Contamination of freshwater fish by the Fukushima Nuclear Accident: Overview of monitoring results

By Jay T. Cullen

Adult Oncorhynchus mykiss or rainbow trout. One of the species monitored for Fukushima derived radionuclides in the Wada et al. (2015) study.

The purpose of this post is to report the results of a monitoring study looking for Fukushima Dai-ichi Nuclear Power Plant (FDNPP) derived contamination in freshwater fishes in Japan.  The peer reviewed study by Wada and colleagues was recently published in Journal of Environmental Radioactivity which is unfortunately behind a publisher paywall.  This post continues a series of posts aimed at communicating the results of scientific investigations of the impact of the FDNPP disaster on public and environmental health.  Wada and colleagues measured the amount of 131-Iodine (131I half life ~ 8 days), 134-Cesium (134Cs half life ~2 years), and 137-Cesium (137Cs half life ~30 years) in freshwater fish species collected from rivers, lakes and aquaculture ponds in Fukushima Prefecture between March 2011 and December 2014.  A total of 16 species and 2692 individual fish were examined.  Between March and June 2011 11 fish were found to have detectable but low activities of 131I (<25 Bq kg-1 wet weight) suggesting that radioactive iodine did not accumulate significantly in fish. 

Given significantly lower radiocesium accumulation in fish reared in aquaculture ponds who were provided with feed compared to fish from lakes and rivers, the major path of concentration for 134Cs and 137Cs into fish was likely through the foodweb. A general relationship was found where higher local air dose rates near a freshwater body were related to higher radiocesium concentrations in fish.  Freshwater fish exceeding the stringent Japanese regulatory limit (100 Bq kg-1 wet for radiocesium) in lakes (39.6%) and rivers (14%) during the 2011-2014 study period were greater than the percentage of marine fish that exceeded the limit (9.9%) indicating that longer term contamination of freshwater fish is likely. Radiocesium activities have been decreasing with time since 2011.  Freshwater fish from Fukushima Prefecture had 1-2 orders of magnitude less radiocesium contamination than fish collected in European and Asian freshwaters after the Chernobyl disaster which is consistent with the lower release and terrestrial deposition of radiocesium from Fukushima compared with Chernobyl.  Given how much lower the deposition of FDNPP derived radionuclides were in North America compared to Fukushima Prefecture it is very unlikely that radiocesium levels in freshwater fish here will be observed to increase significantly from pre-Fukushima levels.

Wada and colleagues collected freshwater fish from rivers, lakes and aquaculture ponds in Fukushima Prefecture between March 2011 and December 2014 at the following locations:

Map of the study area showing main rivers and lakes in Fukushima Prefecture. FDNPP indicates the location of the Fukushima Dai-ichi Nuclear Power Plant.

Time series trends in 131I, 134Cs +137Cs and the 134Cs/137Cs in the various fish species are summarized in the following plot:

 (a) 131-I (Bq kg−1-wet) in freshwater fish  from rivers (grey diamonds), lakes (black circles), and aquaculture ponds (open triangles). Data below the detection limit were placed on or immediately above the x-axis. (b) Radiocesium concentration (134-Cs+137-Cs in Bq kg−1-wet). Hatched and black horizontal lines represent provisional regulatory limit (500 Bq kg−1-wet) and Japanese regulatory limit (100 Bq kg−1-wet) respectively. (c) Ratio of 134-Cs/137-Cs with the line representing the fitted decay trend owing to the shorter half life of 134-Cs.

Major findings were as follows:

  • 131I levels were only detected in early 2011 and was not found to accumulate appreciably in the fish
  • Radiocesium levels decreased with time in freshwater fish with time owing to the decay of 134Cs and generally lower activities in water bodies and surrounding land since major atmospheric and releases ceased in 2011
  • Radiocesium was generally higher in fish caught in lakes and rivers who consumed food naturally present compared to aquaculture fish who were fed less contaminated feed indicating the foodweb as the likely route of radiocesium uptake by fish
  • Freshwater fish were more contaminated than marine fish species indicating that contamination will be a longer term problem in freshwater environments
  • Radiocesium in freshwater fish from Fukushima Prefecture was ~1-2 orders less than levels in freshwater fish caught in the years after the Chernobyl disaster in Europe and Asia which is consistent with the difference in releases of radiocesium between the two nuclear accidents
  • A significant positive relationship between air dose rate near water bodies and radiocesium contamination of resident fish was found

Editorial comment

These results are consistent with the monitoring results we are seeing through the InFORM project in Canada where the levels of radiocesium in marine fish tend to be much lower than radiocesium levels found in freshwater fish.  Given the relationship between deposition of FDNPP derived contamination as measured by air dose rate and freshwater fish radiocesium content and the much, much lower deposition amounts of contamination in North America compared to land in Fukushima Prefecture it is unlikely that radiocesium levels in freshwater fish here in North American will show a significant increase owing to the FDNPP disaster.

For those interested I provide images of the complete data table and useful figures below FYI.

Table 2 Part 1
Table 2 Part 2
Relation between air dose rates (μSv h−1) at 1 m height, measured by the Fukushima Prefectural Government during May–June 2012 and 137-Cs activities in freshwater fish from rivers and lakes in 2012. The positive relationship between the two indicated by the exponential function is significant.


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