Fishing for Answers off Fukushima

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Science  26 Oct 2012:
Vol. 338, Issue 6106, pp. 480-482
DOI: 10.1126/science.1228250

The triple disaster of the 11 March 2011 earthquake, tsunami, and subsequent radiation releases at Fukushima Dai-ichi were, and continue to be, unprecedented events for the ocean and for society. More than 80% of the radioactivity from Fukushima was either blown offshore or directly discharged into the ocean from waters used to cool the nuclear power plants (1). Although offshore waters are safe with respect to international standards for radionuclides in the ocean (2), the nuclear power plants continue to leak radioactive contaminants into the ocean (3); many near-shore fisheries remain closed. What are the prospects for recovery?

Public anxieties in Japan about seafood safety remain high, in part because the Japanese are among the world's highest per capita consumers of seafood. On 1 April 2012, regulators tightened restrictions for cesium-134 and cesium-137 in seafood from 500 to 100 becquerels per kilogram wet weight (Bq/kg wet) in an effort to bolster confidence in the domestic supply. In fact, this measure may have had the opposite effect, as the public now sees more products considered unfit for human consumption.

The Japanese Ministry of Agriculture, Forestry and Fisheries (MAFF) has been monitoring radionuclides in fish and other seafood products since 23 March 2011. They have been releasing these data on a regular basis, most notably in a single annual compilation of more than 8500 samples of fish, shellfish, and seaweeds collected at major landing ports and inland freshwater sites, particularly in the most affected coastal areas near Fukushima (4).

Still radioactive.

Changes in total cesium (137Cs + 134Cs in Bq/kg wet) over time in demersal (bottom-dwelling) fish for five prefectures in eastern Japan closest to Fukushima. Since the nuclear disaster, total cesium levels have been highest in fish caught off Fukushima prefecture.

The MAFF results show that total cesium levels in demersal (bottom-dwelling) fish, including many important commercial species, are highest off Fukushima and lower in four prefectures to the north and south (see the figure). Fishing for these species is currently banned off Fukushima, where 40% of fish are above the new regulatory limit of 100 Bq/kg wet (4).

Demersal fish have higher cesium levels than other marine fish types, grouped here as epipelagic (near-surface), pelagic (open ocean), and neuston (surface-dwelling) fish. Contamination levels of demersal fish are comparable only to those of freshwater fish (see fig. S1). Cesium levels have not decreased 1 year after the accident, except perhaps in neuston, and as of August 2012, fish are still being found with cesium levels above 100 Bq/kg wet (5). The highest total cesium levels found to date, more than 25,000 Bq/kg wet, are from two greenling caught in August 2012 closer to shore off Fukushima (6).

Cesium accumulates in fish muscle tissues with relatively modest concentration factors; the Cs concentration in fish is typically 100 times that in the surrounding seawater (7). The concentration factors increase only slightly as one moves up the food chain (8). Bioaccumulation is much higher in general in freshwater fish because of lower salinities (9) (see fig. S1). Uptake of cesium is balanced by loss back to the ocean, which increases with body size and metabolic rate (8). The loss rate is a few percent per day on average and has been shown to be faster if the cesium supply is pulsed rather than steady (10).

Given these high loss rates and the fact that cesium-134 and cesium-137 remain elevated in fish, particularly in bottom-dwelling species, there must be a continued source of cesium contamination associated with the seafloor. Reports of Fukushima cesium in marine sediments, although not extensive, support the assumption that the seafloor is a possible source of continued contamination (11). Given the 30-year half-life of 137Cs, this means that even if these sources were to be shut off completely, the sediments would remain contaminated for decades to come.

The variability in total cesium levels for any given date and fish type is extremely high, making management decisions of when to open or close a particular fishery more difficult. The wide range of observed cesium levels may be due to variability in the cesium loss rates from fish, the life stages of each species, and differences in habitat. Of course, many fish move over wide spatial scales, which will also affect cesium levels in fish caught at a particular location that may have been exposed elsewhere.

The MAFF data show that the vast majority of fish remain below even the new, stricter regulatory limit for seafood consumption. Many naturally occurring radionuclides appear in fish at similar or higher levels and are not considered a health threat. For example, in fish sampled in June 2011 off Japan, natural levels of potassium-40, a naturally occurring beta emitter like cesium, were more than 10 times those of Fukushima-derived cesium (2). Moreover, because cesium is rapidly lost from muscle after exposure stops, fish that migrate to less affected waters will gradually lose much of their Fukushima-derived cesium, as seen in a report of tuna caught off San Diego (12).

Nonetheless, the fact that many fish are just as contaminated today with 134Cs and 137Cs as they were more than 1 year ago implies that cesium is still being released to the food chain. The Japanese government is using the MAFF results to keep fisheries closed off Fukushima and to closely monitor neighboring areas where levels are approaching the regulatory limits.

Knowledge of the patterns of radionuclide contamination and trends over time for different fish types helps to put risks arising from the released radioactivity in context. However, studies of cesium in fish are not enough. An understanding of sources and sinks of cesium and other radionuclides is needed to predict long-term trends in fish and other seafood. Such knowledge would support smarter and better targeted decision-making, reduce public concern about seafood, and potentially help to revive local fisheries safely, with confidence, and in a timely manner.

Supplementary Materials

Fig. S1


References and Notes

  1. Acknowledgments: Supported by the Gordon and Betty Moore Foundation. I thank S. Clifford for compilation of MAFF data and K. Kostel for assistance in writing.
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