The Rocky Mountain cutthroat trout , formerly lumped in with the cutthroat trout (Oncorhynchus clarkii)Behnke, R. J. 1979. Monograph of the native trouts of the genus Salmo of western North America. U.S. Forest Service, Lakewood, Colorado.Behnke, R. J. 1988. Phylogeny and classification of Cutthroat Trout. Pages 1–7 in R. E. Gresswell, editor. Status and management of interior stocks of Cutthroat Trout. American Fisheries Society, Symposium 4, Bethesda, Maryland.Behnke, R.J. 1992. Native trout of western North America. American Fisheries Society, Monograph 6, Bethesda, Maryland.Behnke, R. J. 2002. Trout and salmon of North America. The Free Press, New York. 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.Page, Lawrence M.; Bemis, Katherine E.; Espinosa-Pérez, Héctor S.; Findley, Lloyd T.; Gilbert, Carter R.; Hartel, Karsten E.; Lea, Robert N.; Mandrak, Nicholas E.; Neighbors, Margaret A. (2023). Common and scientific names of fishes from the United States, Canada, and Mexico. Special publication (Eighth ed.). Bethesda: American Fisheries Society. .
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.Wallace, A. R. 2003. Regional geologic setting of late Cenozoic lacustrine diatomite deposits, Great Basin and surrounding region: overview and plans for investigation. U.S. Geological Survey, Bulletin 2209-B, Reston, Virginia.Wallace, A. R., M. E. Perkins, and R. J. Fleck. 2008. Late Cenozoic paleogeographic evolution of northeastern Nevada: evidence from sedimentary basins. Geosphere 4:36–74 However, gene exchange between these lineages persisted until approximately 3.07 million years ago.Shiozawa, D. K., R. P. Evans, D. D. Houston, and P. J. Unmack. 2018. Geographic variation, isolation, and evolution of Cutthroat Trout with comments on future directions for management and research. Pages 129–172 in P. Trotter, P. Bisson, L. Schultz, and B. Roper, editors. Cutthroat Trout: evolutionary biology and taxonomy. American Fisheries Society, Special Publication 36, Bethesda, Maryland.Arnold, M. L. 2007. Evolution through genetic exchange. Oxford University Press, New York
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.
StudiesToline, C., T. Seamons, and J. Hudson. 1999. Mitochondrial DNA analysis of selected populations of Bonneville, Colorado River, and Yellowstone Cutthroat Trout. Final report of Utah State University, Department of Fisheries and Wildlife to Utah Division of Wildlife Resources, Salt Lake City.Smith, G. R., T. E. Dowling, K. W. Gobalet, T. Lugaski, D. K. Shiozawa, and R. P. Evans. 2002. Biogeography and timing of evolutionary events among Great Basin fishes. Pages 175–234 in R. Hershler, D. B. Madsen, and D. R. Curry, editors. Great Basin aquatic systems history. Smithsonian Institution Press, Smithsonian Contributions to the Earth Sciences 33, Washington, D.C. 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,Loxterman, J. L., and E. R. Keeley. 2012. Watershed boundaries and geographic isolation: patterns of diversification in Cutthroat Trout from western North America. BMC Evolutionary Biology [online serial] 12:38. 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.Metcalf, J. L., V. L. Pritchard, S. M. Silvestri, J. B. Jenkins, J. S. Wood, D. E. Cowley, R. P. Evans, D. K. Shiozawa, and A. P. Martin. 2012. Historical stocking data and 19th century DNA reveal human-induced changes to native diversity and distribution of Cutthroat Trout. Molecular Ecology 21:5194–5207Rogers, K. B. 2012. Piecing together the past: using DNA to resolve the heritage of our state fish. Colorado Outdoors 2012 September/October:28–32Bestgen, K. R., K. B. Rogers, and R. Granger. 2013. Phenotype predicts genotype for lineages of native Cutthroat Trout in the Southern Rocky Mountains. Colorado State University, Larval Fish Laboratory, Fort Collins. Final report to U.S. Fish and Wildlife Service, Colorado Field Office, DenverRogers, K. B., K. R. Bestgen, and J. Epp. 2014. Using genetic diversity to inform conservation efforts for native Cutthroat Trout of the Southern Rocky Mountains. Pages 218–228 in R. F. Carline and C. LoSapio, editors. 2014. Wild Trout XI: looking back and moving forward. Wild Trout Symposium, West Yellowstone, Montana. Available: www.wildtroutsymposium. com/wildTroutXI.php. (January 2018). 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