The relationship between steelhead and salmon can often be a source of confusion for the general public. While both are iconic fish species inhabiting Pacific waters and freshwater streams, their biological classifications and life histories contain distinctions that are not always clear. Understanding these differences helps clarify why they are often grouped together yet remain separate.
The Scientific Classification
Steelhead are the anadromous, or sea-run, form of rainbow trout. This means they are born in freshwater, migrate to the ocean to grow, and then return to freshwater to spawn. Their scientific name is Oncorhynchus mykiss.
Both steelhead/rainbow trout and Pacific salmon species, such as Chinook (Oncorhynchus tshawytscha), Coho (Oncorhynchus kisutch), and Sockeye (Oncorhynchus nerka), belong to the same genus, Oncorhynchus. They are distinct species within this genus. This shared genus reflects a close evolutionary relationship, but steelhead are biologically classified as trout, not as true Pacific salmon species.
Distinguishing Features and Habits
Physical characteristics differentiate steelhead from Pacific salmon. Steelhead have a streamlined, torpedo-like body shape. They often have an iridescent pink or reddish stripe along their lateral line, particularly vivid in breeding males, and fine black spots distributed across their body and tail.
Pacific salmon species, like Chinook and Coho, generally have a thicker body and may develop distinct features during spawning, such as a pronounced hump on their back or a hooked jaw. Spotting patterns also differ; for instance, Chinook salmon have irregular black spots on their back and both lobes of their tail, while Coho have smaller spots primarily on their back and the upper lobe of their tail.
Steelhead prefer cold, clear water with a strong current in their freshwater habitats. They also rely on deep pools and shade to keep water cool. Adult steelhead typically do not feed once they enter freshwater for spawning, but juveniles feed on zooplankton, insects, crustaceans, and small fish. Pacific salmon also inhabit freshwater for spawning and early life stages, but their habitat and feeding preferences vary by species and life stage.
Life Cycles Compared
The primary biological difference between steelhead and Pacific salmon lies in their reproductive strategies. Steelhead exhibit an iteroparous life cycle, meaning they can survive spawning and return to the ocean multiple times to reproduce in subsequent years. While their survival rate after spawning can be low, some individuals do return to spawn more than once. This ability to undertake multiple spawning migrations is a defining characteristic.
In contrast, most Pacific salmon species are semelparous, meaning they undergo a single reproductive event before dying. After their journey from the ocean to their freshwater spawning grounds and reproduction, these salmon expend all their energy and do not survive to spawn again. This difference in their life history, specifically the capacity for repeat spawning in steelhead, distinguishes them from Pacific salmon.