The fish off Fukushima remain radioactive more than a year after the earthquake and subsequent tsunami triggered three meltdowns at the Daiichi nuclear
power plant. In fact, bottom-dwelling greenling fish caught in August 2012 bore
the highest levels of radioactive particles seen to date—25,000 Becquerels per
kilogram. (A becquerel is a unit of the rate of radioactive decay—or radiation
emitted by a substance.) That is 250 times higher than current Japanese safety
standards, a key reason fishing off Fukushima remains prohibited.
The findings suggest that contaminated water is still leaking from the stricken power plant, the sea bottom itself is
now laced with radionuclides, or both. Concentrations in the ocean water itself
remain below any human health concern but they do pass into fish that swim
through those waters. "When fish 'drink' they take [cesium] and other
salts up from the water they are swimming in, that accumulates in the muscle
tissue," explains marine chemist Ken Buessler of the Woods Hole
Oceanographic Institution, who compiled the analysis of publicly released Japanese fisheries data and published it in Science on October 26. But the fish also shed that cesium if they swim in
uncontaminated waters, as has been seen in tuna that migrated from near Japan
to near San Diego, suggesting that levels in fish should decrease over time.
For this reason, most of the fish caught off Japan's northeastern coast are not
radioactive. But roughly 40 percent of fish caught off the coast adjacent to
Fukushima bear radionuclides above the Japanese food safety standard of 100
becquerels per kilogram; the concentration is highest among bottom dwellers,
such as halibut.
that was prepared by staff at the U.S. Nuclear Regulatory Commission,
ingesting fish at that level "would only produce a dose that is a small
fraction of the dose that people receive from natural levels." For
example, as Buessler notes, fish caught off Japan in June 2011 boasted levels
of potassium-40—a naturally occurring radionuclide—10 times higher than those
of radioactive cesium from Fukushima.
Radioactive cesium decays by emitting what's known as a beta-radiation, a negatively charged particle that is easily blocked by metal,
plastic or wood—but not skin. In particular, ingesting beta-emitting
radioactive elements is "a concern," according to the NRC. "Beta
particles released directly to living tissue can cause damage at the molecular
level, which can disrupt cell function." Plus, beta particles are small
enough to travel far in the body, causing damage far and wide.
U.S. safety guidelines from the Food and Drug Administration permit foods to bear 1,200
becquerels per kilogram of radioactive cesium, but the FDA declined to comment
for this article. "The more restrictive action taken by the Japanese seems
reasonable for the population living close to Fukushima because they receive
radiation doses from other sources, including non-fish food, drinking water and
land surface contamination," the NRC staff writes. "Based on the FDA
and [World Health Organization] recommendations, eating fish contaminated at
100 Bq/kg would result in a small and acceptable exposure to radioactive
cesium."
Because it takes decades for radionuclides to decay, fishing off
Fukushima is likely to remain prohibited for many years. Exactly how long is
uncertain, especially as the Tokyo Electric Power Company continues to struggle to contain and clean up millions of litres of contaminated waters at the
Fukushima Daiichi nuclear power plant. And local waterways continue to wash
radionuclides out to sea. "For the record, I was in Japan this past July
and ate all types of seafood," says Buessler, who will hold a public colloquium on the findings November 14 at the University of Tokyo with colleagues. "The fisheries
data like [those] shown here are used to keep certain areas and types of fish
out of the markets. The question we can't answer is when will this no longer be
of concern."
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