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Salmo salar Linnaeus, 1758
Atlantic salmon, often referred to as the ‘king of fish’, is renowned for its vast migrations across the North Atlantic and for its ability to leap over obstacles as it makes its way upstream when it returns to the river of its birth. It is a major resource for angling tourism, and commercial exploitation, as well as having important historical, cultural, aesthetic, and biodiversity significance. In its North Atlantic range the species is now extinct, or in critical condition, in about one-third of rivers, and is endangered or vulnerable in a further third, with substantial declines having also occurred in local rivers.
Fish of the family Salmonidae have a small fleshy fin (adipose fin) on the back just in front of the tail fin, a characteristic which is absent in other European fishes. Adult salmon returning from the sea have a silvery blue-green back, silvery sides and a white belly. After some time in fresh water the silvery appearance is lost and they become brown or greenish mottled with red, orange and purple, especially the male, although some fish become dark grey or black in appearance. In fresh water, juvenile salmon (parr) are dark bluish or brown on the back with scattered black and orange/red spots on the body. They also have a series of dark ‘thumb-print like’ marks (parr marks) on the sides of the body. When the parr move down to sea (smolt stage) they become silvery in appearance for camouflage in the sea.
Spawning usually takes place in November or December and a nest (redd) is excavated in the gravel of the riverbed by the female, the eggs are deposited, fertilised by the attendant male(s), and then covered over with gravel. After some weeks, depending on the water temperature, the eggs hatch into alevins, which, after they have used up the food material in the yolk sac, become fry. These fry move up to the surface of the gravel and become the main juvenile stage of parr. After one to four years in the river, dependent on growth rate, parr become smolts, which move down the river to the sea generally in May or June. The main marine feeding grounds for salmon from Northern Ireland are around the Faroe Islands and off the west coast of Greenland. Most salmon return the following summer after one winter spent feeding at sea, these often being referred to as grilse, and are generally around 2.5–5kg. However, some salmon may spend two or three winters at sea (multi-sea-winter salmon) and often return to fresh water in the early months of the year (‘spring’ salmon), generally 5kg+, with up to 30kg having been recorded. Using ocean currents, magnetic fields and the unique smell of their home river imprinted on them as smolts, salmon can find their way back to the river they left as smolts, and even to the specific tributary. This precise natal homing has resulted in genetic differences among populations in different rivers, or tributaries of larger rivers, due to adaptation to local conditions and as a result of random genetic drift in small populations.
Atlantic salmon can be confused with brown trout in the freshwater juvenile stage and adults returning from sea can be confused with sea trout — the marine migratory form of brown trout. The Atlantic Salmon Trust website shows the main characteristics used to differentiate the two species. Hybrids with brown trout also occur at about one per cent in some rivers and are difficult to identify definitively, except by molecular genetic analysis.
How to see this species
The best time to see Atlantic salmon is in the autumn as the adults make their way upstream to the spawning areas and can often be seen jumping out of the water especially to overcome obstacles such as weirs and waterfalls. Juveniles can be seen in rivers where there is clear still water although they are difficult to differentiate from brown trout at a distance. Polaroid glasses are useful to help get a clearer view.
In Northern Ireland, Atlantic salmon is found in all larger non-polluted river systems without barriers to upstream migration from the sea. The largest populations are present in the Melvin, Foyle, Bann and other north coast river systems together with smaller, but nonetheless important populations in terms of conservation, in the Glens of Antrim and Mourne rivers. Atlantic salmon have been reintroduced to the Lagan, but lack of suitable spawning substrate has prevented re-establishment of a self-sustaining population. Stocking has also been undertaken in the Erne system, once a famous salmon river prior to the construction of hydroelectric dams. Exploitation by both commercial nets and by angling is subject to regulations in terms of methods and season. These regulations are enforced by the Fisheries Conservancy Board for Northern Ireland and by the Loughs Agency in the Foyle and Carlingford systems, together with bailiffs from local angling clubs.
Why is this species a priority in Northern Ireland?
Declining. Atlantic salmon (fresh water only) is listed in Annexes IIa and Va of the EC Habitat and Species Directive and in Appendix III of the Bern Convention.
Threats/Causes of decline
The main threats are pollution in freshwater, habitat degradation including siltation from drainage and over-grazing, water abstraction resulting in low flows, over-fishing (commercial, recreational and illegal fishing), supplemental stocking with non-native salmon, introduction of diseases and parasites through stocking and from salmon farming (especially sea lice), interbreeding with escaped farm salmon, creation of barriers to upstream migration (for example, due to construction of hydroelectric plants), predation by birds and mammals, and increased mortality at sea for reasons not fully understood at present. The potential introduction of the parasite Gyrodactylus salaris is of major concern as it could devastate salmon populations (see link below).
Conservation of this species
What you can do
River pollution and illegal fishing should be reported to the Fisheries Conservancy Board, or Loughs Agency for rivers in the Foyle and Carlingford catchments. Anglers should practice catch-and-release in populations below their conservation limit, that is, populations where the number of returning adults is insufficient to maintain the population at current levels. Anglers and other water users should ensure that all equipment used abroad is properly disinfected prior to use in Northern Ireland waters (see link to precautions against introduction of Gyrodactylus salaris). They should also ensure that all equipment is thoroughly washed in hot soapy water when moving from one river system to another within Northern Ireland.
Text written by:
Professor Andrew Ferguson