A farmer needs about 5.5 pounds of foraged fish for 2.2 pounds of fish to be produced for human consumption. This translates into 2.2 million tons of meal worldwide every year. This large volume of fish meal threatens the survival of some species of fish.
Most salmon farmers decided learned of this problem and decided to help the environment (and their own profit) by replacing some of the fish meal with vegetable oil. However fish fed vegetable oil causes health problems for the consumer. As I stated in the Seafood and You section of this website, fish are the primary meat that gives humans access to the fatty acids in omega 3s. This is beneficial to humans because “they protect against cardiovascular illness, and are necessary for brain development. Therefore, we've tried to make sure that the human diet is rich in these omega threes. And when fish are fed vegetable oils which don't contain omega threes, their flesh contains very little.” (Danger to Your Plate? Farm-Raised and Wild Salmon)
Therefore, though salmon is still healthier than pork, it is significantly lower in omega 3s than it used to be, so the health benefits are lessened.