To sum up the nutritional difference between farm raised and wild salmon: it depends. Omega 3 oils are produced by krill and concentrated up the food chain. So... If the farmed salmon you're eating has been fed primarily a marine diet (fish meal from wild caught species, both caught specifically for feed and trimmings from market-bound wild fish) those oils will be there, but possibly not as much as in wild. If the feed is primarily non-marine they will not be there.
As far as I know, fish cannot convert ALA into omega-3s. Not sure what would happen if there were ALAs in their diet. It's an interesting puzzle. For instance, egg laying hens fed ALA don't convert it - but the ALA ends up in their eggs and we as mammals can convert it (but it varies from person to person and goes down as the proportion of saturated fats in out diet goes down).
Sorry for the verbosity. You *did* ask...