Fukushima Radiation: Study Shows Migratory Fish Not Affected

Fukushima The indications are that migratory fish have not been affected by the Fukushima incident in 2011. Image: Andrew Gray aboard Aoshibi IV

Following the massive 2011 earthquake and tsunami in Japan, the Fukushima Daiichi Nuclear Power Plant in Fukushima Prefecture released vast quantities of radioactive materials into the atmosphere and ocean, raising concerns over the consumption of seafood from the Pacific, reports Bonnie Waycott.

A new study aims to allay such fears. The potential accumulation of radiation in Pacific marine species was recently assessed by an international research team of Zofia Baumann, Owyn E. Snodgrass, Heidi Dewar, Michelle Berman-Kowalewski, Kevin C. Weng, Jun Nishikawa, Peter H. Dutton, and Nicholas S. Fisher, led by Daniel Madigan of Harvard University, following a 2012 pilot study on Pacific bluefin migrating from waters off Japan. The Assessing Fukushima-Derived Radiocesium in Migratory Pacific Predators study indicates that the tuna could have transported radiocesium to waters off California and Mexico.

"The Fukushima accident released several radioactive isotopes, some in greater quantities than others," said Daniel Madigan. "It was our responsibility to assess this. High levels of radioactive exposure can be dangerous but every day we're exposed to naturally-occurring radioactivity. We're still learning how dangerous the Fukushima discharge was."

He and his team concentrated on two water-soluble radiocesium isotopes, 134C and 137C, which were released in large quantities. Both are transported easily by ocean currents and accumulate in fish muscle by uptake through gills, seawater ingestion or consumption of contaminated prey.  Using a method called gamma-spectroscopy with a germanium detector, the team measured the energy spectra of emitted energy of both isotopes in seawater and fish muscle samples. This identified the isotope and amount present.

"We were interested in bluefin because of their trans-Pacific migrations," he said. "But other species travel far, are top predators or are popular as food and/or for conservation. We analysed tuna, swordfish, mahi mahi, salmon, dolphins, whales, sharks and a sea turtle."

All species, except for barely measurable traces in the sea turtle, had no 134Cs, while 137C concentrations were low and consistent with background levels from nuclear weapons testing in the Pacific over half a century ago. Most species also had ~200 to 400 times less 137Cs than 40K, a radioactive isotope that isn't connected to Fukushima. This was meaningful, given that 40K occurs naturally in the earth and would have been found in fish even before humans existed.

As for how long radiocesium remains in the ocean...

"It can stay for decades, as seen by the leftover 137Cs from nuclear weapons testing. This is widely dispersed throughout the Pacific at very low levels," Daniel Madigan said. "It can also become trapped in deep water sediments and decay at different rates. For example, half of the 134Cs from Fukushima will be gone after two years but it will take 30 years for half of the 137Cs to break down."

He said that monitoring will be important. After the accident, most water and fish measurements were taken close to Fukushima and distant regions were only covered by a small number of studies. The Fukushima accident will help generate predictions of dispersal in future accidents and offer guidelines on what to monitor.

"There is also a lot of information," he said. "We recommend peer-reviewed scientific publications or government-sponsored monitoring and data to learn about radiocesium transfer and uptake.  Our research suggests, with reasonable confidence, that seafood far from Fukushima is safe.  But if you're particularly anxious about eating something, avoid it until you have adequate ​information."

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