Rainbow trout are not the same species as salmon, but they are extremely close relatives belonging to the same biological family. Rainbow trout are scientifically classified as a species of trout, yet they share a more recent common ancestor with Pacific salmon than with some other trout types. Similarities in their life cycles, appearance, and market presentation often cause public confusion. Ultimately, rainbow trout are not technically salmon, but they are members of the same diverse fish family.
The Definitive Biological Relationship
Rainbow trout and true salmon are united within the family Salmonidae, which encompasses salmon, trout, char, and whitefish. This family grouping signifies a shared evolutionary history and similar anatomical features, such as the presence of an adipose fin. Scientists separate the fish into different genera, the next level of classification below family.
Pacific salmon (including Chinook, Sockeye, and Coho) are grouped within the genus Oncorhynchus. Rainbow trout (Oncorhynchus mykiss) are also classified here, highlighting their strong genetic link to Pacific salmon. Conversely, Atlantic salmon belong to the genus Salmo, the same genus as the brown trout (Salmo trutta).
Modern classification places Pacific trout, such as the rainbow trout, much closer to Pacific salmon than to Atlantic-based species. This taxonomic grouping reflects genetic and morphological evidence. A rainbow trout is scientifically a trout species within the same genus as Pacific salmon, but distinct from the true salmon species. The term “salmon” generally refers to species that exhibit specific migratory behaviors, regardless of their genus.
Key Differences in Habitat and Life Cycle
The primary distinction between resident rainbow trout and true salmon lies in their migratory patterns. Most true salmon exhibit an anadromous life cycle: they are born in freshwater, migrate to the ocean to mature, and return to their natal stream to reproduce. Nearly all Pacific salmon species die after this single spawning event, making them semelparous.
The typical rainbow trout is a resident fish that completes its entire life cycle exclusively in freshwater. These resident forms usually reach a mature size of between one and five pounds. However, Oncorhynchus mykiss also includes an anadromous form known as the Steelhead. Steelhead follow the salmon pattern, migrating to the ocean and returning to freshwater to spawn, which causes significant confusion.
A key biological difference between Steelhead and Pacific salmon is that Steelhead are iteroparous, meaning they can survive spawning and potentially return to the ocean before spawning again in a later year. This ability to spawn multiple times is rare among Pacific salmon species. The ocean environment allows Steelhead to grow much larger than their resident counterparts, sometimes reaching weights over 20 pounds. Resident trout are generally smaller with a pronounced red stripe, while ocean-dwelling Steelhead develop a silvery, streamlined appearance.
Why Rainbow Trout Are Often Confused with Salmon
The confusion between rainbow trout and salmon stems from factors in the marketplace and common terminology. One significant reason is the visual similarity of their prepared flesh. Wild salmon naturally acquire their characteristic pink or red-orange color from eating a diet rich in crustaceans and krill, which contain the pigment astaxanthin.
Farmed rainbow trout, like farmed salmon, are often given feed containing astaxanthin to achieve the desirable flesh color. Without this supplement, the flesh of farmed trout would be pale white or grey, which is less appealing to consumers. This dietary manipulation makes the fillets of farm-raised trout and salmon visually indistinguishable to the average buyer.
The Steelhead is sometimes marketed using the term “Steelhead salmon” or “ocean trout.” This practice links the two fish directly in the consumer’s mind, especially given the Steelhead’s large size and rich, pink flesh. The similar taste profile and preparation methods for both fish further solidify the common, though inaccurate, belief that rainbow trout are simply a smaller variety of salmon.