All the salmon that i've seen here is Chilean and as far as I know it comes from fish farms. I have know idea about how well they're fed, I remember the news stories that broke a few years ago were about overcrowding rather than feeding practices. Supposedly that's gotten much better in the last couple years after they had outbreak of disease and is one of the better sources for farmed salmon. Someone told me that the chilean farmed pacific salmon is much better than the atlantic ones, although that was probably hearsay. I've seen both varieties for sale here but the overwhelming majority is atlantic salmon. To get the scoop on the feeding and the like, you could probably follow the fish back from the store to the distributor to the exporter to the producer, but as they say, the proof is in the pudding. Unhealthy fish won't taste the same.
Edit: By the way, I don't think most of it's fresh.