Scientists report in a new study that they can use trace amounts of radiation in bluefin tuna to sketch the crisscrossing passage of the fish through the world's largest ocean. Understanding how many fish move back and forth may help with the conservation of bluefin tuna.
During the Fukushima nuclear disaster, two years ago, radioactive particles flooded into the Pacific Ocean. Bluefin tuna swimming through these waters began to store certain radioisotopes, called cesium 134 and cesium 137, in their muscle tissue.
The low levels of these particles don't pose a health danger to either the fish or sushi eaters, said Daniel Madigan, a Stanford University graduate student and an author on the study in the journal Environmental Science & Technology. But scientists can track the chemical mark of Fukushima to recreate a timeline of when bluefin tuna traveled from their spawning grounds, in waters around Japan, to the coast of North America.
Until now, researchers have stuck expensive electronic tagging devices to bluefin tuna to follow their movements through the sea. But it's difficult for scientists to wrestle with a live, torpedo-shaped fish that can weigh more than 1,000 pounds.
With that technology, researchers still don't know where the fish has been before it was
tagged or how many tuna are making the trip each year.
"What the chemical techniques, like radiocesium, can do is tell you where an animal came from," said Madigan, who has gathered samples from more than 400 bluefin in the past two years. Only 50 samples were used in the current study.
Bluefin tuna off the coasts of California and Mexico eventually travel back west to breed. Bluefin tuna bounce back and forth across the Pacific and are part of one large, billion-dollar global fishery.
This January, an almost 500-pound bluefin tuna sold for 1.75 million; the high price is a tradition for the first fish of the season. However, a December 2012 stock assessment by the International Scientific Committee Pacific Bluefin Tuna Working Group found that the bluefin tuna population has dropped more than 96 percent from historical numbers.
To get at these chemical clues, the team of scientists asked recreational bluefin fishermen in Southern California for a slice of their fish -- a chunk of muscle about the size of two decks of cards.
By measuring the levels of radioactive cesium in the steak, the team could pick apart the new arrivals from the tuna that had been swimming in the eastern Pacific for more
than a year. The team was also able to pinpoint when fish of different sizes left the waters around Japan. Most bluefin that make the thousands-of-miles swim to the North American coast, start their journey in June.
"As far as I know, that's the first detailed information on when the fish leave Japan," said Shana Miller, science adviser to The Pew Charitable Trusts. Although Miller agrees that this data will be helpful to fishery managers in the future, she argues that there is enough data available now for fisheries managers to take steps to prevent further declines. "We're dealing with a severely overfished species that has little management in place right now," she said.
Fishermen catch more than 90 percent of bluefin before the fish have a chance to grow big enough to breed. Miller argues that science-based catch limits can make a big impact in reviving this economically important fish by giving enough fish on both sides of the ocean the chance to reproduce.
Knowing how the fish move may provide insight into how to fine-tune those efforts. "If the fish in the east are an important portion of the fish that go back to spawn, then we need to protect [that] group of fish," said Madigan.
This year, Madigan will be collecting more samples of bluefin to build on the team's understanding of the fish's migrations. His team is also applying the radioisotope technique to different animals, including turtles, whales and other tuna.
