Trout are a diverse group of freshwater fish belonging to the family Salmonidae, which also includes salmon and char. All trout species share the characteristic presence of an adipose fin. While the term “trout” is used broadly, scientific classification divides these fish into three distinct genera: Oncorhynchus, Salmo, and Salvelinus. These groups are differentiated by their native geographic ranges, physical appearance, and subtle skeletal features.
The Pacific Trout
The Pacific Trout belong to the genus Oncorhynchus. This group is native to the cold-water tributaries of the North Pacific basin, ranging from Mexico north to the Arctic and west across to Asia. Oncorhynchus is the most species-rich group of trout and includes several species of Pacific salmon, making it highly significant for both ecology and sport fishing.
The most recognized member is the Rainbow Trout (O. mykiss), a species native to the Pacific drainages of North America but now globally introduced. Resident Rainbow Trout are characterized by a broad, iridescent pink or red stripe along the lateral line and numerous small, dark spots scattered over a lighter body, including the fins and tail. Its anadromous form, known as Steelhead, spends several years maturing in the ocean before returning to freshwater to spawn. Unlike Pacific salmon, Steelhead are iteroparous, meaning they can survive to spawn multiple times.
Another significant member is the Cutthroat Trout (O. clarkii), named for the distinct red or orange slash marks found in the fold beneath the lower jaw. This species exhibits the most extensive native distribution of any Oncorhynchus trout in North America, with numerous subspecies adapted to isolated mountain lakes and high-altitude streams. Cutthroat Trout have a spotting pattern similar to Rainbow Trout, but the presence of the crimson cheek marks provides an easy field identifier.
The Golden Trout (O. aguabonita) is a high-altitude member of this genus, known for its brilliant coloration. Native to the high-elevation streams of the southern Sierra Nevada in California, this fish features a deep golden-yellow body, a prominent red lateral stripe, and dark oval “parr marks” visible even in adulthood. Golden Trout primarily inhabit clear, cold water at elevations between 7,000 and 12,000 feet.
The Atlantic Trout
The Atlantic Trout are represented by the genus Salmo. The most prominent species in this genus is the Brown Trout (Salmo trutta). Native to Europe, North Africa, and Western Asia, its popularity as a sport fish led to its widespread introduction across the globe, including North and South America.
Brown Trout are known for tolerating slightly warmer water temperatures than other trout. They typically display an olive-brown to yellowish-brown body color, featuring large, dark spots frequently surrounded by a pale halo, and often including scattered red or orange spots. This pattern provides excellent camouflage in the rocky rivers and streams they inhabit.
Like the Pacific Trout, the Brown Trout exhibits both resident and migratory life histories. The resident form spends its entire life in freshwater. The anadromous form, known as Sea Trout, migrates to coastal waters to feed and grow before returning to spawn. Sea Trout develop a silvery coloration, with distinctive spots becoming less visible, resembling salmon.
The Chars Often Called Trout
The Char belong to the genus Salvelinus. While many species carry “trout” in their common name, they are distinct from the Oncorhynchus and Salmo genera. The most reliable external feature for distinguishing char is their distinctive spotting pattern: Char exhibit light spots (cream, pink, or red) on a dark body, which is the inverse of the pattern found on most true trout.
The most widespread example is the Brook Trout (S. fontinalis). Native to Eastern North America, they thrive in the coldest, cleanest headwater streams. Their olive-green back is covered in distinctive, wavy, worm-like markings called vermiculation, and their flanks feature pale spots with small, blue-haloed red spots. The lower fins are characterized by bright white leading edges, often bordered by a thin black line.
The Lake Trout (S. namaycush) is the largest member of the char genus, native to deep, cold-water lakes across North America. Unlike most other char, Lake Trout possess a deeply forked caudal fin and are adapted to dark, oligotrophic environments often found at great depths. These fish are slow-growing and long-lived, with some individuals reaching massive sizes, exceeding 50 pounds in weight.
Another char is the Bull Trout (S. confluentus), a species native to the Pacific Northwest. Bull Trout lack the red spots of Brook Trout and Arctic Char, instead displaying pale yellow or cream-colored spots on an olive-green to brown body. The classification of Salvelinus as char is confirmed by internal skeletal structures that clearly separate them from the true trout.