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Atlantic salmon, known scientifically as Salmo salar, is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the Atlantic. Most Atlantic salmon follow an anadromous fish migration pattern, in that they undergo their greatest feeding and growth in salt water, but adults return to spawn in native freshwater streams where the eggs hatch and juveniles grow through several distinct stages.
Atlantic salmon do not require salt water, however, and numerous examples of fully freshwater ("landlocked") populations of the species exist throughout the Northern Hemisphere. In North America, the landlocked strains are frequently known as ouananiche.
The freshwater phases of Atlantic salmon vary between 1 to 4 years, according to river location. While the young in southern rivers, such as those to the English Channel, are only one year old when they leave, those further north such as in Scottish rivers can be over four years old. The average age correlates to temperature exceeding 7°C.
The first phase is the alevin stage. During this phase, the fish stays in the breeding ground and uses the remaining nutrients in their yolk sack. During this developmental stage, the young grow and become active hunters. Once they are able to do so, they reach the fry stage. The fish grows and subsequently leaves the breeding ground in search of food. During this time, they move to areas with areas of higher prey concentration. The final freshwater stage is when they develop into parr in which they prepare for the trek to the Atlantic Ocean. During these times, the Atlantic salmon are very susceptible to predation. Nearly 40% are eaten by trout alone. Other predators include other fish and birds.
When parr develop into smolt, they begin the trip to the ocean, which predominantly happens between March and June. Migration usually lasts at least 28 days. During this time, they become preyed upon largely by pike and gulls. The young salmon stage in estuaries and readily acclimate to the changing salinity. Once ready, young smolt leave, preferring an ebb tide.
Having left their natal streams, they experience a period of rapid growth during the 1 to 4 years they live in the ocean. Typically, Atlantic salmon migrates from its home stream to an area on the continental plate off West Greenland. During this time in the salmon's life, they face predation from Greenland sharks, skate, cod, and halibut. Some dolphins have been noticed playing with dead salmon, but it is still unclear whether they consume them.
Once large enough, Atlantic salmon change into the grilse phase where they become ready to return to precise fresh water tributary in which they were born. After returning to its natal stream the salmon will cease eating altogether prior to spawning. Although it is largely unknown how they return to the same spot, it has been suggested that odour — the exact chemical signature of that stream — plays an important rôle in this process. Once above around 250 g, the fish no longer become prey for birds and many fish, although seals do prey upon them. Seals that commonly eat Atlantic salmon are the Grey Seal and Common Seal. Survivability to this stage has been estimated at between 14 and 53%.