With the Cesium radiation we are talking about, the main difference between really concentrated, and really diffuse, is the amount of time it will take to give you cancer...it's half life is really long, it accumulates in fish, and it accumulates in you if you eat it, meaning it doesn't go away when you eat a little, it piles up, until you've eaten enough that you now have a lot in you...and "a lot" doesn't take very much.

I'd say it's a good idea to check, even if it's a low chance of finding it.

Fish on...

Todd