Comments on TEPCO’s Marine Life Rearing Experiments
By Tanimura Nobuko
Tokyo Electric Power Company (TEPCO) announced that on March 31, 2025 it had completed all of its experiments involving raising marine life in ALPS-treated water. The purpose of the rearing tests was to “convey the safety of releasing ALPS-treated water by experimentally raising marine life in it and demonstrating in a ‘visible form’ what the situation will be upon releasing the treated water into the ocean.”
Large volumes of water contaminated with radioactivity are being stored at Fukushima Daiichi Nuclear Power Station. Water that has undergone the radioactive purification treatment process is termed ALPS-treated water. The oceanic release of this treated water began in August 2023, but the reality is that it is water contaminated with radioactivity, including tritium, which cannot be removed using the purification treatment process.
The testing started from raising flounder (about 100 fish) on the nuclear power station (NPS) grounds in March 2022, having the fish get accustomed to the conditions of captivity, and then adding ALPS-treated water with a tritium concentration adjusted to 1,500 becquerels per liter (Bq/L) or less added to the tank on October 3. In addition to flounder, the testing included abalone and also green laver and sargassum (marine algae). Comparison rearing tests were conducted in seawater in the vicinity of the NPP (regarded as having no pollution) and seawater containing ALPS-treated water. Testing was done while changing the tritium concentration in the water during captivity, with some of the organisms removed at random times during the course, while observing the changes over time in tritium concentrations (free-water tritium (FWT) and organic-bound tritium (OBT) [1] in vivo.
TEPCO created a “Treated water portal site” page on its website where it posted test status summaries. That has now been discontinued, but the status of the captive fish is being shown via real-time live camera, and a journal and monthly report on the rearing tests are also being released. IAEA President Grossi visited the test site on July 6, 2023, before the ALPS-treated water was released into the ocean.
The following summary is prominently posted on the “Rearing status results” page:
(1) The internal tritium concentration in flounder does not exceed the tritium concentration in the seawater, including ALPS-treated water, where they are being bred. [Tritium uptake test]
(2) Within a certain period of time, the internal tritium concentration in the fish stays at the same concentration as the seawater in the captive environment. [Tritium uptake test]
(3) Later, when the fish are returned to normal seawater, their internal tritium concentration drops rapidly over time. [Tritium discharge test]
From (1), (2) and (3), it appears that tritium neither accumulates nor is it concentrated internally in flounder.
I want to take an in-depth look of the results based on the monthly report, “Report on the marine life rearing tests as of March[2]” (2025). A look at the graph in Fig. 1 shows it is true that when the flounder were placed in a tank with tritium seawater (at about 1,250 Bq/L), their internal FWT concentration quickly rose, but above a certain value (not exceeding the tritium concentration in the water) no changes could be observed. After that, when the fish were placed in a normal seawater environment, their internal tritium concentration decreased, and after 72 hours it fell below detectable limits. The change in concentration reached a state of equilibrium in about 24 hours for both uptake and discharge.
Fig. 1: Changes in FWT concentration in flounder.
Different results were obtained in the testing for OBT concentration (see Fig. 2). It should be noted that the unit of the horizontal axis differs from that of the FWT test. For this study, the flounder were kept in a tank with tritium seawater (at about 1,250 Bq/l), with several fish removed at a time, totaling 13 times, and their internal OBT concentrations measured. The uptake testing, spanning 650 days, appears to have ended before a constant internal OBT level could be reached, while the concentration change in the upper right-hand corner was continuing. By the last of the 650 days, their internal OBT had risen to 300 Bq/L. After that, in the discharge test, the concentration fell off at a slightly quicker pace until about the 100th day of the testing, but after that the concentration began rising and falling, reaching about 120 Bq/L internally when the testing concluded about a year later [3]. It is thought that the internal OBT concentration in flounder bred in normal seawater is close to zero, so why do they not note here that once elevated, internal OBT does not fall back to normal, even after a year?
Fig.2 Changes in OBT concentration in flounder.
If you rely solely on the summary of “Rearing status results,” you see only that the tritium concentrations in the fishes’ bodies decrease over time, giving the impression that reassuring conclusions have been confirmed. Considering, however, that the fish are quickly distributed by the fisheries for consumption on the day they are caught, the effort that would be needed to keep them in clean seawater while they discharge their tritium would be unreasonable. The results of this survey confirm that fish living in seawater with high tritium concentrations take up that tritium internally in accordance with the tritium concentration in the seawater; they do not demonstrate a lack of effect on fish from marine pollution by ALPS-treated water. It is therefore not possible thereby to “convey the safety” of the released water, which was the original purpose of the testing.
And that’s not all. It is important to note that this testing by no means reproduced a complex natural environment. The testing involved only seawater and target organisms, which were placed in tanks and raised, while in actual sea environments, various kinds of organisms coexist as part of a food chain. The monthly report referenced here does not contain any information on the feed provided, but if they want to investigate internal concentrations of radioactive substances, they should also consider the feed as an important factor. I therefore had a look at the photos published in one of the daily reports for clues, and found several showing the fish being fed. It seems they were being given a commercially available artificial dry food.
Fig. 3: Flounder being fed (from a daily report).
The flounder is a carnivorous fish that hides in the sand to ambush prey, snatching approaching fish. In the larval stage, they eat small zooplankton, and switch to small fish when they get bigger. Supporting the base of the marine food chain are phytoplankton. Phytoplankton are what change FWT to OBT through photosynthesis and carry it into the body. The substances contained in phytoplankton are taken up by organisms higher in the food chain. In an environment contaminated with radioactive substances, it is impossible for fish to obtain only uncontaminated prey. It must be noted that the captivity experiment described here was conducted in an unnatural environment, focusing on a snippet of the natural world that is easy for humans to manipulate.
The safety of ALPS-treated water, to which TEPCO is appealing, cannot be shown from these tests. In addition, accusations will inevitably arise that these captivity tests using non-radioactive feed were conducted for the purpose of obtaining results favorable to TEPCO’s interests.
It is difficult for humans both to control and to predict what will happen in nature—a complex system. I wish to make an appeal for this matter to be faced sincerely and that there should be no more deliberate environmental pollution than has already been perpetrated.
[1] Free-water tritium (FWT): tritium present in the body of an organism in the form of water.
Organic-bound tritium (OBT): tritium organically bound to molecules containing carbon and other elements in the body of an organism.
[2]www.tepco.co.jp/decommission/information/newsrelease/breedingtest/pdf/2025/202503.pdf
[3] Note that although the detection limit is shown as 188 Bq/L in the FTW test graph, it is not shown in the OBT test graph. If the testing was conducted with the same detection limit, the experimental design could be said to be faulty.
