For my fish choices, I look at Seafood watch
http://www.mbayaq.org/cr/seafoodwatch.asp from the Monterey Bay aquarium. I use it as a guideline for healthy and environmentally responsible choices. Not to say that I'm perfect but it gives me a good idea which fish are better than others. I used to buy Salmon from Costco but stopped because it advised against farmed salmon.
Seafood guide A-N
http://www.mbayaq.org/cr/SeafoodWatc...factsheet.aspx
Seafood guide O-Z
http://www.mbayaq.org/cr/SeafoodWatc...factsheet.aspx
Here is what it says about farmed salmon specifically:
Health Alert
Salmon farmers may use pesticides and antibiotics to control outbreaks of disease among the fish. When consumers eat this fish, the residues from the chemicals may affect their health or interfere with medicines they’re taking. The Environmental Defense has issued a health advisory for Farmed salmon due to high levels of PCBs and dioxins. For more information, visit their Ocean's Alive web site.
Summary
Salmon are farmed in open pens and cages in coastal areas. Waste from most farms is released directly into the ocean. Parasites and diseases from farmed salmon can spread to wild fish swimming near the farms. And feeding farmed salmon actually uses more fish than it produces, which puts more pressure on wild populations. Farmed salmon generally uses three pounds of wild fish to grow one pound of farmed salmon.
Segments of the salmon farming industry are improving their practices but the environmental impact is still increasing because production has risen more than 400% in the last decade. In the market, there is currently no way to tell which salmon are coming from more-sustainable farms, so for now we ask you to avoid farmed salmon and choose wild-caught salmon instead.