Part of the reason for the Atlantic salmon's renown is undoubtedly the epic migration it undertakes from the river it grew up in, to distant oceanic feeding grounds, the return to its 'home' river and then on to spawn in the gravel beds of its birth. Salmon from Scottish rivers mainly leave between April and June, when they are called 'smolts'. Once in the sea, they begin an arduous journey to feeding grounds around Greenland and the Faroes. There they feed, along with salmon from North America and Scandinavia, on a diet that includes shrimps, squid, sprats and sand eels.
Salmon remain at sea for an average of one to two years, although some stay longer. Those returning after one year are known as 'grilse'. Losses at sea are high - they are taken by a variety of predators including humans, seals and large fish such as sharks, but sufficient numbers are able to return to the river to produce the next generation.
Little is known about the habits of salmon at sea but in order to complete such an impressive migration and return to the river of their birth, salmon are thought to use several methods of navigation. In the oceans they may use currents; closer to the river mouth chemicals in the water are likely to be important, and when ascending the river further, chemical and visual information guides them to their spawning grounds.
Salmon in Scottish rivers are unique in that they return from the sea almost all year round. Those that return early in the year tend to have been at sea for more than one year and will be migrating to spawning grounds in the uppermost reaches of their 'home' river. Once back in the river, adult salmon generally do not feed, resulting in a prolonged fast and a loss of 40% of their body weight by the time they spawn. Yet they retain sufficient reserves of strength to ascend waterfalls up to 3m high, and can frequently be seen negotiating these spectacular and seemingly impassable obstacles, such as Rogie Falls on the Black Water and the Falls of Shin, Sutherland, during the summer.
When returning from the sea, both male ('cock') and female ('hen') salmon have silver sides and a silver/white belly, but as spawning time approaches the male in particular will change markedly both its colour and appearance. Such male fish can be known as 'Tartan Fish' due to the mixture of red, brown and purple colours which develop on the sides and back. The male also develops a prominent upturned hook ('kype') on the lower jaw.
The spawning grounds to which the salmon are returning are typically gravel beds in relatively shallow, fast-flowing water. Salmon reach their spawning grounds during October to December when the female will excavate a depression in a gravel bed by lying on her side and repeatedly swishing her tail. This action lifts the gravel and carries it downstream. Then, when the spawning site (or 'redd') is about 15cm deep, she releases her eggs into it and they are immediately fertilised by an accompanying male. The female then fills in the redd, burying the eggs. During spawning the male fish aggressively chases away other fish. The splashing associated with spawning behaviour can sometimes be seen, especially when it happens in very shallow water and the backs of the fish are exposed.
After spawning the male fish remain near the redds in order to mate with any other females; however, they eventually become very weak and die. The majority of female fish also eventually die, but a small proportion are able to migrate back to the sea to feed ('kelts') and survive to spawn again.