Atlantic Salmon

The Atlantic salmon (Salmo salar) is a species of ray-finned fish in the family Salmonidae. It is the 3rd largest of the Salmonidae and can grow up to a meter in length.  It is found in the northern Atlantic Ocean and in rivers that flow into this ocean. Most populations of this fish species are anadromous, hatching in streams and rivers but moving out to sea as they grow where they mature, after which the adult fish seasonally move upstream again to spawn.

When the mature fish re-enter rivers to spawn, they change in color and appearance. Some populations of this fish only migrate to large lakes, and are "landlocked", spending their entire lives in freshwater. Such populations are found throughout the range of the species. Unlike Pacific species of salmon, S. salar is iteroparous, which means it can survive spawning and return to sea to repeat the process again in another year -such individuals can grow to extremely large sizes, although they are rare.

Atlantic salmon are the largest species in their genus, Salmo. After two years at sea, the fish average 71 to 76 cm (28 to 30 in) in length and 3.6 to 5.4 kg (7.9 to 11.9 lb) in weight. But specimens that spend four or more winters feeding at sea can be much larger. An Atlantic salmon netted in 1960 in Scotland, in the estuary of the river Hope, weighed 49.44 kg (109.0 lb), the heaviest recorded in all available literature. Another netted in 1925 in Norway measured 160.65 cm (63.25 in) in length, the longest Atlantic salmon on record.

The colouration of young Atlantic salmon does not resemble the adult stage. While they live in fresh water, they have blue and red spots. At maturity, they take on a silver-blue sheen. The easiest way of identifying them as an adult is by the black spots predominantly above the lateral line, though 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, except the adipose fin, are bordered with black.

The natural breeding grounds of Atlantic salmon are rivers in Europe and the northeastern coast of North America. In Europe, Atlantic salmon are still found as far south as Spain, and as far north as Russia. The species distribution is easily influenced by changes in freshwater habitat and climate. Atlantic salmon are a cold-water fish species and are particularly sensitive to changes in water temperature. 

Young salmon spend one to four years in their natal river. When they are large enough (c. 15 centimetres (5.9 in)), they smoltify, changing camouflage from stream-adapted with large, gray spots to sea-adapted with shiny sides. They also undergo some endocrinological changes to adapt to osmotic differences between fresh water and seawater habitat. When smoltification is complete, the parr (young fish) now begin to swim with the current instead of against it. With this behavioral change, the fish are now 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. 

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 kilometres at sea; instead they surf through sea surface currents.  It is possible they find their natal river by smell, although this is not confirmed; only 5% of Atlantic salmon go up the wrong river. The range of an individual Atlantic salmon can thus be the river where they are born and the sea surface currents that are connected to that river in a circular path.

Atlantic salmon are the only salmon native to the Atlantic Ocean. There are three groups of wild Atlantic salmon: North American, European, and Baltic. The North American group, including the Canadian and U.S. populations, was historically found from northern Quebec southeast to Newfoundland and southwest to Long Island Sound. In the United States, Atlantic salmon were once native to almost every river north of the Hudson River. Due to the effects of industrial and agricultural development (including habitat destruction, dams, and historic overfishing), most populations native to New England were eradicated. Now, the only native populations of Atlantic salmon in the United States are found in Maine.

Atlantic Salmon - by NOAA

Atlantic Salmon - by US Fish & Wildlife Service

Fish Facts: Atlantic Salmon (Salmo salar) - by Orvis

Fly Fishing for Atlantic Salmon - by Orvis