The two contaminants of most concern with fish are mercury and PCBs (Polychlorinated biphenyls).
A study from a few years back revealed that farmed raised salmon had about 10 times more PCBs, dioxins, and pesticide residues than wild salmon. The Food and Drug Administration claims that the levels of PCBs are not high enough to recommend limiting the consumption of farmed salmon. However, I think it’s probably best to avoid this whenever possible. Choose wild/organic whenever you can!!
That really shouldn’t cause you to limit your salmon intake though. It is one of the best muscle building/fat loss foods out there! It’s packed with healthy omega 3’s and protein!