Fish

Salmon

Oncorhynchus and Salmo · also called salmonid

Not Evaluated ↓ carnivore Ocean Coasts & shorelines Rivers & wetlands

A salmon spends its life crossing a boundary most fish never cross. It hatches in river gravel, rebuilds its body to live in salt water, grows huge in the ocean, and then rebuilds itself again to fight back up the same river it came from — often the exact tributary — to spawn. For most Pacific salmon, that arrival is also the end.

Overview

Salmon are the migratory members of the family Salmonidae, in the genera Oncorhynchus (the Pacific salmon and the rainbow trout) and Salmo (the Atlantic salmon and the brown trout). They live in the cool Northern Hemisphere, are anadromous — born in fresh water, grown at sea — and range from the 1 to 2 kg pink salmon to the Chinook, which can exceed 30 kg.

Two bodies, one fish

A young salmon, or smolt, remodels its gills, kidneys, and gut to switch from fresh water to salt, then reverses the change to come back. The homing journey uses the Earth’s magnetic field across the open ocean and then the learned smell of the natal stream to pick the right river and fork. Straying rates are often just a few per cent.

The spawning run

Approaching fresh water, males grow a hooked kype, big teeth, a humped back, and strong colour. The fish stops feeding and burns its own body for fuel. Pacific salmon die within days of spawning, their immune systems collapsing; Atlantic salmon and steelhead hold back enough to return another year.

Salmon as a keystone

Spawned-out carcasses pour marine nitrogen and phosphorus into rivers and, via bears, eagles, and floods, onto the land, fertilising streamside forests measurably. Salmon have fed people around both northern oceans for millennia, and farmed Atlantic salmon is now a major aquaculture product, though sea-cage farming raises concerns over sea lice, escapes, and pollution.

Status

Salmon as a group are Not Evaluated, and the overall trend is decreasing. Many Alaskan and Russian runs remain strong, but numerous Atlantic salmon populations are depleted or gone, and several Pacific Northwest Chinook and sockeye runs are listed as threatened or endangered.

Salmon: common questions

How do salmon find their way back to their home river?

In two stages. Out in the ocean, where there are no landmarks, salmon appear to navigate using the Earth's magnetic field, sensing both its direction and its strength to work out roughly where they are and which way the coast lies. Once they reach the coast and the river mouth, they switch to smell. As young fish leaving fresh water, salmon imprint on the specific blend of chemicals, from soil, plants, and other fish, that makes their home stream smell unique, and they follow that scent trail upriver, choosing the correct fork at each junction. The system is accurate enough that most fish return not just to the right river but to the right tributary.

Why do salmon die after spawning?

For Pacific salmon, spawning is a one-way trip built into their biology. The upstream migration is fuelled entirely from body reserves, since the fish stops eating, and at the same time a flood of stress and reproductive hormones drives the body to break down its own muscle and organs to supply energy and eggs or milt. The immune system collapses, fungus spreads over the skin, and the fish dies within days of laying or fertilising the eggs. This seems to be an evolved strategy, pouring everything into a single huge reproductive effort and then leaving a nutrient-rich carcass that helps feed the next generation. Atlantic salmon hold back more reserves and can survive to spawn again.

What is the difference between salmon and trout?

Less than the names suggest, since they are close relatives in the same family and several species blur the line. In general, salmon are the species that make long migrations to the sea and grow large, while trout are more often freshwater residents that stay smaller, but this is not a firm rule. The rainbow trout and the sea-run form of the same species, the steelhead, are one animal with two lifestyles, and brown trout also have sea-run populations. Genetically, Pacific salmon and rainbow trout are all in the genus Oncorhynchus and are more closely related to each other than the Pacific salmon are to the Atlantic salmon, which sits with the brown trout in the genus Salmo.

Is farmed salmon bad for wild salmon?

It carries real risks that the industry and regulators are still working on. Open sea cages concentrate parasites, especially sea lice, which can spread to wild smolts passing nearby and kill them. Farmed fish that escape can interbreed with wild populations and weaken their fitness, or compete for spawning space. Waste feed and faeces enrich the seabed under farms, and disease can pass both ways. Against this, farming takes pressure off wild stocks by supplying most of the salmon people eat, and land-based and offshore systems are being developed to reduce the problems. The picture depends heavily on how and where a given farm is run.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published November 6, 2026. Last updated November 6, 2026.

Quick facts

Scientific name: Oncorhynchus and Salmo
Diet: Plankton and insect larvae in the river as fry, then a rich marine diet of small fish, squid, krill, and amphipods at sea, and in most species nothing at all during the final spawning run, when the fish lives off its own body reserves
A life split between fresh water and salt: Salmon are anadromous, hatching in gravel-bottomed rivers, spending months to years in fresh water, then physically remodelling their gills, kidneys, and gut to move to the sea, where the real growth happens, before switching back again to breed
They find the stream they were born in: A returning salmon navigates the open ocean partly by the Earth's magnetic field, then locks onto the unique chemical smell of its home river, learned as a young fish, and can pick out its natal tributary from among many, with straying rates often only a few per cent
For Pacific salmon, breeding is fatal: All five North American Pacific salmon species die within days to weeks of spawning, their bodies breaking down even as they defend the nest, whereas Atlantic salmon and steelhead can survive and return to spawn a second or even third time
The body transforms for the river: On the spawning run males in particular develop a hooked jaw called a kype, enlarged teeth, a humped back, and vivid colours, changes driven by a surge of hormones that also shut down feeding and immune defence
Dying fish that feed a forest: The carcasses of spawned-out salmon deliver large amounts of marine nitrogen and phosphorus into rivers and onto land, carried by bears, eagles, and floods, and this ocean-derived fertiliser can be traced in the wood of streamside trees
Prized enough to be farmed in millions of tonnes: Wild salmon have been a staple food for thousands of years around the North Pacific and North Atlantic, and farmed Atlantic salmon is now one of the largest aquaculture products in the world, though sea-cage farming raises concerns about sea lice, escapes, and pollution
Range: The cooler waters of the Northern Hemisphere, with Atlantic salmon in the North Atlantic and the rivers of eastern North America and western Europe, and Pacific salmon around the rim of the North Pacific from California and Japan north to Alaska and the Russian Far East; salmon have also been introduced to the Great Lakes, Patagonia, and New Zealand.
Conservation status: Not Evaluated ↓ ≈ Not assessed as a group, and status varies widely by species and river, with many Alaskan and Russian Pacific salmon runs still large and well managed, several Atlantic salmon populations in Europe and eastern North America seriously depleted or lost, and a number of Pacific Northwest runs, including several Chinook and sockeye populations, listed as threatened or endangered under United States law in the wild