Lahontan cutthroat trout, Oncorhynchus henshawi, (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. 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 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. 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. 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. 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.
Relationship to other lineages The Lahontan cutthroat trout group is considered sister to the Westslope cutthroat trout in many analyses. 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. 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
Description Lahontan cutthroat trout are known for being distinct and diverse, reflecting a history of long isolation and evolution. Distinguishing characteristics include medium-sized, roundish spots distributed across the body, head, and often the abdomen; the highest number of gill rakers among cutthroat trout (21–28, averaging 23–26); and abundant pyloric caeca (40–75, typically more than 50). Typical vertebral counts are 61–63, and lateral line scales range from 150–180. They are also known for their potential to grow to large sizes, with records of 18.6 kg and reports of 28 kg specimens from Pyramid Lake. Lahontan cutthroat trout and its subspecies share a 64-chromosome karyotype. Genetic evidence, such as diagnostic allozyme loci and SNPs, also characterizes them. Phenotypically, they exhibit both large lacustrine and smaller fluvial forms.
Lifecycle
Most Lahontan cutthroat trout live a fluvial lifecycle. However, the broad diversity of environments, perhaps greater than any other cutthroat, influenced by factors such as elevation and stream characteristics, leads to a wide array of life history patterns and survival strategies. Likewise, the specific timing and duration of these stages can vary among different populations and subspecies. For example, populations at higher elevations may have shorter growing seasons and delayed maturity compared to those in lower-elevation streams. Additionally, factors such as stream flow, temperature, and food availability can significantly influence growth rates and survival throughout the lifecycle. Populations that are born and raised in small streams tend to be less migratory and move short distances. Lacustrine populations tend to be the largest, with the Lahontan cutthroats residing in Pyramid Lake being the largest of any cutthroat trout. Historically, this population moved great distances to reach ideal spawning habitats prior to the damming of the Truckee River and other tributaries.
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