Lahontan cutthroat trout

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. 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. 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. The closure of the Lahontan Basin around 4 million years ago may have severed connections with Coastal cutthroat trout and initiated independent evolution. 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==

, Nevada. on the taste of Nevada trout. 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. Large Lacustrine forms These are a defining feature of Lahontan cutthroat trout, found in the western Lahontan Basin, including areas like the Truckee, Walker, and Carson River basins, along with Summit Lake. These lake-associated populations were historically known to grow to enormous sizes. These large lacustrine Lahontan Cutthroat Trout spawned in tributary habitats, reared in the river environment, and returned to the lake to mature. Here, Lahontan cutthroats became a large (up to 1 m or 39 in) and moderately long-lived predator of chub suckers and other fish as long as 30 or . The trout was able to remain a predator in the larger remnant lakes where prey fish continued to flourish. The record-size cutthroat trout of any subspecies was a Lahontan caught in Pyramid Lake, weighing , although anecdotal and photographic evidence exists of even larger fish from this lake. == Ecology ==

Range The Lahontan cutthroat trout are concentrated within the Lahontan hydrographic basin. Similar to other cutthroat trout, Lahontan cutthroats exhibit an opportunistic feeding behavior with a varied diet that expands as they grow. Their primary forage, often mimicked by fly fishers, include macroinvertebrates such as caddisflies, stoneflies, mayflies and aquatic dipterans (including midges and craneflies) in adult, larval, and pupal stages. Other prey include adult terrestrial insects such as ants, beetles, spiders, grasshoppers and crickets, and aquatic items including crayfish, freshwater molluscs, fish eggs, shrimp and other crustaceans, and other fish. Their diet and feeding habits shift in response to factors like food availability, habitat conditions, age, and size. In large endorheic lakes, Lahontan cutthroats essentially evolved in inland ocean-like conditions and became largely piscivorous, with a diet centered on the Cui-ui lakesucker, which is endemic to Pyramid Lake, the Tui chub. Here, Lahontan cutthroats became a large (up to 1 m or 39 in) and moderately long-lived predator where prey fish continued to flourish. == Population threats ==

The extirpation of Lahontan cutthroat trout in Lake Tahoe and Pyramid Lake was primarily caused by a combination of human activities that disrupted their habitat and spawning cycles. Overfishing, habitat degradation from logging and erosion, the construction of dams and water diversions, and the introduction of non-native species all contributed to their decline. See also Pyramid Lake (Nevada)#Water quality. Lake Tahoe was historically one of several oligotrophic lakes in the Truckee River watershed that supported the large lacustrine forms of Lahontan cutthroat trout known for growing to enormous sizes. The sources indicate that these populations were extirpated from Lake Tahoe in the mid-20th century. This extirpation occurred due to the combined pressures of overharvest and the loss of access to the tributary habitats necessary for spawning. The discovery of the Comstock Lode in 1859 led to extensive deforestation in the Tahoe Basin, which resulted in sedimentation that covered the rocky riverbeds essential for spawning. Climate change Climate change poses significant threats to Lahontan cutthroat trout and related lineages. Wildfire, which can also alter habitats and stream temperatures, is expected to be influenced by climatic changes. Temperature barriers are potentially a range-wide anthropogenic impact, which aligns directly with the threat of rising water temperatures due to climate warming. adds challenges for populations already threatened by fragmentation, disease, and nonnative species. The sources explicitly list habitat degradation and fragmentation and nonnative trout (which pose risks through hybridization, predation, and competition) as major rangewide anthropogenic impacts to Lahontan cutthroat trout. These interactions are critical, as climate change can make habitats less suitable, potentially concentrating native fish and making them more vulnerable to nonnative predators or competitors, or reducing the connectivity necessary for metapopulation persistence. ==Human history==

The Lahontan cutthroats of Pyramid and Walker Lakes were of considerable importance to both the Paiute tribe and the Washoe tribe of Nevada and California. These trout, as well as cui-ui (Chasmistes cujus), a sucker now found only in Pyramid Lake, were dietary mainstays and were used by other tribes in the area. When John C. Frémont and Kit Carson ascended the Truckee River on January 16, 1844, they called it the 'Salmon Trout River', after the huge Lahontan cutthroat trout that ran up the river from Pyramid Lake to spawn. American settlements in the Great Basin nearly extirpated this species. During the 19th and early 20th centuries, Lahontan cutthroats were caught in tremendous numbers and shipped to towns and mining camps throughout the West; estimates have ranged as high as annually between 1860 and 1920. A dam in Mason Valley blocked spawning runs from Walker Lake. By 1905, Derby Dam on the Truckee River below Reno interfered with Pyramid Lake's spawning runs. A poorly designed fish ladder washed away in 1907, and then badly timed water diversions to farms in the Fallon, Nevada, area stranded spawning fish and desiccated eggs below the dam. By 1943, Pyramid Lake's population was extinct. Lake Tahoe's population was extinct by 1930 from competition and inbreeding with introduced rainbow trout (creating cutbows), predation by introduced lake trout, and diseases introduced along with these exotic species. Upstream populations have been isolated and decimated by poorly managed grazing and excessive water withdrawals for irrigation, as well as by hybridization, competition, and predation by non-native salmonids. This is important, as although Lahontan cutthroat trout can inhabit either lakes or streams, they are obligatory stream spawners. ==Pyramid Lake and Truckee River water quality==

Pyramid Lake, the second-largest natural lake in the Western United States—prior to construction of the Derby Dam, which diverted water from the lake—has been the focus of several water quality investigations, the most detailed starting in the mid-1980s. Under the direction of the U.S. Environmental Protection Agency's comprehensive dynamic hydrology transport model, the Dynamic Stream Simulation and Assessment Model (DSSAM), was applied to analyze impacts of a variety of land use and wastewater management decisions throughout the Truckee River Basin. These analyses allowed more competent decisions to be made regarding the watersheds, as well as the management of treated effluent discharged to the Truckee River. ==Conservation==

Lahontan cutthroat trout currently occupy a small fraction of their historic range. In Independence Lake, precariously low spawner numbers have recently increased along with five years of brook trout removal. Pyramid and Walker Lakes have been restocked with fish captured in Summit Lake in Nevada and Lake Heenan in California, and those populations are maintained by fish hatcheries. Unfortunately, the Summit Lake strain does not live as long or grow as large as the original lacustrine strain of fish. However, in April 1977, fish believed to have been stocked almost a century ago from the Pyramid Lake strain were discovered by Kent Fisher, a biologist for the Utah Division of Wildlife Resources, in a small unnamed stream above the western edge of the Bonneville Salt Flats and are a genetic match to the original strain. This Pilot Peak strain is now integral to the reintroduction and planting programs maintained by the U.S. Fish and Wildlife Service. Another strain of the subspecies from Independence Lake is available, and a broodstock is managed by the California Department of Fish and Wildlife at their hatchery on Hot Creek, though less is known about the suitability of Independence Lake fish to other systems. Preservation of highly complementary habitats is crucial for the survival of the different age classes of cutthroat trout, with clean gravels needed for spawning, slow-moving side channel habitats used by juvenile fish, and deeper pool habitats such as beaver ponds for larger adult fish. They were classified as endangered species between 1970 and 1975, then the classification was changed to threatened species in 1975, and reaffirmed as threatened in 2008. Because it tolerates water too alkaline for other trout, Lahontan cutthroats are stocked in alkaline lakes outside its native range, including Lake Lenore (alternately Lenore Lake), Grimes Lake and Omak Lake in central Washington and Mann Lake in Oregon's Alvord Desert east of Steens Mountain.