The Rocky Mountain cutthroat trout (Oncorhynchus virginalis), formerly lumped in with the cutthroat trout (Oncorhynchus clarkii) as one species with multiple subspecies, is a fish species of the family Salmonidae native to cold-water tributaries of the northern and southern Rocky Mountains, as well as into portions of the Great Basin in North America. As a member of the genus Oncorhynchus, it is a part of the Pacific trout group, which includes the widely distributed rainbow trout. Cutthroat trout are popular gamefish, especially among anglers who enjoy fly fishing. The common name "cutthroat" refers to the distinctive red coloration on the underside of the lower jaw.
Taxonomy Historically, cutthroat trout was considered one species (Oncorhynchus clarkii). However, recent genetic, taxonomic, and geologic evidence has determined that cutthroat trout should be divided into four species, with each (except for the coastal cutthroat) having multiple subspecies corresponding to the evolutionary lineages found within major river basins. In 2023, the American Fisheries Society formerly reclassified all Cutthroat Trout from one species into four distinct species: Coastal, Lahontan, Westslope, and Rocky Mountain Cutthroat Trout.
Subspecies and uniquely identifiable evolutionary units During the annual meeting of the American Fisheries Society (AFS) in 2015, the Western Division of AFS (WDAFS) organized a special workshop with a panel of experts to evaluate the validity of the currently recognized subspecies, considering the available evidence both supporting and challenging the existing classification system. The panel found the current classification scientifically indefensible, and proposed a revised phylogeny and classification that aligns with the comprehensive evidence. In that discussion, the panelists were unable to resolve how to discuss specific subsets of the population. One side argued that subspecies are valuable taxonomic ranks, because they represent important evolutionary and ecological information that should be recognized to preserve biodiversity. The other side argued that subspecies do not always align with true evolutionary entities, and are arbitrary designations of geographic lineages within a species. To resolve the impasse, the panel coined the term "uniquely identifiable evolutionary unit" (UIEU) to describe subunits that have diverged from species. These UIEUs are distinct population groups that show evolutionary independence but do not meet all the criteria to be considered full species. The creation of this term allowed the panel to acknowledge and discuss these units without getting caught up in the subspecies debate.
Evolutionary lineages
The upper Snake River/Yellowstone evolutionary lineage The upper Snake River/Yellowstone evolutionary lineage, exhibiting nine uniquely identifiable evolutionary units (UIEUs), shows complex patterns of divergence and dispersal. Genetic evidence suggests that the lineage initially diverged from the Lahontan Basin lineage around 9 million years ago, potentially facilitated by drainage connections between the two basins. However, gene exchange between these lineages persisted until approximately 3.07 million years ago. Among the nine recognized UIEUs within this lineage are an upper Snake/Yellowstone UIEU, a Bear River UIEU, and a Bonneville Basin UIEU. Notably, the Fine-spotted Snake River Cutthroat Trout, previously designated as O. c. behnkei, lacks support as a distinct UIEU in the current molecular evidence, despite its ecological and morphological differences. Studies indicate a closer ancestral relationship between Bear River cutthroat trout and Yellowstone Cutthroat Trout than with Bonneville cutthroat trout, suggesting a complex branching within the upper Snake River/Yellowstone lineage. Additionally, mitochondrial DNA sequence data point to a genetic affinity between the Bonneville UIEU and Cutthroat Trout in the Colorado River and nearby watersheds, indicating dispersal routes beyond those traditionally proposed.
Southern Rocky Mountain lineage The Southern Rocky Mountain region, previously thought to harbor four cutthroat trout subspecies, has been revealed to contain six distinct UIEUs. This includes three distinct clades within the Colorado River cutthroat trout (O. v. pleuriticus), referred to as the blue and green lineages in the scientific literature, as well as the San Juan cutthroat trout (O. v. ssp.), the Greenback cutthroat trout (O. v. stomias), the extinct Yellowfin cutthroat trout (O. v. macdonaldi), and the Rio Grande cutthroat trout (O. v. virginalis). These findings, along with the misidentification of the original specimen used to describe the Greenback cutthroat trout, demonstrate the changing nature of the understanding of cutthroat trout diversity and the importance of continuing research.
Uniquely identifiable evolutionary units of Rocky Mountain cutthroat trout
Description Like all other cutthroat, Rocky Mountain cutthroat trout display a high degree of variability in terms of size, color, and habitat selection. These trout exhibit a range of colors from yellowish-brown, silvery, brassy, to brilliant orange-red and golden. They may have bright golden-orange, red, or rose tints. Spots vary from medium-large and rounded to small and pepper-like, and can be concentrated on the caudal peduncle or evenly distributed over the sides. They typically have between 60 and 63 vertebrae and a number of lateral scales ranging from 150 to 200. Adult weight ranges can vary between approximately 10 grams to over 7 kilograms. The largest known Rocky Mountain cutthroat was the Yellowfin. Due to historical introductions and hybridization with other subspecies, it can be difficult to unambiguously separate some Rocky Mountain cutthroat trout from other closely related subspecies based on appearance alone. All have distinctive red, pink, or orange linear marks along the underside of their mandibles in the lower folds of the gill plates. The common name "cutthroat" comes from these marks, coined in an 1884 article in The American Angler by outdoor writer Charles Hallock.
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