It was the most serious disaster since the Chernobyl disaster in 1986. The crippled reactors released an estimated 22 x 10E 15 Becquerel of Cesium-137 into the ocean, which was then picked up by the Pacific bluefin tuna, Thunnus orientalis and carried all the way across the northern Pacific.
And they got caught by fishermen from Mexico and Southern California.
And sure enough, they did find traces of Cesium-134 and Cesium-137 in all 15 fish, and the level was ten times higher than those caught in previous years.
In addition, tuna also carries other contaminants which are far more potent than Cesium. One example is methyl mercury, a neurotoxin that could harm developing fetuses.
So if you seek to kill yourself by eating radionuclide-contaminated sushi, you would have to, according to Robert Emery at the University of Texas Health Science Centre, gorge down 2.5 to 4 tons of tuna to acquire the amount of Cesium that exceeds health limit.
Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California.Daniel J. Madigana, Zofia Baumannb, and Nicholas S. Fisherb. PNAS June 12, 2012 vol. 109 no. 24. doi: 10.1073/pnas.1204859109