The cutthroat trout (Oncorhynchus clarkii clade) is a clade of four fish species of the family Salmonidae native to cold-water tributaries of the Pacific Ocean, Rocky Mountains, and Great Basin in North America. These four species are the Coastal (O. clarkii), Westslope (O. lewisi), Lahontan (O. henshawi), and the Rocky Mountain (O. virginalis). As a member of the genus Oncorhynchus, it is in 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. The specific name clarkii was given to honor explorer William Clark, coleader of the Lewis and Clark Expedition.
Cutthroat trout usually inhabit and spawn in small to moderately large, clear, well-oxygenated, shallow rivers with gravel bottoms. They reproduce in clear, cold, moderately deep lakes. They are native to the alluvial or freestone streams that are typical tributaries of the rivers of the Pacific Basin, Great Basin and Rocky Mountains. Cutthroat trout spawn in the spring and may inadvertently but naturally hybridize with rainbow trout, producing fertile cutbows. Some populations of the coastal cutthroat trout are semi-anadromous.
Several subspecies of cutthroat trout are currently listed as threatened in their native ranges due to habitat loss and the introduction of non-native species. Two subspecies, and , are considered extinct. Cutthroat trout are raised in hatcheries to restore populations in their native range, as well as stock non-native lake environments to support angling. The cutthroat trout type species and several subspecies are the official state fish of seven western U.S. states.
Taxonomy
Cutthroat trout were the first New World trout encountered by Europeans when in 1541, Spanish explorer Francisco de Coronado recorded seeing trout in the Pecos River near Santa Fe, New Mexico. These were most likely Rio Grande cutthroat trout . The species was first described in the journals of explorer William Clark from specimens obtained during the Lewis and Clark Expedition from the Missouri River near Great Falls, Montana, and these were most likely the westslope cutthroat trout . As one of Lewis and Clark's many missions was to describe the flora and fauna encountered during their expedition, cutthroat trout were given the name in honor of William Clark. In 1836, the type specimen of was described by naturalist John Richardson from a tributary of the lower Columbia River, identified as the "Katpootl", which was perhaps the Lewis River, as there was a Multnomah village of similar name at the confluence. This type specimen was most likely the coastal cutthroat trout subspecies . Until the 1960s, populations of westslope cutthroat trout and Yellowstone cutthroat trout were lumped into one subspecies; Salmo clarkii lewisii. Biologists later split the group into two subspecies, christening the name westslope cutthroat trout with the lewisii name which honors explorer Meriwether Lewis and renaming the Yellowstone cutthroat trout Salmo bouvierii, the first name given to the Yellowstone cutthroat trout by David Starr Jordan in 1883 honoring a U.S. Army Captain Bouvier.
In 1989, morphological and genetic studies indicated trout of the Pacific Basin were genetically closer to Pacific salmon (Oncorhynchus species) than to the Salmos–brown trout or Atlantic salmon of the Atlantic Basin. Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat and other Pacific Basin trout into the genus Oncorhynchus.
This single species (O. clarkii) classification is now changed. Genetic, taxonomic, and geologic studies have determined that cutthroat trout should be divided into four distinct species, with each having multiple subspecies corresponding to the evolutionary lineages found within major river basins, except for the Coastal cutthroat trout.
Subspecies and uniquely identifiable evolutionary units Behnke in his salmon and trout handbook of 2002 recognized 14 subspecies of cutthroat trout that are each native to a separate geographic area. Not all of them were scientifically described, and different views on the taxonomic identities have been presented.
During the annual meeting of the American Fisheries Society (AFS) in 2015, the Western Division of AFS (WDAFS) hosted an expert-panel workshop to evaluate the validity of the currently recognized subspecies, using evidence both for and against. The panel found the Behnke classification scientifically indefensible, and proposed an updated phylogeny and classification that aligns with the corpus of evidence.
One specific detail the panelists were unable to resolve was how to discuss specific subsets of the population. One side favored subspecies as a valuable taxonomic rank, because they represent important information about their evolutionary and ecological history that should be recognized to preserve biodiversity. The other side disagreed, arguing that subspecies do not necessarily align with actual evolutionary units, and are simply designations of lineages within a species based on geography.
To facilitate further discussion without needing to resolve that detail, the panel used the term "uniquely identifiable evolutionary unit" (UIEU) to describe various evolutionary subunits within a species. In short, UIEUs are recognizable population groups that show independence in an evolutionary sense, but do not meet all the criteria to be considered full species.
The evolutionary history of the cutthroat trout is complex and not fully agreed upon by all scientists. What is known is that cutthroat trout were present in the then-forming Lahontan basin 10 million years ago. This, coupled with new information about drainage patterns in western rivers, suggests that the interior radiation and colonization pathways of the cutthroat trout may be substantially different than what was previously thought.
Previously, the first evolutionary theory, based on the work of David Starr Jordan and later Robert J. Behnke, proposed that an ancestral cutthroat trout from Asia entered North America and dispersed inland through the Columbia River basin. The formation of Shoshone Falls on the Snake River then isolated groups of trout, leading to the variety of subspecies seen today. Behnke later modified this theory, suggesting that the Colorado River system was invaded from the Yellowstone and not the upper Arkansas River.
More recently, an alternative theory demonstrates that cutthroat trout originated in the Bonneville Basin and then radiated outwards approximately 4 million years ago. This theory was based on their interpretation of the fossil record and DNA analysis. New evidence, particularly DNA analysis, is providing new insights into the origins and relationships of the different lineages. However, there is still much to be learned about this complex history. What is known is that repeated glacial and interglacial periods that would have caused repeated fracturing and isolation of cutthroat trout populations, eventually resulting in the different cutthroats found today.