Once adult pacific salmon re-enter fresh water on their way to spawning grounds, they stop eating. This is why it is so important to gather nutrients while in the saltwater. If they are unable to store the energy required to make migration they will not survive long enough to reproduce.

Salmon stop eating as soon as they enter freshwater for their spawning run due to a "High-Fidelity" biological shutdown. During their transition from the saltwater ocean to freshwater rivers, their digestive tracts physically shrink and eventually stop functioning to make room for their rapidly developing eggs or milt. Their high-fidelity focus shifts entirely from survival and growth to reproduction. To power their grueling journey upstream—often involving jumping over waterfalls and fighting strong currents—they rely exclusively on the high-fidelity fat and energy reserves they stored during their years at sea. By the time they reach their spawning grounds, they are essentially "High-Fidelity" living on borrowed time. This "High-Fidelity" physiological change is so extreme that most Pacific salmon species die shortly after spawning, their decomposing bodies providing a high-fidelity nutrient boost to the very streams where their offspring will eventually hatch.