The same factor that put the Fukushima power plants at risk—proximity to the ocean—ensured that a sizable fraction of the radioactivity liberated from the plants ended up in the Pacific. That helped ensure that the contamination was diluted back to safe levels rapidly, although radioactive isotopes were detectable in fish caught near the plants. But fish don't sit still, and a new study has also detected low levels of radioactivity from Fukushima in tuna that were caught off the coast of California.

The study takes advantage of the fact that there is an isotope of cesium, 134Cs, that is both short lived and only produced through nuclear processes, making it an excellent tracer of contamination from Fukushima. A second isotope, 137Cs, is present at very low levels due to historic nuclear tests thanks to its longer half-life. These can be contrasted with 40K, a potassium isotope that is naturally present throughout the world's oceans.

The cesium isotopes were detected in sea life near Japan and that gave researchers good reason to look into the tuna population elsewhere. The bluefin tuna of the northern Pacific breed along its western shores, including near Japan. Immature fish stay in that area for a year or two before migrating across the Pacific to mature near California. Thus, younger fish caught near California had a good chance of having been near Fukushima when the reactors melted down.

By selecting for young tuna caught near California, the authors were able to identify a population that had picked up radioactive cesium from Fukushima. At the time of the sampling in August 2011 (about a half-year after meltdown), the average for 134Cs was four decays/second in each kilogram of muscle. Elevated levels of 137Cs were detectable as well. Tuna that wouldn't have been present near Fukushima during the accident, as expected, had no (134) and low (137) levels of these isotopes.

Extrapolating things back to the time of contamination, the authors compensated for radioactive decay and the rapid growth of the fish to get an estimate of where things stood when the fish were near Japan. Their numbers, while between two and 15 times higher than the levels registered in US waters, were consistent with actual measurements taken in Japan. That helps support the contention that these fish were carrying radioactive isotopes across the Pacific.

The levels were well below safety limits set by Japan, and the radioactivity caused by the cesium was about 30 times below that caused naturally by radioactive potassium. So, the tuna don't present a health threat to anyone involved.

The authors suggest there are two lessons here. The first is that, although it's reasonable to focus attention on Fukushima and its surroundings, the presence of migratory species in that area means that we can't limit our attention exclusively to those areas. And the second is that Fukushima has provided, "an unexpected tool for examining migratory origins of apex predators in the Pacific Ocean."

PNAS, 2012. DOI: 10.1073/pnas.1204859109 (About DOIs).