This article by George Mateljan does a great job of laying out the case for avoiding farmed salmon. I urge you to take a moment to read it.
Funny enough, the salmon in this picture, which I snagged from George Mateljan's wonderful website, World's Healthiest Foods, looks farmed. One way to tell is the nearly perfect square shape of the fish, which means it has been cut into shape in processing. I've never seen this preparation for wild caught salmon.