Fish
Atlantic Salmon
Salmo salar · also called the leaper, Atlantic salmon
The Atlantic salmon lives one of the most demanding lives of any fish. It is born in a gravel-bedded river, transforms its body to survive salt water, crosses an ocean to feeding grounds off Greenland, and then finds its way back — often to the very pool where it hatched — to spawn and, in most cases, die.
Overview
Atlantic salmon are anadromous: they breed in fresh water and grow to maturity in the sea. Their spawning rivers ring the North Atlantic, from Maine and the Canadian Maritimes across to Iceland, Ireland, Britain, Norway, and Arctic Russia, while the adults share common feeding areas in the cold, plankton-rich waters near Greenland and the Faroe Islands.
A life in stages
The salmon changes form repeatedly:
- Alevin and fry — newly hatched fish in the spawning gravel, then tiny free-swimming fish.
- Parr — a camouflaged juvenile that spends one to several years in the river eating insects.
- Smolt — at a set size the parr’s body re-tunes for salt water: it turns silver, its gills and kidneys adapt, and it migrates downstream to the sea.
- Adult — one to four years at sea, feeding and growing fast.
- Return and spawn — the mature fish stops feeding, re-enters fresh water, and battles upstream, leaping waterfalls, to the spawning grounds, where the female cuts a nest (“redd”) in the gravel and the eggs are fertilised.
- Kelt — unusually among salmon, some survive spawning, return to the sea, and come back to breed again.
Navigation
Getting home requires two navigation systems. At sea, salmon use a geomagnetic map, sensing the Earth’s field to work out where they are and which way the coast lies. In fresh water they switch to olfactory memory, following the distinctive scent of their birth stream — imprinted when they were parr — through every tributary junction to the exact reach where they were spawned.
Threats and conservation
Although the species as a whole is currently rated Least Concern, wild Atlantic salmon have declined dramatically — roughly 70% since 1980 — and runs in many rivers of southern Europe and the northeastern US have collapsed. The causes stack up: barriers to migration (dams, weirs), degraded rivers (pollution, siltation, warming), a sharp fall in survival at sea, and the impacts of salmon farming (sea lice, disease, and genetic contamination from escapees). Recovery efforts focus on removing dams, restoring spawning habitat, sharply limiting netting, reforming aquaculture, and international management of the shared ocean feeding grounds.
Atlantic Salmon: common questions
How does a salmon find its way back to the river where it hatched?
In two stages. Out in the open ocean, hundreds or thousands of kilometres from any coast, salmon use a magnetic sense — reading the intensity and angle of the Earth's magnetic field like a map — to steer toward the right stretch of coastline. Once they reach fresh water they switch to smell — each stream has a unique chemical signature from its rocks, soil, and vegetation, and the salmon imprinted on that smell as a young fish, following it upstream past every fork until it reaches its home reach. A small percentage "stray" to other rivers, which is how salmon colonise new habitat.
What is the difference between Atlantic and Pacific salmon?
Atlantic salmon are a single species, Salmo salar, more closely related to trout than to the five or so Pacific salmon species (genus Oncorhynchus). The biggest behavioural difference is that Pacific salmon always die after spawning once, whereas some Atlantic salmon survive, drop back to the sea as "kelts," recover, and return to spawn a second or even third time. Nearly all "Atlantic salmon" sold today is farmed.
Why are wild Atlantic salmon declining?
Many pressures acting together. Dams and weirs block or delay migration; rivers have been polluted, warmed, and silted; forestry and farming degrade spawning gravels. At sea, marine survival has dropped sharply for reasons still being worked out, linked to warming water and shifting food webs. Salmon farming adds sea-lice infestations and disease to wild smolts passing farms, and escaped farmed fish interbreed with and weaken wild stocks. Overfishing, historically severe, is now tightly restricted.
Do salmon really stop eating when they enter the river?
Largely, yes. Once an adult salmon leaves the sea to spawn it feeds little or not at all, living off fat and muscle reserves built up during the ocean feeding years. Everything is directed toward reaching the spawning grounds and reproducing. This is why river anglers use lures and flies that provoke an aggressive or reflexive strike rather than genuine hunger.