how concerened should I be with fish and murcury levels?

I have just recently started following some of the advice in eating for your blood type. I am a b and am supposed to eat fish 3 times or so a week. salmon, cod and sardines are my top picks, but can I eat them 3 times a week and not become ill with all the murcury that is laiden on them nowadays?
I get my fish from the whole foods store but I dont know if that makes a difference...
advice|?