Salmon are anadromous, meaning they migrate from freshwater to the ocean before returning to reproduce. Juvenile salmon are given a new name with almost every change in their early life. These distinct names mark their physical changes, nutritional source, and eventual readiness to tolerate the vast difference between river and ocean habitats.
Newly Hatched and Free Swimming
The first developmental stage after hatching is known as the alevin, a tiny fish still nestled within the gravel nest, or redd. Alevins possess a large, visible yolk sac attached to their underside, which supplies all the nourishment they need during this initial period of immobility. Remaining hidden within the protective confines of the gravel protects them, as they are highly vulnerable if washed out into the open current. They remain in this stage until the yolk sac is fully absorbed and they are ready for independent movement.
Once the yolk sac is consumed, the fish emerges from the gravel and is recognized as a fry, signaling the start of its active, free-swimming life. The fry must immediately begin foraging for itself, feeding on small invertebrates and aquatic insects. They are typically one to two inches in length and actively seek out calm, protected areas of the stream to begin their growth. This transition marks a profound shift in behavior and survival strategy.
Developing in the River
Following the fry stage, the young salmon enters the parr stage, often the longest phase of its freshwater existence. The most distinctive feature is the series of dark, vertical bars along its sides, known as parr marks. These markings provide excellent camouflage against the mottled riverbed, helping the fish hide from predators while establishing a small territory. The parr actively feeds on insect larvae and other small aquatic life, growing steadily as it prepares for the journey ahead.
The length of the parr stage varies significantly across different salmon species, reflecting diverse life history strategies. For example, some Pacific salmon like Chinook may spend only a few months as parr before migrating downstream. In contrast, species such as Coho, Sockeye, and Atlantic salmon can remain in the river for one to four years, depending on factors like food availability and water temperature. This extended freshwater residence allows them to reach a larger size, increasing their chances of survival once they enter the ocean environment.
Transition to Saltwater
The final juvenile stage before entering the ocean is the smolt, which undergoes a complex physiological change called smoltification. This preparatory transformation equips the salmon to survive the drastic change from freshwater to saltwater. Externally, the parr marks fade, and the fish develops a bright, reflective silver coloration and a more streamlined body shape. This silvering provides necessary camouflage for life in the open ocean water.
Internally, smoltification involves a complete reprogramming of the fish’s osmoregulatory system, centered in the gills and kidneys. In freshwater, the salmon works to excrete excess water and retain salts. In saltwater, it must reverse this function, actively drinking water and excreting salt through specialized chloride cells in the gills. This complex hormonal change is typically triggered by increasing day length and water temperature in the spring. Once transformed, the smolt ceases territorial behavior, begins to school, and instinctively starts its downstream migration toward the estuary.