Salmon is the common name for several species of euryhaline ray-finned fish belonging to the subfamily Salmoninae of the family Salmonidae, order Salmoniformes. It primarily refers to six species of the genus Oncorhynchus in the North Pacific Ocean (Chinook salmon, coho salmon, masu salmon, pink salmon, sockeye salmon and chum salmon), as well as the Atlantic salmon, the only species in the genus Salmo that is referred to as "salmon", inhabiting the North Atlantic Ocean. In China’s consumer market, the classic salmon generally refers to Atlantic salmon, commonly known as Norwegian salmon. As a world-renowned precious cold-water fish species, salmon enjoys the reputation of "the King of the Icy Sea"

Their life cycle represents an ultimate cycle of life: born in fresh water, growing in the ocean, and returning to fresh water. Within a lifespan of only a few years, they travel back and forth between freshwater and marine environments, undergoing multiple morphological and physiological transformations. The vast majority of wild individuals exhaust their vitality after fulfilling their reproductive mission, concluding a life journey of rebirth through death. A full understanding of the salmon life cycle reveals the unique life force of this species.

The life of a salmon begins in the upper reaches of clear, cold freshwater streams. Every autumn and winter, sexually mature wild salmon migrate thousands of kilometers back to their birthplaces. The female fish swings her powerful caudal fin to dig redds in the riverbed gravel and lays thousands of orange-red eggs. Males then perform external fertilization. Afterwards, the female stirs the gravel again to cover the eggs with a thick layer, shielding them from water currents and predators.

The hatching rate of eggs depends on water temperature. Under suitable cold conditions, the incubation period lasts 2 to 4 months. Buried in crevices among gravel, the eggs develop slowly with continuous oxygen supply from running water. Due to multiple pressures including predation, sharp fluctuations in water temperature and water pollution, less than 10% of the eggs can hatch successfully. Life faces harsh natural selection right from the very start.

When the eggs hatch, the young fish produced are called alevins, also known as yolk-sac fry. At this stage, the tiny fish carry a large yolk sac attached to their abdomen — their innate "personal food reserve". Newly hatched alevins cannot feed on their own and survive entirely by absorbing nutrients from the yolk sac, with no need to seek food outside.

To avoid predation by birds and small fish, alevins remain hidden deep within gravel crevices, staying dormant and growing quietly. After 1 to 2 months of growth, the nutrients in the abdominal yolk sac are fully consumed. The juveniles’ internal organs and swimming capabilities become fully developed. They emerge from hiding and enter the parr stage.

Once the yolk sac is fully absorbed, the young fish develop into salmon fry. Slender and agile at this stage, they begin active feeding, preying on plankton, aquatic insects and tiny crustaceans.

This is a critical period for salmon to adapt to the external environment. Freed from the shelter of gravel, the fry stay active in shallow stream riffles. While feeding and growing, they hone survival skills such as swimming against the current and evading predators, building up sufficient energy reserves to physiologically prepare for their future journey to the ocean.

After several months of growth, the fry enter the parr stage, the longest phase of the salmon’s life spent in freshwater. The fish develop distinctive dark vertical bars and spots, which act as natural camouflage. This patterning blends seamlessly with the light, shadows and gravel of the stream, reducing the risk of predation.

Parr live in their natal freshwater rivers for 1 to 3 years, gradually reaching a body length of 10–25 cm. They keep feeding to build strong bodies and adapt to complex flow conditions. Better living conditions enable faster development for parr, bringing them sooner to the critical transformation for downstream migration to the ocean.

This is the most critical transitional metamorphosis in a salmon’s life, scientifically termed smoltification. Once parr reach sufficient size and physical condition, their bodies undergo comprehensive physiological changes: the original dark markings fade completely, their scales turn silvery and smooth all over, and their bodies become more slender and streamlined, bringing a striking change in appearance.

The more essential transformation lies in the reconstruction of bodily functions. Gills, kidneys and the osmoregulatory system are fully upgraded. The fish evolve from freshwater-only dwellers into organisms capable of tolerating high-salinity seawater, perfectly solving the survival challenge for freshwater fish entering the ocean. This process is known as silvery metamorphosis or smoltification. After completing smoltification, the young fish swim downstream in schools, leaving the freshwater habitat where they have lived for years, heading for the vast ocean.

1.Ocean Growth Phase

After entering the ocean, salmon embark on a period of rapid growth. The open ocean abounds with food sources such as krill, small fish and crustaceans, supplying the salmon with ample nutrition. During their 2–4 years at sea, their body size increases dramatically. They grow from small fish of more than ten centimeters into adult fish weighing several kilograms, with firm muscles and plump fat deposits. This is the primary reason why salmon flesh is rich, tender and nutrient-dense for human consumption.

Ocean-dwelling salmon are silvery all over with robust bodies, adept at long-distance roaming. They build up abundant energy reserves across vast waters until they reach sexual maturity, when the ultimate mission of their life quietly begins.

2. Swimming Upstream: A Journey Toward Life Through Death

Sexually mature salmon embark on one of nature’s most spectacular anadromous migrations. Guided by powerful spatial memory, they cross the vast ocean to return precisely to the stream where they were born. Once they enter freshwater, salmon cease feeding entirely and rely solely on fat reserves built up during their time at sea for all physical exertion.

The journey home is fraught with dangers. They struggle against strong upstream currents, leap over waterfalls and dams, and face multiple threats from brown bears, water birds and human fishing. During this arduous trek, the appearance of adult salmon transforms once again: their silvery bodies turn deep red and dark green, their head morphology changes, their skin becomes cracked, and their bodies steadily deteriorate from constant energy depletion.

3. Spawning Completes, and the Cycle of Life Continues

After enduring countless hardships to reach the spawning grounds, male and female salmon mate and lay eggs, repeating this life ritual generation after generation. Having exhausted all physical energy, the vast majority of wild salmon die within one to two weeks after spawning. Their bodies sink into the stream, serving as nutrients for aquatic plants and microorganisms, nourishing the entire water system and creating living conditions for the new generation of eggs and fry. Only a tiny number of individuals can survive and return to the ocean, which remains a rare occurrence. With this, the life cycle of one generation of salmon comes to an end, and a new cycle begins.

Note: Farmed salmon for human consumption are harvested during the ocean grow-out phase. They do not undergo the exhausting upstream migration, which explains their tender flesh and rich fat content.