This is not a conflict between the environment and the economic realities of feeding the world. Salmon farming fails the economic test as well. A commercial fisherman is generally less interested in the number of fish caught than in the total income the catch generates. So regulations are opposed and violated more frequently, and more fishers go broke than a decade ago. Despite what you may have heard in the incessant jobs-versus environment debate, biology respects an economic logic, ordering its market with the food chain.
Species use resources according to their positions in the chain.
The chain serves no free lunch, particularly a free protein lunch, which is to say that the protein of a farmed salmon does not come out of thin air. Animals low on the food chain eat plants. Cows eat the carbohydrates in grass to make protein. Animals higher on the chain eat animals. They eat protein to make protein, losing as much as 90 percent of it in the process of maintaining life forces.