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.