Five months after the Fukushima disaster, Daniel J. Madigan a marine ecologist at Stanford’s Hopkins Marine Station in Pacific Grove (Monterey County), Nicholas Fisher, a marine scientist internationally known as a specialist in radiation hazards at Stony Brook University on Long Island, Zophia Baumann, a staff scientist in Fisher’s laboratory, and their team decided to test highly prized Pacific bluefin that were caught off the coast of San Diego last August. The finding was wholly unexpected Madigan admitted later.
Madigan had collected samples of muscle tissue from 15 2-year-old tuna given to him by San Diego fishermen in August, and when tests detected radioactivity in one sample he sent all 15 samples to Fisher in Long Island, he said.
To their surprise, tissue samples from all 15 tuna captured contained levels of two radioactive substances — ceisum-134 and cesium-137 — that were higher than in previous catches. The contamination reportedly stems from the Fukushima nuclear disaster.
“We were frankly kind of startled,” said Fisher.
The report shows that samples from the tuna contained 4 becquerels of cesium 134 per kilogram and 6.3 becquerels of cesium 137 per kilogram. A becquerel is a unit of radioactivity equal to one nuclear disintegration per second.
Judging by the size of the bluefin tuna they sampled – they averaged about 15 pounds (6 kg) – the researchers knew these were young fish that had left Japanese water about a month after the accident.
The levels of radioactive cesium were 10 times higher than the amount measured in tuna off the California coast in previous years. Bluefin tuna caught in the same waters in 2008 reportedly carried no cesium 134 and only negligible levels of cesium 137.
The results “are unequivocal. Fukushima was the source,” said Ken Buesseler of the Woods Hole Oceanographic Institution.
Since the fish tested were born about a year before the disaster, “This year’s fish are going to be really interesting,” Madigan said. “There were fish born around the time of the accident, and those are the ones showing up in California right now,” he said. “Those have been, for the most part, swimming around in those contaminated waters their whole lives.”
Madigan said, he is preparing to collect samples from a new group of bluefin tuna that have recently migrated to the waters off San Diego in order to determine their levels of radioactive cesium.
They will have lived in Fukushima’s contaminated ocean for a full year longer than the first fish he collected, and the scientists will seek to know whether radiation levels in the tunas’ bodies have increased or decreased, he said.
“This year’s study will be much higher sample size across a greater range of fish, ages and sizes,” Madigan said. And if any fish are found with dangerous levels of radioactive material in their tissue, “It would be our responsibility to report it right away,” he said.
Madigan said the concentrations were likely higher in smaller fish, but shrank as the bluefin grew during their migration and processed some of the cesium in their bodies. Japanese government figures estimate cesium levels in fish caught off its shores at between 61 and 168 bq/kg.
Bluefin tuna absorbed radioactive cesium from swimming in contaminated waters and feeding on contaminated prey such as krill and squid, the scientists said. Now that scientists know that bluefin tuna can transport radiation, they also want to track the movements of other migratory species including sea turtles, sharks and seabirds.
To rule out the possibility that the radiation was carried by ocean currents or deposited in the sea through the atmosphere, the team also analyzed yellowfin tuna, found in the eastern Pacific, and bluefin that migrated to Southern California before the nuclear crisis. They found no trace of cesium-134 and only background levels of cesium-137 left over from nuclear weapons testing in the 1960s.
Previously, smaller fish and plankton were found with elevated levels of radiation in Japanese waters after a magnitude-9 earthquake in March 2011 triggered a tsunami that badly damaged the Fukushima Dai-ichi reactors.
The Japanese consume 80 percent of the world’s Pacific and Atlantic bluefin tuna, where the fatty muscle in the tuna is particularly prized as a delicacy, sliced and eaten raw as sushi. Considering that after the tsunami, a Japanese boat was found drifting near the West Coast, it’s not surprising to find radiation traveling just as fast to the other side of the Pacific.
Source: NBC San Diego
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