in Pacific salmon and the health risk associated with the consumption of these fish will be correspondingly lower.  Thus far the salmon monitoring results from the Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) project are in keeping with the scientific communities understanding of ¹³⁷Cs bioconcentration in fish outlined in the Tucker study and references therein.

Radionuclides in Atlantic Salmon: Bioconcentration and migration routes

When I was an undergraduate student at McGill University in Montreal, Dr. Joe Rasmussen (he is now at the University of Lethbridge in Alberta) headed up a freshwater ecology group that used radionuclides to understand energy and contaminant cycling in the aquatic environment. I remember learning about gamma spectrometry and the processing of samples for radionuclide determinations through conversations with his graduate students.  Their work made an impression on me and highlighted the utility of radioisotopes for understanding rates of processes and pathways of contaminant transport in natural waters.  The paper I will summarize here roughly dates to my time at McGill and is the work of Strahan Tucker who along with Marc Trudel works for Fisheries and Oceans Canada at the Pacific Biological Station in Nanaimo BC and part of the team of scientists working on the InFORM project.  Tucker and colleagues exploited the east to west gradient in ¹³⁷Cs activity in seawater in the North Atlantic to determine how much radiocesium was present in Atlantic salmon returning to the Ste. Marguerite River in Canada and by extension where they had migrated and fed during their growth and development.  The figure below shows the high levels of seawater ¹³⁷Cs contamination in the Irish Sea and eastern Atlantic compared to the western Atlantic near Canada owing to release of the isotope from fuel reprocessing plants in the UK and France and deposit of Chernobyl derived contamination in 1986.

¹³⁷Cs distribution (Bq m^–3) in waters of the North Atlantic with levels >10 in marginal seas of the eastern Atlantic and 0-1.5 in waters of the west near to Canada.

Given that salmon tend to bioconcentrate radiocesium about 130 times relative to the seawater in which the live (through the prey they consume) the predicted range in ¹³⁷Cs in salmon from the eastern Atlantic would be 1.3 — 4.0 Bq kg^-1 while fish living in the less contaminated western Atlantic would have 0.15 — 0.65 Bq kg^-1.  The range of ¹³⁷Cs measured by Tucker and colleagues in salmon returning to the St. Marguerite River in Quebec, Canada, predicted ranges given seawater activities in the figure above and activities in fish harvested from different areas of the North Atlantic are summarized in the figure below.

Frequency distribution of ¹³⁷Cs concentrations (Bq kg^–1) in Atlantic salmon from the Ste. Marguerite River, Que. Tissue samples were obtained fish caught in the sport fishery during the summers of 1995 (n = 33) and 1996 (n = 28) and measured by gamma spectrometry. Dashed vertical lines denote the expected range in 137Cs concentrations in salmon based on a mean bioaccumulation factor of 130 from waters in the North Atlantic outlined in the color bar above (water ¹³⁷Cs concentrations color coded as in the first figure). Horizontal lines denote the observed ranges in ¹³⁷Cs concentrations in salmon and other fish (cod, whiting, haddock, hake, mackerel, and plaice) caught in those same waters.

The range of activities found in the migratory Atlantic salmon is similar to the range seen for other species of fish across the North Atlantic and suggests that almost half of the Ste. Marguerite salmon spent their lives feeding in waters near to Norway and the UK.  This is an amazing result and suggested that more fish spend more time in the eastern Atlantic than was thought at the time.  The levels seen in the salmon agree well with predictions based on seawater activities and the expected bioconcentration factor in the food web to salmon of ~130.  You can read more about bioconcentration of radionuclides and concentration factors in marine organisms in one of my earlier posts here.

What does this tell us about expected contamination from Fukushima in Pacific Salmon and health risks to consumers?

The maximum seawater concentration of ¹³⁷Cs in the central and eastern North Pacific we have measured through the InFORM project is about 7 — 10 Bq m^-3. Given the bioconcentration expected from previous studies of salmonid species like Tucker and colleagues above we might expect maximum contamination levels in Pacific salmon of ~ 1.3 Bq kg^-1 wet weight.  The range of values we have detected in Pacific species returning to British Columbia rivers and streams since the Fukushima disaster in 2011 is ~0.20-0.60 Bq kg^-1 suggesting that these fish have consumed prey and lived in waters with seawater activities <10 Bq m^-3.  At present the levels of Fukushima derived contamination do not lead to ionizing radiation doses to consumers that remotely approach the dose attributable to naturally occurring radioisotopes like ⁴⁰K and ²¹⁰Po.  The ionizing radiation dose from the naturally occurring isotopes do not approach levels where significant risks to the health of consumers are to be expected.  Given what the scientific community understands about bioconcentration of the most radiologically significant isotopes released from Fukushima and measured and forecast levels of these isotopes in the expansive North Pacific the community has confidence that levels in Pacific salmon species will not approach levels were risk to consumers will become significant. The InFORM project will continue to monitor contamination levels in seawater and the marine biota to provide accurate information and useful, scientifically derived assessment of risk to the public.