Anadromous

Anadromy requires osmoregulatory flexibility (euryhalinity) — salmon undergo a developmental transformation (smoltification) in which gill Na+/K+-ATPase activity increases dramatically, enabling the kidneys to switch from conserving salt (freshwater mode) to excreting it (seawater mode). Navigating back to natal streams relies on olfactory imprinting during the smolt stage. Pacific salmon of genus Oncorhynchus are semelparous — dying after a single spawning event — while Atlantic salmon (Salmo salar) can spawn multiple times (iteroparity). The carcasses of dead Pacific salmon fertilize stream ecosystems with marine-derived nutrients.