Best-known for its remarkable migration upstream back to its river of birth, the Atlantic salmon (Salmo salar) is a vital link between aquatic and forest ecosystems.
Atlantic salmon occurs in the region bordering the North Atlantic Ocean, from Portugal and Spain northwards throughout northwestern Europe, including Scandinavia and parts of Russia, as well as the countries bordering the Baltic Sea, such as Latvia and Estonia. Further west, it occurs in Iceland, Greenland, Quebec and the Maritime Provinces of eastern Canada, and the northeastern USA, from Maine to Long Island Sound in Connecticut. However, in many areas (eg the New England states in the USA, the Rhine in Germany), salmon populations have been reduced or eliminated, due to factors such as damming of rivers, over-exploitation and pollution. The species is now considered regionally extinct in Switzerland and the Czech Republic.
Atlantic salmon is raised in fish farms in various parts of the world outside its natural range, including western Canada, Chile, Australia and New Zealand. Substantial numbers often escape from their holding pens, and Atlantic salmon have been found in 77 rivers and streams in British Columbia in Canada, and in marine waters in Alaska, where fish farms have been banned since 1990.
Distribution in Scotland
The Atlantic salmon was originally native to almost all the rivers in Scotland, with the exception of those with waterfalls that are too high to jump up (eg the Affric River in Glen Affric). Today, it is still widely distributed throughout mainland Scotland and on Hebridean Islands such as Lewis, although it no longer occurs on Orkney. In parallel with the situation in many other countries, salmon numbers decreased significantly in the 20th Century, and wild salmon populations were extirpated from many rivers.
In the west of Scotland, an area where rivers were not subjected to the levels of pollution associated with the collapse of fish stocks across much of lowland Europe, the decline has been shown to correlate with the proliferation of fish farms. Salmon that escape from fish farms compete for food and inter-breed with wild fish, producing less well-adapted offspring that have lower survival rates in the wild. Juvenile salmon with artificial pigments (transferred to them from escaped farmed mother salmon in eggs) were found in many rivers on the west coast in the 1990s. Fish farms can also produce high concentrations of parasites such as sea lice, which can affect juvenile wild salmon that pass the farms during their migrations.
The Atlantic salmon is listed in annex II of the European Union’s Habitats Directive as a species of European importance. In Britain, it was added to the UK Biodiversity Action Plan (BAP) list in 2007 as a priority species for conservation action, and a number of Special Areas of Conservation (SACs) have been established to protect the species in response to the EU Habitats Directive. In the USA, Atlantic salmon is classified as Endangered.
Atlantic salmon is a fish in the Salmonidae family, and is anadromous, meaning that it breeds and spends part of its life in freshwater, and the other part in saltwater. However, the species does not require saltwater, and there are various instances of populations throughout its range that are confined to freshwater. These are known as landlocked salmon, and in eastern North America they are considered a separate subspecies (Salmo salar sebago).
Adult salmon normally range from 2 to 10 kg. in weight and 50 to 100 cm. in length. Some individuals grow larger than this, and the largest recorded Atlantic salmon, a male caught in Norway's Tana River, weighed 35.89 kg. and was over 150 cm. in length. Females reach a maximum of 20 kg. in weight and 120 cm. long.
At sea, salmon are silvery in colour, with a series of black spots on their sides above the lateral line, which is a sensory organ that runs lengthwise down each side of the body. As they mature, usually after returning to freshwater, female salmon become darker and male salmon become redder developing a characteristic 'tartan' pattern. There is a prominent triangular dorsal fin midway along the fish's back, and a small fleshy adipose fin between it and the tail. Other fins include the pectoral fins, positioned just behind the gills on each side of the body, the two small pelvic fins, on the underside of the fish below the dorsal fin, and the larger anal fin, which is situated between the pelvic fins and the tail. The fins consist of bony spines that are covered in skin, and are used for propelling and steering the fish through the water.
The Atlantic salmon has very good senses of smell, hearing and taste, and it can also detect the movement of water through its lateral line. Its sense of smell has been reported as being 1,000 times more sensitive than that of a dog, and it is believed that this olfactory ability is crucial in guiding salmon back to their natal rivers for spawning. The salmon has a moderately large mouth, and when a male is ready to spawn, its head elongates and a conspicuous hook or 'kype' forms on the lower jaw.
Spawning takes place in late autumn and winter in rivers and tributary streams, some of which are just a few metres wide, in areas with gravel beds. Turning on her side, a female salmon creates a hollow 'nest' in the gravel with her tail, and secretes ovarian fluid as she does so, to attract male fish. Adult male salmon compete with each other, using their sharp-toothed kypes for intimidation, and may be seen splashing or 'bow-waving' with their backs out of the water in shallow spawning areas. The successful male will engage in foreplay with the female, stimulating the simultaneous release of sperm and eggs into the nest. A mature female will lay between 3,000 and 12,000 eggs altogether. These are spherical, about 5mm in diameter, and orange in colour. While a mature pair are mating, small male parr (salmon that have not gone to sea, but which have reached sexual maturity) will also attempt to fertilise the eggs with their sperm – up to a third of the eggs can be fertilised in this way. To cover the fertilised eggs with gravel, the female moves forward and flexes her tail, and in so doing creates another hollow, which may become the next nest. A group of nests created in this way by the female as known as a 'redd' .
After over-wintering in the gravel, the eggs hatch in the spring, as 'alevins', which stay in the redd, using up the nutrients in the yolk sac that remains attached to their body. After two or three weeks they emerge from the redd to actively seek food. They are then known as 'fry' and feed mainly on the larvae of aquatic invertebrates, such as mayflies, caddisflies and stoneflies. As they grow a pattern of camouflaging vertical stripes develops on their sides. By their second spring they are known as 'parr'.
The parr remain in the river of their birth for one to four years before becoming elongated and silvery in colour, in a process known as 'smoltification', which prepares them for life at sea. The smolts, typically 10 to 15 cm. in length, move downstream, entering the sea in spring and head north for the cold, rich waters between Norway and Greenland. There, they feed on crustaceans and fish such as capelin (Mallotus villosus) and sandeels (Ammodytidae family). Salmon that return to their natal river to breed after one winter at sea are known as grilse, but most will remain at sea for longer – up to four winters – before returning to their birthplace.
The anadromous nature of the salmon's lifecycle enables it to take advantage of the relatively protected river waters when it is young, and also the much richer food source of the sea as an adult. This is particularly important because each adult ceases feeding when it re-enters freshwater. It relies instead on its accumulated fat for nutrients for the tremendous physical exertion of the journey upstream to its birthplace.
Many Atlantic salmon, particularly male fish, die after spawning. Those that survive are known as kelts, and after a period of recovery, ranging from a few weeks to the whole winter, they descend to the sea. There, they feed again, before making their remarkable journey to spawn once more.
The Atlantic salmon is host to a number of parasites. These include a freshwater parasitic worm (Gyrodactylus salaris), which attaches itself to the fish’s skin and is native to the Baltic Sea. When this parasite was accidentally introduced to Norwegian rivers, populations of wild salmon with no natural resistance were decimated. Elimination of the parasite to enable the recovery of some Norwegian salmon populations has involved poisoning whole rivers at huge expense. The sea louse (Lepeophtheirus salmonis) is another external or ectoparasite and is a serious problem in many fish farms, where it has multiplied and spread to infect wild salmon in nearby waters.
The freshwater pearl mussel (Margaritifera margaritifera), a mollusc that is included on the UK’s BAP list, is partially parasitic on Atlantic salmon. The mussel’s tiny larvae encyst on the salmon’s gills in the summer, and remain there before dropping off the following spring, but they appear to cause no permanent damage to the salmon. Indeed, recent research in Russia suggests that the relationship between pearl mussels and salmon may be a symbiotic one, in which the fish provides an essential stage in the mussel’s life cycle, while the adult mussels improve the water quality for the salmon by their filter feeding.
As a predator, Atlantic salmon plays a key role in regulating the numbers of the species it preys upon, particularly in freshwater. However, salmon populations themselves are dependent on the abundance of their prey. The decrease in numbers of salmon returning to their natal rivers that has been recorded throughout their range in recent decades is due at least in part to depletion of their food species by human overfishing, particularly harvesting of ‘industrial’ species such as sandeels, blue whiting (Micromesistius poutassou) and capelin.
Salmon are themselves prey for other species. Fry and parr are eaten by brown trout (Salmo trutta) and other fish, and birds such as herons. As young adults in the sea, salmon are fed upon by cod (Gadus morhua), skate ( Rajidae family ) and Greenland sharks (Somniosus microcephalus). When they are larger their main predators are the common seal (Phoca vitulina) and grey seal (Halichoerus grypus). Until recently, a majority of salmon were taken by people before returning to freshwater, by being netted either in the sea, or in estuaries. As a result of international agreements, the amount of salmon netting is currently greatly diminished, and many netting stations no longer operate. In rivers, anglers are increasingly aware of conservation concerns, with many returning rod-caught fish to the river alive, so that they can spawn.
Otters (Lutra lutra) take many salmon from some spawning streams, pulling carcasses onto stream banks. Weak and dying fish are most easily caught. An otter is only able to eat part of a larger fish in a single meal, and other animals, including fox (Vulpes vulpes), badger (Meles meles ), pine marten (Martes martes), wood mouse (Apodemus sylvaticus) and many birds, may feed on a carcass before the otter returns to it for a second time.
In the past, brown bears (Ursus arctos) would also have fed on salmon that return to spawn in Scotland, as they still do elsewhere in the world today. The extinction of bears here has caused a disruption to the cycling of nutrients from the sea back to the land, as the salmon scraps left by bears in other forests are an important source of nitrogen and protein for the trees and many other organisms. In some streams the otter still performs this ecological role. Salmon benefit too from the presence of riparian trees – they provide shelter and shade, and their leaf litter feeds many of the aquatic invertebrates that the fry and parr eat. In turn, salmon that die after spawning provide a source of nutrients for riverside trees and other plants, thereby illustrating that the relationship between these remarkable fish and riparian vegetation is a mutually dependent one.