By Jay T. Cullen

@JayTCullen and @FukushimaInFORM

1. Fish released to a lake on Honshu Japan accumulated significant radiocesium in the muscle tissue demonstrating that contamination of freshwater fish was still an ongoing problem at the time of the work 1.5 years after the disaster
2. 90 day tank experiments where the water and feed activities of radioactive cesium were controlled showed that no transfer of contamination directly from water to fish was observed while fish fed contaminated feed were themselves contaminated
3. Contamination of these fish will occur through the food chain while direct intake of radioactive cesium from water (through skin, gut, and gills) has little impact on muscle tissue concentrations

Yamamoto and colleagues from the Japanese Fisheries Research Agency conducted experiments in and using water from Lake Chuzenji which is about 160 km from the Fukushima Dai-ichi Nuclear Power Plant:

Studies of lakes and streams after the Chernobyl disaster in 1986 have demonstrated that radiocesium contamination of freshwater fish is can be significant and persist for more than the 10 years. The same is true for Japan where elevated activities of radionuclides have been detected in freshwater fish more than 3 years after the triple meltdowns and subsequent contamination of the central and northern Honshu region of Japan.

The goal of the Yamamoto study was to investigate what the main way that 134-Cs (half life ~ 2 years) and 137-Cs (half life ~30 years) were taken up by the salmonids (e.g salmon, trout) in lakes after the 2011 disaster. The species of fish they focused on were kokanee (Oncorhynchus nerka) and masu salmon (Oncorhynchus masou). To do this they conducted two experiments that are described below.

Experiment #1 Radiocesium uptake rate of salmonids
Two large cages (6 x 6 x 5 height) were deployed in Chuzenji Lake and filled with 500 uncontaminated, juvenile kokanee or masu salmon respectively. Fish were not fed but allowed to grow from food naturally present in the lake. Over ~ 5 months (Nov. 22, 2012 – April 10, 2013) 20 fish from each cage were collected every 14 days and the radiocesium activity of their muscle tissue was measured.

Experiment #2 Radiocesium uptake pathway of the fish: water or food
To determine whether uptake from surrounding water through skin and gills or from their food was most important for radiocesium accumulation by the fish tank experiments were set up in the laboratory. Two 500 liter tanks were set up. One was continually filled and flushed with filtered (60 micron to remove particles and plankton which salmon might eat) water drawn directly from the lake. Radiocesium activity of lake water was 42 milliBq L^-1 (42 Bq m^-3). 200 juvenile kokanee were released into the tank and fed uncontaminated feed for the duration of the experiment. Over the next 90 days 20 fish were removed every two weeks to be analyzed for radiocesium activity. The other 500 L tank was filled and flushed with pure spring water that had no detectable radiocesium activity. The 200 juvenile kokanee released to this tank were fed a feed prepared from Cs contaminated protein (smallmouth bass) with a mean radiocesium activity of 445 Bq kg^-1. Fish were collected every 14 days for the duration of the experiment and their muscle tissue radiocesium levels measured.

Results What did they find?

Experiment #1
The following figure summarizes the results of the experiment and shows the change in radiocesium activity in the kokanee and masu salmon muscle tissue with time in the cages in Lake Chuzenji.

Significant accumulation of ¹³⁴Cs + ¹³⁷Cs occurred by the first sampling only 14 days after the fish were introduced to the lake. Kokanee had 2.2 Bq kg^-1 wet weight while masu had 3.9 Bq kg^-1 wet weight. Radiocesium activities increased more or less linearly after that at ~ 0.14 Bq kg^-1 day^-1 for kokanee and 0.22 Bq kg^-1 day^-1 for masu. Masu had higher activity after 90 days (30.9 Bq kg^-1) thank kokanee (19.2 Bq kg^-1).

Experiment #2
The results of the experiment to examine uptake from water versus food are shown in the figure below:

What does it mean?

The experiments demonstrate that kokanee and masu salmon in freshwater are accumulating contamination from Fukushima Dai-ichi primarily from their food (zooplankton) even 1.5 years after the major releases from the site. The activity of zooplankton sample from the lake (12.6 Bq kg^-1) was lower than maximum activities in the salmon suggesting bioconcentration in the fish relative to their prey. Continued monitoring of food chains in freshwater systems like lakes and rivers will help to better determine concentration factors for radiocesium between water –> plankton and –> fish.

Comparison to marine fish off the coast of North America

The radiocesium activities in Chuzenji Lake is about a factor of 7 higher than ¹³⁴Cs + ¹³⁷Cs activities being measured in offshore waters in the northeast Pacific Ocean. The kokanee and masu salmon in the Yamamoto study accumulated ¹³⁴Cs + ¹³⁷Cs activities that are about 50 to 100 times higher than fish being harvested off the coast of British Columbia Canada. The radiocesium currently being detected in fish in the Pacific is all 137-Cs from legacy sources, primarily nuclear weapons tests in the 20^th century, rather than from Fukushima to this point.

Editorial Comment/Opinion
It would be quite interesting to measure freshwater fish from various locations in North America to look at radiocesium levels that still might be elevated from legacy sources. It would not be surprising to find that freshwater fish here have significantly higher radiocesium than marine species such that avoiding marine fish as a source of protein in favor of freshwater species might increase ones exposure to these isotopes.