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.
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 tid.
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 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 odor — the exact chemical signature of that stream — plays an important role 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 coloration 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 coloration. 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.
Fry and parr have been said to be territorial, but evidence showing that they guard territories is inconclusive. While they may occasionally be aggressive towards each other, the social hierarchy is still unclear. Many have been found to school, especially when leaving the estuary.
Adult Atlantic salmon are considered much more aggressive than other salmon and are more likely to attack other fish than others. Where they have become an invasive threat it has become a concern that they are attacking native salmon such as Chinook salmon and Coho salmon.
Atlantic salmon breed in the rivers of: Western Europe from Northern Portugal north to Norway, Iceland, Greenland, and the east coast of North America from Connecticut in the United States north to northern Labrador and Arctic Canada. Atlantic salmon which have escaped from the aquaculture industry have also been found breeding in rivers tributary to the Pacific Ocean in British Columbia on Canada's west coast. At sea, the species is found mainly in the waters off Greenland and in migrations to and from its natal streams. Until the early 1800s, Atlantic salmon were native to the waters of central New York. When dams were constructed on the Oswego River their spawning areas were cut off and they went extinct in the area.
Unlike the various Pacific salmon species, the Atlantic salmon is iteroparous, which means the fish does not automatically die after spawning, and may recondition themselves, return to the sea to repeat the migration and spawning pattern several times. Nevertheless, migration and spawning exact an enormous physiological toll on the individual fish, such that repeat spawners are the exception rather than the norm.
|
