One of the problems faced by salmon, and other resources, is being forced to "diet shift". They may prefer X, say herring, for oil content, calories per gram, and so on. Switch to, say, sand lance or anchovy, may provide the value but less food value (calories, fats, whatever).

Currently, large adult salmon are dieing on their return to the river because the foods they are eating cannot meet metabolic needs. Full stomach, but starving.