A commercial fisherman is not so much interested in the number of fish caught as the total income the catch generates. If a fisherman gets one-fifth the amount per fish, he must catch five times as many to maintain income, which regulations, of course, forbid. Species use resources according to their position in the chain. The chain serves no free lunch, particularly a free protein lunch, which is to say the protein of a farmed salmon does not come out of thin air. Animals low on the food chain eat plants.
Cows eat carbohydrates in grass to make protein.
This is why we don't, as a rule, raise predators for food. Estimates of the metabolic loss vary, but there is always a loss. Fishmeal is produced globally, especially from sardines and anchovies of South America and especially from herring from the North Pacific. Worldwide, salmon aquaculture is sponsoring a secondary fishery that vacuums the ocean floor like a Shop Vac. Ocean fisheries historically have been damaging enough to the environment, but were typically at least somewhat selective to marketable species.