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. This is why we don't, as a rule, raise predators for food. We don't farm lions because it would be stunningly inefficient. But we do farm salmon, and salmon are predators.
Estimates vary, but there is a metabolic loss in each step up the food chain.
Fishmeal is produced globally, especially from sardines off South America and especially from herring in the North Pacific. Worldwide, aquaculture is sponsoring a secondary fishery that vacuums the ocean floor like a Shop Vac. Ocean fisheries historically have depleted fish stocks, but until recently were at least somewhat selective to marketable species. However, when the end product is fishmeal, most of what shows up in a net can be ground into the mix, setting the stage for a decimation of the ecosystem the way markets for wood pulp set the stage for clearcuts. Wild salmon graze this ecosystem selectively, efficiently harvesting its protein for us.