Cesium 137 is a product of nuclear reactions.

[Old Crank]

It has a half life of 30 years. Articles I have read don't give any indication of where the radiation comes from. The US did a lot of nuke testing in the pacific islands. That may or may not have released more cesium into the oceans than Fukishima.
Articles that I read had no information about the amount of radiation other than low, or extremely low, not really helpful. Especially since we can detect at smaller and smaller amounts. Same with chemical contamination.