Atlantic salmon

Name and taxonomy

Carl Linnaeus named the species Salmo salar in 1758. Salmo is the Latin word for salmon. Salar is often translated as leaper, though it more likely meant a dweller in salt water. A Latin dictionary of 1879 reads it as a kind of trout, from a poem by Ausonius in the 4th century. The young fish look so different from the adults that they were once thought to be separate species.

The salmon belongs to the genus Salmo, which it shares with the brown trout, within the order Salmoniformes. It is the largest species in its genus and the third largest salmonid, after the Siberian taimen and the Chinook. English has a name for every stage of its life: alevin, fry, parr, smolt, grilse and kelt. Fishermen also call it bay salmon, black salmon, fiddler, silver salmon and winnish. In North America a population that never goes to sea is called ouananiche, or landlocked salmon.

Evolution and relatives

The salmon family also holds the trouts, chars, whitefishes and graylings. The closest relative is the brown trout, Salmo trutta, and the two breed together where they share a river. In two of four rivers studied in northern Spain, 2 to 3% of the salmon were salmon-trout hybrids. That is the highest rate of natural hybridisation reported anywhere in Europe.

Fish scales from a coastal pond in New Jersey show that salmon lived there 10,000 years ago. Like other salmonids, the species keeps a small fleshy adipose fin on the back near the tail. The anal fin has under 13 rays, while the Pacific salmons have more. Landlocked populations across the Northern Hemisphere show that the sea phase is not essential to the species.

Appearance

The salmon has a torpedo-shaped body, a small pointed head and well-developed teeth. The body is deepest under the dorsal fin and narrows to a slim tail stalk with a slightly forked tail. Every fin except the adipose fin has a black edge. At sea the flanks and belly are silver, and the back is brown, green or blue. Black spots shaped like an X sit mostly above the lateral line, and the tail fin usually has none.

Young parr carry 8 to 11 dark bars along each side, with a row of red spots between them. Those marks vanish when the fish turns silver as a smolt at about 15 centimetres. In the river before spawning, both sexes turn bronze to purple, and red spots appear on the head and body. A male grows a hooked lower jaw called a kype, and his head lengthens. Spent fish, the kelts, go so dark that anglers call them black salmon.

After two winters at sea a fish is 71 to 76 centimetres long and 3.6 to 5.4 kilograms. Fish that feed at sea for four or more winters, or that spawn again, grow far larger. The longest on record, from Norway in 1925, measured 160.7 cm. The heaviest, from the River Hope estuary in Scotland in 1960, weighed 49.4 kg.

Habitat and range

The salmon breeds in rivers on both sides of the North Atlantic. In Europe it runs rivers from northern Portugal and Spain to Norway, Iceland, Greenland and the Pechora River in Russia. In North America it breeds from Connecticut north to Labrador and Arctic Canada. Warming seas let it colonise Svalbard, at 78 degrees north, between 2002 and 2006. GBIF holds records from 55 countries, and that count includes farm escapes in the Pacific.

The young need large, cool rivers with wide gravel beds in the headwaters. At sea the fish prefer water of 4 to 12 degrees Celsius. They survive a short spell in water just below freezing, or as warm as 27.8 degrees, but no longer. Most sea-going fish from both continents gather on the continental shelf off West Greenland. The species is a cold-water fish, and it reacts quickly to a change in river temperature.

Landlocked populations live in large lakes and never see the sea. One lived in Lake Ontario until logging, dams and mills wiped it out by 1896. Farm escapes from net pens have reached Puget Sound, British Columbia and Alaska since the 1980s. One river on Vancouver Island has seen them breed, but no lasting Pacific population has formed.

Diet and feeding

A newly hatched alevin lives on its yolk sac and starts to hunt within days of using it up. In the river the young eat the larvae of blackflies, stoneflies, caddisflies, mayflies and midges. Land insects that fall into the water matter too, above all in late summer. Parr sometimes eat small fish, and a few have been seen eating salmon eggs.

At sea the young fish first take plankton such as krill, plus amphipods and shrimps. Larger salmon switch to fish, and capelin is the favourite. They also take herring, alewives, smelt, sand lance, small mackerel and small cod. The shrimp in the diet is what turns the flesh pink. Once a salmon enters fresh water to spawn it stops eating, even though it will still strike at an angler's fly.

Behaviour and social life

Fry and parr may hold small territories, but the evidence is unclear. They can be aggressive to each other, and many school together, above all when they leave the estuary. Adults are more aggressive than other salmon and attack other fish more readily. Salmon born in many different rivers share the same feeding grounds at sea.

Smolts leave the river between March and June, and they prefer to go on an ebb tide. Once at sea they follow the surface currents and feed as they drift. The idea that salmon swim thousands of kilometres is a misconception: they ride the currents. After a year of good growth they move into currents that carry them back towards home. Their range is the home river plus the loop of currents linked to it.

On the way upstream a salmon can leap about 3 metres up a waterfall. Beaver dams rarely stop them, and parr have been found upstream of the dams. In one restored Nova Scotia stream the dams only blocked fish in the smallest reaches in dry years. Beaver ponds help: two-year-old parr in Canadian beaver ponds grew faster and were in better condition than parr elsewhere.

Communication and senses

The salmon has a keen sense of smell, hearing and taste, which it uses to find food and to sense danger. The lateral line along each flank feels water movement, so a fish senses a predator's approach as pressure waves. Iron in the lateral line lets the fish sense changes in the Earth's magnetic field at sea. Like other fish, it probably uses that magnetic sense together with the currents to navigate.

As a smolt heads downstream it memorises the chemical signature of its home water, a process called imprinting. Years later the adult follows that scent back up the coast and into the right river. Scientists are not certain how the return works, and some suggest the sun and stars help. Most agree that the smell of the parent stream is the main guide. Salmon do not call to each other, and any signalling between fish at the redd is thought to be visual.

Reproduction and life cycle

Spawning runs from October into November, with a peak in late October. The female picks a gravel riffle above a pool and digs a nest, the redd, with strong beats of her tail. The male courts her and drives off rivals while she works. When the redd is ready the pair release eggs and milt together, and she covers them with gravel. Small precocious parr sometimes dart in and fertilise some of the eggs.

A female lays about 1,500 eggs per kilogram of her body weight. The eggs are pale orange, round and briefly sticky, and they lie 12.7 to 25.4 centimetres deep in the gravel. She rests, then digs another redd, and a full spawning can take a week or more. The eggs develop for about three months and hatch in April. The alevins stay in the gravel until the yolk is gone and emerge in May or June.

Parr grow slowly and stay in the river for one to four years, or up to eight in Ungava Bay. At 12 to 15 centimetres they turn silver, become smolts and head for the sea. A fish that returns after one winter at sea is a grilse. Some fish do not die after spawning, and 5 to 10% go back to sea and return to spawn again. A very few spawn three times or more, and the oldest known fish reached 13 years.

Predators and threats

The young are very vulnerable, and trout alone eat nearly 40% of them. Other fish and birds take the rest. Once a salmon passes about 250 grams, birds and most fish leave it alone. At sea the predators are seals, Greenland sharks, skates, cod, halibut and people. Grey seals and common seals often eat adult salmon, and survival to the return is put at 14 to 53%.

Dams and mills block the way to the spawning gravel, and over half of North America's runs were lost by 1850. In one region of Nova Scotia 31 of 33 salmon streams were dammed by lumber mills. Overfishing, pollution and acid rain add to the pressure, and warming rivers hurt a cold-water fish. Escaped farm fish breed with wild fish and pass on parasites. In the Northwest Atlantic 27.1% of fish in 17 of 18 rivers studied were stocked fish or farm hybrids.

Population and conservation

The IUCN Red List rates the Atlantic salmon as Near Threatened. A regional assessment found the European population Vulnerable. The runs along the Maine coast are listed as endangered under the US Endangered Species Act. Canada declared the inner Bay of Fundy runs endangered in 2000. Finland lists the species as Endangered, Portugal and Estonia as Critically Endangered. It is extinct in Czechia, the Netherlands, Poland, Slovakia and Switzerland.

In the 1950s fishermen found the sea feeding grounds off Greenland and the Faroes and set drift nets there. Catches fell from four million fish to 700,000 between 1979 and 1990. Around 1990 the death rate at sea in the western Atlantic more than doubled, and runs in Maine and Nova Scotia collapsed. NASCO, an international council based in Edinburgh, was set up in 1983 to protect the stocks.

Returns picked up after 2008. The Penobscot River in Maine saw about 940 fish in 2007 and 1,938 by mid-July 2008. In 2011 more than 3,100 returned, the most since 1986. Ontario's restoration programme, begun in 2006, has stocked over 6,000,000 young salmon in Lake Ontario and its rivers. In 2024 salmon spawned in the upper River Derwent in England for the first time in 100 years.

Relationship with humans

People have netted and trapped salmon for thousands of years, with wood and stone weirs in New England rivers. Medieval Scotland prized the fish so highly that a second conviction for poaching on royal land could bring death. King Robert I set a minimum mesh for fish traps in 1318 so that the young could pass. From the 15th century Aberdeen salted salmon in barrels and shipped them as far as the Baltic. In England the 1861 Salmon Fisheries Act followed a royal commission, and the 1975 act banned blocking the migration.

Fly anglers prize the salmon above almost every other river fish. The United States bans all fishing for wild sea-run salmon, and 11 rivers in Maine are on the endangered list. Quebec sets a limit per river, and some rivers allow only catch and release with 3 fish a day. Even catch and release adds stress to a fish already pressed by warming and predation.

Wild-caught salmon has almost vanished from shops, and nearly all fish on sale come from farms. Norway, Chile, Canada, the UK, Ireland, the Faroes, Russia and Tasmania raise them in sea cages. Fry grow in freshwater tanks for 12 to 20 months, then spend up to two years in cages at sea. Escapes are common: up to 300,000 fish broke out of a Puget Sound farm in 2017. Washington then voted to phase out salmon farming by 2025.

In culture and myth

In Irish tradition the Salmon of Knowledge ate nine hazelnuts that fell into the Well of Segais. The poet Finegas caught it, and the boy Fionn mac Cumhaill burnt his thumb while cooking it. Fionn sucked the thumb and gained all the salmon's wisdom. Two other figures of Irish legend, Fintan mac Bochra and Tuan mac Cairill, lived for ages in the shape of a salmon.

In the Welsh Mabinogion the Salmon of Llyn Llyw is the oldest of all animals, and it carries Arthur's men on its back. In Norse tradition Loki turned himself into a salmon to hide from the other gods. Across northern Europe the fish stands for wisdom, return and persistence. Its Latin name is often read as the leaper, and the leaping salmon appears on many Scottish and Irish crests.