The Atlantic salmon (Salmo salar) is a salmon in the family Salmonidae. It is found in the northern Atlantic Ocean, in rivers that flow into the north Atlantic and, due to human introduction, in the north Pacific.
Other names used to reference Atlantic salmon are: bay salmon, black salmon, caplin-scull salmon, Sebago salmon, silver salmon, fiddler, or outside salmon. At different points in their maturation and life cycle, they are known as parr, smolt, grilse, grilt, kelt, slink, and spring salmon. Atlantic salmon that do not journey to sea, usually because of past human interference, are known as landlocked salmon or ouananiche.
Atlantic and Chinook salmon are the largest salmon. After two years at sea they average 71 to 76 cm (28 to 30 in) in length 3.6 to 5.4 kg (7.9 to 11.9 lb) in weight. But specimens can be much larger. An Atlantic salmon netted in 1960 in Scotland, in the estuary of river Hope, was recorded as weighing 49.44 kg (109 lb), while another netted in 1925 in Norway was recorded at 160.65 cm (63.25 inches) in length.
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 adult, the easiest way of identifying them is by the black spots predominantly above the lateral line, although the 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 the adipose, are bordered with black.
The distribution of Atlantic Salmon is temperature dependent. Because of global warming, some southern populations in Spain and other warm countries are expected to be extirpated soon. Before human influence, the natural breeding grounds of Atlantic Salmon were rivers in Europe and the Eastern coast of North America. When North America was settled by Europeans, eggs were brought on trains to the West coast and introduced in the rivers there. Other attempts to bring Atlantic salmon to new settlements were made; e.g. New Zealand. But since there are no suitable ocean currents on New Zealand, most of these introductions failed. There is at least one landlocked population of Atlantic salmon on New Zealand, where the fish never go out to sea.
Young salmon spend 1-4 years in their natal river. When they are large enough (ca. 15 cm), they smoltify which means they undergo a physiological change where they change camouflage from stream-adapted with large gray spots to sea-adapted with shiny sides. They also undergo some endocrinological changes which means they adapt for the change in osmosis from freshwater to saltwater. Finally, the parr (young fish) will finish smoltification by swimming with the current instead of swimming against the current. When this change of behavior occurs, they are no longer called parr, but are referred to as smolt. When the smolt reach the sea, they follow sea surface currents and feed on plankton or fry from other fish species such as herring. During their time at sea, they can sense the change in the Earth magnetic field through iron in their lateral line.
When they have had a year of good growth, they will move to the sea surface currents that transport them back to their natal river. It is a major misconception that salmon swim thousands of kilometers at sea; instead they surf through sea surface currents. When they reach their natal river they find it by smell; only 5% of Atlantic salmon go up the wrong river. Thus, the habitat of Atlantic salmon is the river where they are born and the sea surface currents that are connected to that river in a circular path.
Wild salmon disappeared from many rivers during the twentieth century due to overfishing and habitat change. By 2000 the numbers of Atlantic salmon had dropped to critically low levels.
The freshwater phases of Atlantic salmon vary between one and eight 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, and in Ungava Bay, northern Quebec, smolts as old as eight years have been encountered. The average age correlates to temperature exceeding 7 ºC (45 ºF).
The first phase is the alevin stage, when the fish stay in the breeding ground and use the remaining nutrients in their yolk sacs. During this developmental stage, their young gills develop and they become active hunters. Next is the fry stage, where the fish grow and subsequently leave 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 allows acclimation 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 one to four years they live in the ocean. Typically, Atlantic salmon migrate from their home streams to an area on the continental plate off West Greenland. During this time, they face predation from humans, seals, 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, when they become ready to return to the same freshwater tributary they departed from as smolts. After returning to their natal streams, the salmon will cease eating altogether prior to spawning. Although largely unknown, odor – the exact chemical signature of that stream – may play an important role in how salmon return to the area where they hatched. Once heavier than about 250 g, the fish no longer become prey for birds and many fish, although seals do prey upon them. Grey and common seals commonly eat Atlantic salmon. Survivability to this stage has been estimated at between 14 and 53%.