From the Web....
The only real difference is that “organic” salmon is stocked in open net cages at a lower density. ... “Organic” salmon farmers still use all the same chemicals, including lice treatments, thus killing other crustaceans in the vicinity.
Compared with wild salmon, standard farmed salmon has much higher levels of PCBs and other man-made pollutants, and there's no evidence that organic farmed salmon are any better in this regard. So, compared with standard farmed salmon, “organic” farmed salmon are not substantially healthier to eat.