SYDNEY: The Pacific bluefin tuna is responsible for transporting radionuclides from the 2011 Fukushima disaster across the entire north Pacific Ocean, according to U.S. researchers.
This research demonstrates that cesium radionuclides can be successfully used to trace migration patterns and that it is possible for migratory ocean species to carry radionuclides over great distances.
“While the levels of radionuclides we found were low, this may not always be the case for other animals, in other places and for other accidents,” said paper author Daniel Madigan, from Stanford University, in California.
Radiocesium increase in post-Fukushima fish
There have always been low levels of radionuclides in ocean waters (the West Pacific, for instance, has low concentrations of cesium-137 (Cs-137) leftover from past nuclear weapons testing). And there have always been levels of naturally occurring radionuclides such as potassium-40 in the muscles of fish from evolving in such waters.
“But the striking finding here is that the tuna accumulated cesium from Japanese waters and are transporting them across the entire Pacific Ocean,” said co-author Nicholas Fisher, professor at the State University of New York.
The researchers collected muscle tissue from 15 Pacific bluefin tuna caught off the California coast, whose sizes indicated they were specifically from the West Pacific around Japan. They tested the tissue for Fukushima radionuclides (particularly Cs-134 and Cs-137) and found both (4.0 Bq kg-1 of Cs-134 and 6.3 Bq kg-1 of Cs-137) in all 15 fish.
These artificially radioactive levels in the bluefin were much lower than levels of the naturally occurring K-40, but still significant in its increase. To control for non-Fukushima sources of radiocesium, they also referenced data from Pacific bluefin collected in 2008 (before the Fukushima disaster) and yellowfin tuna (fish who do not swim near Japan). Neither of these fish showed significant or elevated amounts of Cs.
“An unfortunate serendipitous tool”
The study’s findings indicate that using the Cs isotopes as a chemical ‘signal’ may help “distinguish animals that have recently migrated from waters around Japan and may even help us determine the general timeframe at which they left Japanese waters,” said Madigan. The presence of Fukushima-based Cs in the bluefin also suggests the merit of investigating other types of highly migratory species – such as whales and pinnipeds – for the potential to be biological carriers of radionuclides.
Craig Franklin, zoology professor at the University of Queensland, commented that this study is important in its use of radionuclides to trace migration – “an unfortunate serendipitous tool.”
“It is interesting that tuna carry and transport radionuclides – but almost expected, given their migration around the Pacific Ocean. For me, using the presence of Cs-134 as a tool to trace migration is the most significant,” said Franklin.
While the presence of radionuclides was detected in these bluefin, the researchers want to stress that the levels are nowhere near concerning.
“Just because contaminants can be measured in seafood – in this case, the radioactive cesium in tuna – does not mean that the contaminant will be toxic to either the tuna or to human consumers. In this case, the Cs levels were higher but still below recommended safety levels and below levels of naturally occurring radionuclides,” said Fisher.
On a broader scale, Madigan said their findings demonstrate the impressive ability of nature to link very distant ecosystems.
“In this case, a fish that swims across the largest ocean basin on Earth linked an accident in Japan with an ecosystem off the coast of California, thousands of miles away.”