To be honest; I would imagine that the Cs-137 is a product of illegal fishing. Fishermen are told not to fish in contaminated areas; but may do so anyways out of desperation for a catch they can sell.
I remember nuclear disasters near SE Asia and Japan over the last decade. It is also true that Shrimp are also, not so unimportantly, Bottom-Feeders. As Bottom of the Ocean feeders. They literally hoover up the soil at the bottom of the ocean and eat the microbiological life living there. With recent nuclear disasters; I think it's possible some orphan Cesium got down there.
Naturally this is just my theory; based on my limited college educated knowledge of biology and the food web.