Complex fish migrations call for Fukushima radioactivity monitoring beyond marine systems

Japan started to intentionally release treated, radionuclide-contaminated wastewater to the North Pacific Ocean on 24 August 2023, 12 years after the Fukushima nuclear disaster. Radionuclides in released water have raised concerns regarding radioactive transport and its potential impacts on marine ecosystems (Nogrady, 2023). Oceanic transport and the redistribution of radionuclides from Fukushima have been investigated, and short-term monitoring has shown that radionuclides, including radiostrontium, radiocesium and radioiodine (strontium-90, cesium-134 and -137, and iodine-129), have accumulated in marine and freshwater fish near the power plant (Miki et al. 2017, Teien et al. 2023). It was reported in 2012 that migratory Pacific bluefin tuna (Thunnus orientalis) transported radionuclides across the North Pacific Ocean from Japan to California, demonstrating the importance of migratory animals as radionuclide transport vectors (Madigan et al. 2012). Although many highly migratory fish species inhabit Japan's waters, studies have shown little to no Fukushima-derived radiocesium in open-ocean fish across the broader North Pacific (Madigan et al. 2017). Nevertheless, these studies have not considered more complex ecosystem links and multicompartment fish migration cycles. Anadromous fishes diverge from exclusively oceanic lifestyles by returning to freshwater to spawn, presenting the potential to export Fukushima-derived radionuclides to distant inland regions. Because some anadromous fish species, including salmons and lampreys, die in large numbers after spawning in rivers, high biomass of fish carcasses will inevitably deposit radionuclides to the spawning habitat after decomposition. The chum salmon (Oncorhynchus keta) is one such potential vector, as a highly abundant and widely distributed anadromous salmonid in the Pacific Ocean that travels thousands of kilometers to headwater streams of the Yukon River, Amur River, and other river basins to spawn. The high biomass of salmon carcasses in certain locations could plausibly serve as a larger and more concentrated source of biota-bound radionuclides to rivers and the surrounding soil, in which longer radionuclide residence time causes further remediation challenges, as was seen at Chernobyl. Unique dynamics of nutrient transport from salmon carcasses to forest growth via redeposition by predators (bears, wolves, eagles) provides the possibility of absorption through plant roots or leaves, resulting in an additional reservoir of radionuclides. These transport dynamics will be complex, depending on the natal origin of the salmon, their extent of residence in Fukushima-contaminated areas, and the degree of homing to their natal rivers. Fish species close to the release site are expected to show the largest long-term radionuclide activities and may now accumulate more radionuclides (Alava and Gobas 2016). Dilution may be the (unfortunate and unescapable) solution in tidally flushed marine systems, but transport dynamics and implications in more complex systems and species remain to be seen. Long-term integrated monitoring should be conducted by countries that border the Pacific Ocean, focused not exclusively on marine systems but also on freshwater systems, particularly those used by high numbers of anadromous fish species, to more fully assess the aquatic ecosystem effects of the Fukushima aftermath (Yoshida and Kanda 2012). Jingrui Sun ([email protected]) is affiliated with the Yunnan Key Laboratory of International Rivers and Transboundary Eco-Security and with the Institute of International Rivers and Eco-Security, at Yunnan University, in Kunming, in China. Martyn C. Lucas is affiliated with the Department of Biosciences at Durham University, in Durham, England, in the United Kingdom. Daniel J. Madigan is affiliated with the Department of Integrative Biology at the University of Windsor, in Windsor, Ontario, in Canada.

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