The question of whether a Steelhead is a salmon or a trout is a common source of confusion stemming from its unique biology and migratory behavior. The definitive answer is that the Steelhead is a sea-run Rainbow Trout, meaning it is genetically a trout that adopts a salmon-like lifestyle. It belongs to the same species, Oncorhynchus mykiss, as the non-migratory Rainbow Trout, but the name “Steelhead” is used exclusively for individuals that migrate to the ocean.
The Taxonomy of Salmonids: Where Steelhead Fits
Steelhead and resident Rainbow Trout are grouped with Pacific Salmon within the biological family Salmonidae, which encompasses all salmon, trout, and chars.
The key to understanding the Steelhead’s classification lies in its scientific genus, Oncorhynchus. This genus includes all Pacific Salmon species, such as Chinook (O. tshawytscha) and Coho (O. kisutch), as well as O. mykiss, the scientific name for both the Rainbow Trout and the Steelhead. This classification places the Steelhead firmly within the group commonly referred to as Pacific Salmon, even though it is biologically a trout.
The shared genus explains the strong biological relationship and physical similarities between Steelhead and true Pacific Salmon. However, unlike most Pacific Salmon species, which are semelparous (spawning once and dying), Steelhead are iteroparous, meaning they can spawn multiple times over their lifespan. This ability to survive spawning and return to the ocean is a characteristic shared with Atlantic Salmon and many trout species, highlighting its trout identity within a salmon genus.
The Defining Lifestyle: Anadromous vs. Resident
The difference between a Rainbow Trout and a Steelhead is purely an ecological distinction based on a migratory behavior known as anadromy. Anadromous fish, like the Steelhead, hatch in freshwater, migrate to the ocean to feed and grow, and then return to freshwater to spawn. The resident Rainbow Trout, by contrast, completes its entire life cycle in freshwater, never venturing into the saltwater environment.
Juvenile Oncorhynchus mykiss preparing for sea migration undergo a physiological transformation called smoltification, which allows them to survive in saltwater. This process involves changes in their gills, kidneys, and metabolism, adapting them from a freshwater existence to a marine one. The decision to migrate is influenced by genetics, individual condition, and environmental factors like water temperature and food availability.
The two different life histories are not reproductively isolated, meaning that resident Rainbow Trout and anadromous Steelhead can and do interbreed. A female Steelhead can produce offspring that become resident Rainbow Trout, and a resident female can produce offspring that migrate to the ocean and return as Steelhead.
Physical Differences Between Steelhead and Resident Trout
The Steelhead’s time in the ocean results in dramatic changes to its physical appearance, causing it to resemble a salmon more than a trout. The most immediate difference is coloration: while resident Rainbow Trout have colorful, multi-hued bodies, prominent black spots, and a distinctive pink or red horizontal stripe, the Steelhead loses much of this vibrant pigmentation.
Upon returning from the ocean, the Steelhead is predominantly bright silver or chrome-colored, which gives the fish its name “steel head.” This silvery sheen is a form of countershading that provides camouflage in the open ocean environment. As the Steelhead spends more time back in freshwater, this silvery color gradually fades, and the trout’s classic colors may begin to reappear.
The ocean environment also influences the fish’s body shape and size. Steelhead are generally much larger than their resident counterparts, often reaching lengths of 18 to 24 inches and sometimes exceeding 45 inches, with weights potentially over 50 pounds. This significant growth is directly attributed to the abundance of high-protein food sources available in the open ocean. Furthermore, the Steelhead develops a more streamlined, torpedo-like body shape, an adaptation for efficient, long-distance swimming.