Fukushima Radiation in Migrating Bluefin Tuna Expected to Fall

Radioactive material found in bluefin tuna that swam or fed in waters off the crippled Fukushima nuclear plant in Japan is likely to decrease over time as the material dilutes in the ocean, scientists said.

A study of 15 Pacific bluefin caught off San Diego in August last year found levels of radioactive cesium 10 times higher than in fish caught in previous years and provide “unequivocal evidence” that the radiation came from Fukushima, researchers including Daniel Madigan and Nicholas Fisher said in a study published yesterday in the Proceedings of the National Academy of Sciences.

Contamination levels, which the authors say are not a danger to public health, are likely to decline even though cesium has ‘biomagnification’ characteristics, meaning the concentration increases from prey fish to predators when cesium is consumed. That would be offset by the bluefin’s metabolism, which should excrete cesium at a rate of about 2 percent per day, Fisher wrote in an e-mail.

“Much will depend on the concentration in the prey fish, which in turn is ultimately dependent on the water concentration,” wrote Fisher, a professor at Stony Brook University in New York, in response to e-mail questions. “If concentrations in water will eventually decline, as we would expect, due to dilution and dispersion, then concentrations in living organisms will eventually decline as well.”

Prized Sushi

The bluefin is the most prized among tuna for making sushi or sashimi in Japan, where a 5-centimeter (2-inch) piece of the fattiest part of the fish, called otoro in Japanese, can cost over 1,000 yen ($12.58) in a Tokyo restaurant.

Kiyomura K.K., a Tokyo-based sushi chain operator, paid a record 56 million yen ($705,000) for a 269 kilogram (593 pound) bluefin tuna at Tokyo’s Tsukiji fish market on Jan. 5, the year’s first bidding session.

Bluefin can grow as long as 3 meters (9.8 feet), and have been proven to travel as far as 45,000 miles in 16 months, according to Tagging of Pacific Predators, a research program co-managed by Stanford University’s Hopkins Marine Station.

The group plans to conduct another bluefin tuna study this year to determine whether cesium concentrations have changed, Fisher wrote.

“This probably shows the need for international monitoring of marine life,” Japan’s Chief Cabinet Secretary Osamu Fujimura told reporters today in Tokyo. “We are studying the best way to collect information on the issue.”

About 3,500 terabecquerels of cesium 134 and 3,600 terabecquerels of cesium 137 may have leaked into the sea between March 26 and Sept. 30 last year after the meltdown of three reactors at the Fukushima Dai-Ichi nuclear plant, operator Tokyo Electric Power Co. said in a statement last week.

Weapons Testing

The study compared bluefin with fish caught before the disaster, which showed no trace of cesium 134 and only background levels of cesium 137 from nuclear weapons testing.

Cesium levels found in the post-Fukushima bluefin were less than the radioactive doses naturally occurring in the fish from isotopes including potassium 40, the study found.

“It is unlikely” that the level will rise in tuna, Daniel Madigan, the study’s co-author and researcher at Hopkins Marine Station at Stanford, wrote in an e-mail.

“However, certain small fish around Japan showed very high levels after the accident. If certain larger predators happen to feed on these prey, higher levels than we observed may be possible.”

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