Salmon’s unique life cycle often raises questions about their habitat: are they freshwater or saltwater fish? The truth about salmon is more intricate than a simple classification. Their lives span vastly different aquatic environments, showcasing biological adaptations.
Salmon’s Dual Habitat
Salmon are neither exclusively freshwater nor exclusively saltwater fish. Instead, they are classified as “anadromous,” a term derived from the Ancient Greek words ana, meaning “up,” and dromos, meaning “running.” Anadromous fish hatch in freshwater, migrate to the ocean to mature, and then return to freshwater to reproduce. Most salmon populations exhibit this migratory pattern, spending the majority of their adult lives in the ocean before returning to their natal freshwater streams for spawning.
The Salmon Life Journey
A salmon’s life begins as an egg, laid in gravel beds within cool, clean freshwater rivers or streams. After hatching, the young are known as alevin, remaining in the gravel and absorbing nutrients from their yolk sacs. They then develop into fry, emerging from the gravel to feed on small insects and plankton in the freshwater environment. As they grow, they become parr, residing in freshwater for a period.
The next stage is smoltification, where juvenile salmon, now called smolts, undergo physiological changes to adapt for marine life. After this transformation, smolts migrate downstream to estuaries, which serve as transitional zones, before entering the ocean. They spend years in the ocean, growing significantly by feeding on smaller fish, squid, and crustaceans. Once mature, adult salmon journey back to their freshwater birthplace to spawn.
Physiological Adaptations for Migration
Salmon possess physiological adaptations that enable them to transition between freshwater and saltwater environments, a process governed by osmoregulation. In freshwater, a salmon’s body has a higher salt concentration than its surroundings, leading to water constantly entering its body and salts diffusing out. To counteract this, freshwater salmon produce large volumes of dilute urine to expel excess water and actively absorb salts through their gills. They also minimize drinking water in this environment.
Conversely, in the ocean, the salmon’s body has a lower salt concentration than the surrounding seawater, causing water to leave its body and salts to enter. To maintain balance, ocean-dwelling salmon drink seawater and excrete excess salt through specialized cells in their gills, which actively pump it out. The kidneys also produce a small volume of concentrated, salt-rich urine. During smoltification, the direction of these ion pumps in the gills reverses, preparing the fish for the change in salinity. Hormonal changes also facilitate these physiological shifts.