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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 north Atlantic.

Most Atlantic salmon follow an anadromous fish migration pattern, in that they undergo their greatest feeding and growth in salt water, however 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.

The freshwater phases of Atlantic salmon vary between 1 to 5 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 gills develop and they 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 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 acclimatise 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 humans, 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 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%.

The colouration of young Atlantic salmon does not resemble their adult stage. While they live in freshwater they have blue and red spots. While they mature they take on a silver blue sheen. When they are adults the easiest way of identifying them is by the black spots predominantly above the lateral line, although its caudal fin is usually unspotted. When they reproduce males take on a slight green or red colouration. The salmon has a fusiform body, and well developed teeth. All fins, save for the adipose, are bordered with black.

After hatching, young salmon begin a feeding response within a couple days. After the yolk sac is absorbed by the body, they begin to hunt. Juveniles start with tiny invertebrates, but as they mature they may occasionally eat small fishes. During this time they hunt both in the substrate, and also those in the current. Some have been known to also eat salmon eggs. The most commonly eaten foods include caddisflies, blackflies, mayflies, and stoneflies.

In adulthood, fish feed on much larger food: Arctic squid, sand eels, amphipods, Arctic shrimp, and sometimes herring. During this feeding time the fish's size increases dramatically.