Head to head
Atlantic Salmon vs Rainbow Trout
Salmon and trout are both members of the family Salmonidae, and telling them apart is harder than it sounds. Traditionally, salmon are the big migratory fish that run from the sea up rivers to spawn, and trout are the ones that mostly stay in fresh water, but there are trout that go to sea and salmon that never leave a lake, and the genetics cut across the names. The Atlantic salmon and the rainbow trout are the two most important species commercially, and they sit on different branches of the family.
| Atlantic Salmon | Rainbow Trout | |
|---|---|---|
| Species | Salmo salar (Atlantic salmon) | Oncorhynchus mykiss (rainbow trout) |
| Genus | Salmo, with the brown trout | Oncorhynchus, the Pacific salmon genus |
| Natural range | Rivers and seas of the North Atlantic | Cold waters of western North America and NE Asia |
| Goes to sea? | Yes, then returns to its birth river | The steelhead form does; many stay in fresh water |
| Dies after spawning? | Not necessarily; can spawn again | No; can spawn several years running |
| Typical size | Often 3 to 10 kg returning adults | 0.5 to 5 kg; steelhead larger |
| Farmed | Enormously, in sea cages (Norway, Chile, Scotland) | Enormously, mostly in freshwater tanks and raceways |
| Wild status | Many wild river populations declining or endangered | Secure as a species; some native runs threatened |
Salmon vs Trout: common questions
What is the actual difference between salmon and trout?
There is no single clean rule, because the names are older than the science. As a rough guide, "salmon" refers to species that undertake long sea migrations and return to fresh water to spawn, and "trout" to species that spend most or all of their lives in fresh water, but exceptions are everywhere. The rainbow trout has a sea-going form; the Atlantic salmon has landlocked lake populations; the brown trout, a true trout, is in the same genus as the Atlantic salmon, while the rainbow and cutthroat trout are in the Pacific salmon genus. In practice the distinction is partly about lifestyle, partly about size and market category, and only loosely about ancestry.
Is a steelhead a salmon or a trout?
It is a trout, specifically a rainbow trout that has gone to sea. Steelhead and stream rainbow trout are the same species, Oncorhynchus mykiss; the difference is that a steelhead's young migrate to the ocean, feed and grow there for one to four years, and then return to fresh water to spawn, turning chrome-silver in the process and often growing several times larger than a fish that stayed in the river. Because that genus is the Pacific salmon genus, a rainbow trout or steelhead is genetically closer to a Chinook or sockeye salmon than an Atlantic salmon is. On menus, "steelhead trout" is usually farmed rainbow trout raised to a large size.
Why is farmed salmon pink, and is it dyed?
The flesh colour comes from a carotenoid pigment called astaxanthin, the same compound that makes shrimp and flamingos pink. Wild salmon and trout get it by eating krill, shrimp, and other crustaceans. Farmed fish are given it in their feed, because without it the flesh would be pale grey and consumers expect pink; the pigment used is chemically identical to the natural one, whether it is sourced from algae, yeast, or synthesis, so it is fortification rather than dye in the sense of a surface colour. Farmed salmon is also generally fattier than wild, which affects flavour and the amount of omega-3 and of contaminants it carries.