Trout are a diverse group of freshwater fish belonging to the family Salmonidae, which also includes salmon and char. They are widely distributed across the Northern Hemisphere, inhabiting cool, clear streams, rivers, and lakes. Determining the precise number of trout species is complicated because the answer depends entirely on how a scientist defines a “species.” Therefore, it is more accurate to explore the taxonomic groups that contain fish commonly referred to as trout rather than attempting to provide a single, fixed number.
Why Defining Species Is Complicated
The difficulty in tallying a definitive number of trout species stems from the fluid nature of biological classification. A species is generally a population that can interbreed and produce fertile offspring, but within trout, there are numerous distinct populations called subspecies or strains. For instance, the Brown Trout (Salmo trutta) is a single species, yet it has distinct morphs—river trout, lake trout, and the anadromous sea trout.
Genetic studies frequently reveal that what was once considered a single species is actually a complex of isolated units, or vice versa. This ongoing research continually shifts the lines between a recognized species and a mere subspecies. Furthermore, trout are highly prone to both natural and human-induced hybridization, which further blurs the taxonomic boundaries.
The interbreeding of closely related species can produce fertile hybrid offspring, such as the “cuttbow” trout, a cross between a Rainbow Trout and a Cutthroat Trout. This phenomenon threatens the genetic purity of native populations, which is a major concern for conservation biologists. The resulting genetic variability makes it challenging for scientists to agree on a final, authoritative list of distinct species.
The True Trout Genera
The fish universally recognized as “true trout” belong to two primary genera within the Salmonidae family: Salmo and Oncorhynchus.
The genus Salmo is the Old World group, with its species originally native to the Atlantic basin, Europe, North Africa, and Western Asia. The most famous member is the Brown Trout (Salmo trutta), which has been introduced globally. Salmo species are characterized by relatively large scales compared to their char relatives and typically feature dark spots on a lighter body color.
The second major group is the genus Oncorhynchus, known as the Pacific trouts, native to the Pacific basin of North America and Asia. This genus includes the Rainbow Trout (Oncorhynchus mykiss), whose sea-run form is known as the Steelhead. The Cutthroat Trout (Oncorhynchus clarkii) is another prominent species, notable for the distinctive reddish slash under its lower jaw. Like Salmo, these Pacific trouts are characterized by dark spots covering their bodies.
Fish Often Mistaken for Trout
A large part of the popular confusion surrounding the number of trout species comes from the common names given to fish of the genus Salvelinus. These fish are scientifically classified as char, not true trout, despite names like Brook Trout, Lake Trout, and Bull Trout. Char belong to the same family, Salmonidae, but they are genetically distinct from the Salmo and Oncorhynchus genera.
The most reliable way to physically distinguish a char from a true trout is by observing its spotting pattern. Char, such as the Brook Trout (Salvelinus fontinalis) and Arctic Char (Salvelinus alpinus), are characterized by light-colored spots, often cream, pink, or red, on a darker background. In contrast, true trout have dark spots on a lighter background.
Char also tend to have much smaller scales than true trout, which are nearly imperceptible to the touch. The genus Salvelinus is highly diverse, with over 50 recognized species and numerous isolated populations. The widespread use of the word “trout” for these char species adds a substantial, though technically incorrect, number to any casual count of trout species.