Radiation from the nuclear reactor disaster in Japan has been found in bluefin tuna in waters off San Diego by researchers from Stanford and Stony Brook University. It marks the first instance of radioactive materials being transported through the sea by migrating animals.
The radiation levels in the fish, caught by recreational anglers, are not considered to be a threat to human health.
The researchers reported in the Proceedings of the National Academy of Sciences how they used the radioactive cesium in the tuna to understand the origin and timing of their migration across the Pacific, showing that the radioactivity emitted by the disaster can be used as a new tool for tracking migration patterns.
On March 11, 2011, a tsunami flooded the Fukushima Dai-ichi nuclear power plants in Japan, leading to the failure of cooling equipment inside. The plants overheated, spilling their radioactive cooling water into the sea. It was the largest radioactive release into the ocean in the history of nuclear accidents.
The spill left behind two radioactive isotopes of cesium. One was previously undetectable in the ocean off the California coast, and the other had previously existed only in very low levels in radiation left behind from the fallout of atmospheric tests of nuclear weapons.
Using radioactivity to retrace tuna route
After the Fukushima spill, the Japanese government monitored the levels of radioactivity in seafood caught near shore, and standards were set for contamination. But radioactivity was not being tracked in the migratory species that pass through Japanese waters. Discovering radioactive cesium isotopes in Pacific bluefin was an unexpected discovery for the scientists involved in the study.
“We never thought to study radioactivity in migratory species,” said Zofia Baumann, a postdoctoral investigator at Stony Brook and coauthor of the study. In June of last year, Baumann was measuring the levels of cesium in the zooplankton and small fish that live near the spill site. The researchers began to wonder if they would see the same radioactive cesium isotopes show up in Pacific bluefin tuna that had traveled from Japan. It is believed that all Pacific bluefin tuna are reared in the coastal waters of Japan and the Philippines.
When they are full size, tuna can weigh more than 1,000 pounds, measure 10 feet in length and live up to 26 years; as 1-year-olds, they are strong enough to swim across the Pacific Ocean. Some unknown fraction of the population embarks on this transoceanic journey from Japan to the California Current.
“The area of the Fukushima spill is an area of high use for juveniles,” said Daniel Madigan, a Stanford doctoral student in marine biology at Hopkins Marine Station and the lead author of the study. Many tuna spend their first year in the coastal waters off Japan feeding and gathering strength for the long migration ahead of them. So, he thought, there was a possibility that some of the tuna feeding in the coastal areas around Japan had accumulated radioactive cesium.
In order to test this hypothesis Madigan collected tissue samples from young bluefin tuna that were caught off the coast of San Diego in August 2011. He sent them to the laboratory at Stony Brook to be tested by Baumann for radioactive cesium.
“We thought that it was unlikely that we would find anything,” said Professor Nicholas Fisher of Stony Brook, a coauthor of the study, “We were very surprised at the results – all of them had clearly detectable levels.”
The radiation provides “unequivocal evidence” that these fish transported radioactivity across the Pacific Ocean, the researchers said. Furthermore, the ratio between the two isotopes of cesium allowed the scientists to determine that this particular group of Pacific bluefin tuna had left Japan approximately four months earlier, after spending less than a month in the contaminated waters near Japan.
Not a public health concern
“All living things are radioactive,” said Fisher, “primarily attributable to the naturally occurring potassium-40. The potassium-40 radioactivity in the bluefin tuna was over 30 times higher than that from the radioactive cesium. So, the radioactivity from the spill really only adds 3 percent more radioactivity than the background level.”
This study opens up the door for radioactivity from the Fukushima disaster to serve as a valuable tool in mapping the paths of little-understood migratory species.
“We now know that we can use these isotopes to trace biological movements from Japan across long distances,” said Fisher, “just like scientists have used isotopes in the past to track ocean currents.” This new tool can be used alongside other tools like incidental catch reports and electronic tagging to piece together the journeys of the creatures that travel the oceans.
“This is an example of how events don’t happen in a vacuum,” said Madigan, “These tuna carried this radiation across the entire Pacific, the largest ocean on the planet.”