Lahontan cutthroat trout, Oncorhynchus henshawi,Markle, D. (2018). An interim classification of the cutthroat trout complex, Oncorhynchus clarkii Sensu Lato, with comments on nomenclature. In Trotter P., Bisson P., Schultz L., & Roper B. (Eds.), Cutthroat trout: Evolutionary biology and taxonomy (special publication 36, pp. 181–197). American Fisheries Society. (formerly, O. clarkii henshawi) formerly all grouped together as the cutthroat trout under a single species Oncorhynchus clarkii with many subspecies, is a fish species of the family Salmonidae native to cold-water tributaries of the Basin and Range province of Nevada, as well as adjoining areas of southeast Oregon and northeastern California.Trotter, Patrick; Bisson, Peter; Roper, Brett; Schultz, Luke; Ferraris, Carl; Smith, Gerald R.; Stearley, Ralph F. (2018), Trotter, Patrick; Bisson, Peter; Shultz, Luke; Roper, Brett (eds.), "A Special Workshop on the Taxonomy and Evolutionary Biology of Cutthroat Trout", Cutthroat Trout: Evolutionary Biology and Taxonomy, American Fisheries Society, doi:10.47886/9781934874509.ch1, , retrieved 2024-08-12Trotter, Patrick; Bisson, Peter; Roper, Brett; Schultz, Luke; Ferraris, Carl; Smith, Gerald R.; Stearley, Ralph F. (2018), Trotter, Patrick; Bisson, Peter; Shultz, Luke; Roper, Brett (eds.), "A Special Workshop on the Taxonomy and Evolutionary Biology of Cutthroat Trout", Cutthroat Trout: Evolutionary Biology and Taxonomy, American Fisheries Society, doi:10.47886/9781934874509.ch1, , retrieved 2024-08-12Behnke, 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. 2002. Trout and salmon of North America. The Free Press, New York.Peacock, M.M., H.M. Neville and A.J. Finger. 2018. The Lahontan basin evolutionary lineage of cutthroat trout. Pages 231–259 in P. Trotter, P. Bisson, L. Schultz and B. Roper editors. Cutthroat trout: evolutionary biology and taxonomy. American Fisheries Society Special Publication 36. American Fisheries Society. Bethesda, Maryland.Love Stowell; Metcalf; Markle; Stearly (2018), Trotter; Bisson; Shultz; Roper (eds.), "Species Conceptualization and Delimitation: A Framework for the Taxonomic Revision of Cutthroat Trout", Cutthroat Trout: Evolutionary Biology and Taxonomy, American Fisheries Society, doi:10.47886/9781934874509.ch2, , retrieved 2024-08-13 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 fly fishing anglers. The common name "cutthroat" refers to the iconic red slash on the underside of the lower jaw. This species includes the type subspecies, the giant Lahontan cutthroat trout found in Pyramid Lake.

Taxonomy Historically, cutthroat trout were considered a single species (Oncorhynchus clarkii). However, a review of genetic, taxonomic, and geologic evidence lead biologists to the conclusion that cutthroat trout should be divided into four species, with each (except for the coastal cutthroat) having multiple unique populations corresponding to the evolutionary lineages found within major river basins. In 2023, the American Fisheries Society formally reclassified all cutthroat trout into the following four distinct species: Coastal, Lahontan, Westslope, and Rocky Mountain cutthroat trout.

Subspecies and uniquely identifiable evolutionary units During the 2015 meeting of the American Fisheries Society (AFS), 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 long-held classification"Cutthroat trout Oncorhynchus clarki". Trout and Salmon of North America. The Free Press. pp. 138–139. .Behnke, Robert J.; Williams, Ted (2007). About Trout: The Best of Robert J. Behnke from Trout Magazine. Globe Pequot. p. 12. ISBN 978-1-59921-203-6.Smith, Gerald R.; Stearley, Ralph F. (1989). "The Classification and Scientific Names of Rainbow and Cutthroat Trouts" (PDF). Fisheries. 14 (1). American Fisheries Society: 4–10. Bibcode:1989Fish...14a...4S. doi:10.1577/1548-8446(1989)014<0004:TCASNO>2.0.CO;2. hdl:2027.42/140998. scientifically unsupportable, and proposed a revised classification and phylogenic tree that aligns with the preponderance of genetic, geologic, and fossil evidence.

To break the deadlock, the panel introduced the term "uniquely identifiable evolutionary unit" (UIEU) to describe subunits that have diverged from a species. These UIEUs represent distinct population groups that exhibit evolutionary independence but do not fully meet the criteria for classification as separate species. By adopting this terminology, the panel was able to recognize and discuss these units without becoming entangled in the debate over subspecies.

Origin Cutthroat trout have a long history in the Lahontan Basin, supported by fossil records dating back at least 10 million years.Smith, G. R., and Ralph F. Stearly. 2018. The Fossil Record of Cutthroat Trout: Implications for Evolution and Conservation. Pages 77–101 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.Stearley, R. F., and G. R. Smith. 2016. Fishes of the Mio-Pliocene western Snake River Plain and Vicinity, volume: salmonid fishes from Mio-Pliocene lake sediments in the western Snake River Plain and the Great Basin. Miscellaneous Publications Museum of Zoology University of Michigan 204.Reheis, M. C., A. M. Sarna-Wojcicki, R. L. Reynolds, C. A. Repenning, and M. D. Mifflin. 2002. Pliocene to Middle Pleistocene lakes in the western Great Basin: ages and connections. Pages 53–108 in R. Hershler, D. B. Madsen, and D. R. Currey, editors. Great Basin aquatic systems history. Smithsonian Institution, Washington, D.C. Fossil evidence from the Nevadaplano suggests cutthroat trout originated in high-elevation areas more than 10 million years ago, predating the origins of Rainbow and Redband trout.Hadly, E. A. 1996. Influence of late-Holocene climate on northern Rocky Mountain mammals. Quaternary Research 46:298–310. Oncorhynchus cyniclope, a fossil trout from Nevada dated to around 9 million years ago, is described as an early member of the cutthroat trout clade with jawbones resembling modern O. h. henshawi. Fossil evidence, including O. cyniclope and Pliocene fossils, suggests that descendants of O. belli (another fossil form) persisted in western Nevada and are ancestral to the Lahontan cutthroat trout.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. The closure of the Lahontan Basin around 4 million years ago may have severed connections with Coastal cutthroat trout and initiated independent evolution. Molecular clock estimates place the divergence of the Lahontan lineage from the Snake River lineage at approximately 2 million years, and from the upper Columbia River lineage at about 2.8 million years.

Evolutionary lineages Based on the sources, the Lahontan Basin evolutionary lineage is a distinct species of cutthroat trout. The diversification patterns and timing within the Lahontan Basin lineage have been significantly shaped by the dynamic hydrographic landscape, particularly the history of pluvial Lake Lahontan.Benson, L. V., and R. S. Thompson. 1987. Lake-level variation in the Lahontan Basin for the past 50,000 years. Quaternary Research 28:69–85.Carter, D. T., L. L. Ely, J. E. O'Connor, and C. R. Fenton. 2006. Late Pleistocene outburst flooding from pluvial Lake Alvord in the Owyhee River, Oregon. Geomorphology 75:346–367.

Relationship to other lineages The Lahontan cutthroat trout group is considered sister to the Westslope cutthroat trout in many analyses.Young, M.K., K.S. McKelvey, T. Jennings, K. Carter, R. Cronn, E.R. Keeley, J.L. Loxterman, K.L. Pilgrim and M.K. Schwartz. 2018. The phylogeography of westslope cutthroat trout. Pages 261–301 in Trotter P, Bisson P, Schultz L, Roper B (editors). Cutthroat Trout: Evolutionary Biology and Taxonomy. Special Publication 36, American Fisheries Society, Bethesda, Maryland.Thorgaard, G. H., J. P. Brunelli, N. R. Smith and B. M. Johnson. 2018. Y chromosome marker evolution in Cutthroat Trout. Pages 173–179 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. The Lahontan Basin evolutionary lineage is also recognized as the sister lineage to the Coastal cutthroat trout lineage. Some studies using mitochondrial DNA and Y chromosome markers suggest closer associations between the Westslope and Lahontan cutthroat trout lineages, with the Coastal cutthroat trout less closely related.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. Karyotype data suggests that the Yellowstone complex of cutthroat trout originated from a 64-chromosome ancestor of the Lahontan cutthroat trout lineage.

Uniquely Identifiable Ecological Units