Most farmed salmon eat a diet heavy in fish meal, derived from wild caught but "undesirable" fish. Eating farmed salmon may not harm wild salmon populations directly, but they still have an impact on wild fish stock. Eventually, there could very well be food competition between wild salmon and farmed if fisheries continue to seek fish meal from wild sources to feed an increasing population of farmed fish.
In the PNW wild salmon is usually only about 40% more per lb and definitely worth it. Also most of it is pretty sustainably fished in Alaska (though in the puget sound region the population is reduced from what it should be). You shouldn't eat it raw but it tastes incomparably better cooked
Same is true for a number of other staples: Tilapia, Shrimp, Catfish, etc.
That said, similar to land based CAFO's you don't really want to know what they're fed (o;