By Jay T. Cullen

The purpose of this post is to report on a recent peer-reviewed study that investigated the radionuclide content of fish caught in the harbor of the Fukushima Dai-ichi Nuclear Powerplant (FDNPP) in 2012 and 2013. The post is also written in part to address questions like:

Why don’t you measure ⁹⁰Sr in fish you catch off of North America?

This post is part of an ongoing series dedicated to summarizing results from scientific research into the impact of the FDNPP disaster on the environment. Fujimoto and colleagues measured the activity of Cesium-134 (¹³⁴Cs half life ~2 years), Cesium-137 (¹³⁷Cs half life ~30 years) and Strontium-90 (⁹⁰Sr half life ~29 years) in fish collected from the FDNPP harbor and just outside the port in 2012 and 2013. Fish were most contaminated in the harbor and had radiocesium activity concentrations (in whole body without internal organs, Bq kg^-1 – wet weight) that were ~200-330 times higher than measured ⁹⁰Sr levels. The much lower ⁹⁰Sr levels compared to radiocesium in the fish is consistent with much lower releases of ⁹⁰Sr to the Pacific Ocean compared to radiocesium in the aftermath of the meltdowns at FDNPP (see here, here and here for example). The activity of radiocesium in fish diminishes dramatically with distance from the harbor and as of April-June 2015 none of the fish caught in Fukushima prefecture waters exceeded the stringent 100 Bq kg^-1 Japanese safety standard. Across the Pacific, we have yet to detect Fukushima derived radiocesium in salmon and steelhead trout caught in British Columbian waters as part of the Fukushima InFORM monitoring effort. ⁹⁰Sr is much more difficult and costly to analyze in environmental samples than are the cesium isotopes. The results of the Fujimoto study suggest that ⁹⁰Sr from Fukushima is unlikely to be found at detectable levels in marine organisms in the northeast Pacific and that resources to monitor the impact of the disaster on our marine environment should focus on the detection of the cesium isotopes.

Fujimoto and colleagues collected Japanese rockfish, brown hakeling and fat greenling using small cage traps and gill nets inside the harbor at the FDNPP and at locations in the coastal ocean nearby. Sampling locations are shown on the map below.

Full results of the study can be accessed in pdf here.

Radiocesium in Fish Near to FDNPP

The highest concentration of radiocesium was found in fat greenling caught at the harbor entrance on February 12, 2013 which was measured at 202,000 Bq kg^-1 wet weight. The lowest was a brown hakeling caught near the south jetty on January 30, 2013 at 104 Bq kg^-1 wet weight. The results comparing values for fish caught at the FDNPP compared with fish caught in other parts of Fukushima prefecture are shown in the figure below.

Rockfish consistently had the highest radiocesium activities which may reflect diet choice differences among fish species. Activities of ¹³⁴Cs and ¹³⁷Cs in the fish dropped dramatically outside the harbor as the results for “Pod” stations below indicate.

This is consistent with radionuclide activities that drop dramatically in seawater and sediments with distance from the FDNPP site owing to ocean transport and mixing processes.

Comparing Radiocesium and ⁹⁰Sr in Fish Near FDNPP

The figure below shows the relationship between radiocesium and ⁹⁰Sr in the fish.

The ⁹⁰Sr activity in the fish in the fish tended to be about 300 fold less intense than the radiocesium activity. This likely reflects the lower activity of ⁹⁰Sr in seawater and sediments in close proximity to the FDNPP. The maximum activity of ⁹⁰Sr in the fish from the harbor was 170 Bq kg^-1 which is approximately 4 orders of magnitude higher than values measured before the FDNPP accident. All fish caught outside the harbor and measured had ⁹⁰Sr levels -1. Given that ⁹⁰Sr was released in much lower total activity when compared to radiocesium these relative values in fish with distance from the FDNPP site make sense.

The lack of significant concentrations of ⁹⁰Sr compared to Cs in fish close to Japan highlight the fact that deploying significant resources to test samples near to the coast of North America would not likely provide useful information about the impact of the disaster on our marine ecosystem. Indeed, given the amounts of ⁹⁰Sr released to this point by the disaster there is likely to little in the way of measurable impact on levels in seawater or fish in the eastern Pacific. This is why monitoring efforts like the Fukushima InFORM project tend to focus on the cesium isotopes as together they were released in significant quantities, serve as an unequivocal tracer of Fukushima impact and tend to bioconcentrate in marine organisms. They provide the most information relative to the cost of sampling and analysis.

In Case You’re Interested (ICYI): Methods to detect radiocesium and ⁹⁰Sr in fish

Radiocesium – Fish samples (~2-300 grams) are frozen and then minced with a knife before placing in gamma counting vials. Samples are kept frozen until counting. Gamma emissions are counted using a high purity germanium detector coupled to a multichannel spectroscope. Samples are counted for 7200 – 16000 seconds depending on radiocesium activity to achieve desired detection limits.

⁹⁰Sr – Fish, minus internal organs, are dried at 105 C for 48 hours and then ashed for a further 48 hours at 500 C. 20 grams of ash are dissolved in 6 milliliters of nitric acid and then digested at high temperature and pressure in a laboratory microwave. To isolate Sr from this digest solution divalent cations are concentrated in a precipitate using carbohydrate and oxalic acid precipitation steps. The precipitate is redissolved in 200 milliliters of 0.5 M hydrochloric acid and then loaded onto a cation ion exchange resin column (Dowex). Strontium is purified by subsequently washing the column with ammonium acetate/ethanol and ammonium acetate solutions. ⁹⁰Sr is detected by measuring the beta decay of its progeny isotope yttrium-90 (⁹⁰Y half life ~64 hours) two weeks after the first preconcentration steps are taken. ⁹⁰Sr activity is decay corrected to the time of sampling.

This highlights the fundamentally different effort/resources that must be deployed to monitor ⁹⁰Sr in biological samples compared to radiocesium and why ⁹⁰Sr measurements are more rare in the literature.