About Oncorhynchus virginalis (Girard, 1856)
Rocky Mountain cutthroat trout, which has the scientific name Oncorhynchus virginalis (Girard, 1856), shares the high variability in size, coloration, and habitat selection seen in all other cutthroat trout. Their body colors range from yellowish-brown, silvery, and brassy, to brilliant orange-red and golden, and they may also have bright golden-orange, red, or rose tints across their bodies. Their body spots vary in size and shape, from medium-large and rounded to small and pepper-like; spots can either be concentrated on the caudal peduncle or evenly distributed over the body sides. This subspecies typically has between 60 and 63 vertebrae, and has between 150 and 200 lateral scales. Adult weights vary broadly, from approximately 10 grams to over 7 kilograms; the largest known Rocky Mountain cutthroat trout is the Yellowfin variety. Because of historical human introductions and hybridization with other cutthroat subspecies, it can be difficult to definitively distinguish some Rocky Mountain cutthroat trout from closely related subspecies based on physical appearance alone. All members of this group share a distinctive trait: red, pink, or orange linear marks along the underside of their mandibles, in the lower folds of the gill plates. These markings give the group the common name "cutthroat", a term coined in an 1884 article in The American Angler by outdoor writer Charles Hallock. Historically, Rocky Mountain cutthroat trout were native to a broad range of river drainages and basins across the western United States. Their native range extends from the Rio Grande, Canadian, and Pecos River drainages in New Mexico and southern Colorado, north to the Yellowstone and Upper Snake River drainages in Montana, Idaho, Wyoming, and parts of Utah and Nevada. They are also native to the Bear River watershed on the border of Idaho, Utah, and Wyoming, the Dolores, Colorado, Gunnison, White, Yampa, and upper Green River watersheds across Colorado, Wyoming, and Utah, and the South Platte and Arkansas drainages. At one point, they were also likely established in the Davis Mountains in Texas. Today, their range is broadly restricted to the headwater reaches of these original watersheds. Like other salmonids, Rocky Mountain cutthroat trout require cold, clean, well-oxygenated water to survive and grow. They most often live in creeks, streams, and small rivers with gravelly bottoms, or in cold, moderately deep lakes. Streamside vegetation provides shade, stabilizes stream banks to reduce erosion and limit silt sediment loads, which would otherwise harm trout habitat and spawning grounds. Beaver ponds provide refuge during droughts and serve as overwintering habitat. Like other cutthroat trout, Rocky Mountain cutthroat trout are opportunistic feeders with diverse diets, and their food size increases as the trout grows. Their primary forage includes macroinvertebrates such as caddisflies, stoneflies, mayflies, and aquatic dipterans including midges and craneflies, in their adult, larval, and pupal stages; these prey types are frequently imitated by fly fishers. Additional prey includes adult terrestrial insects such as ants, beetles, spiders, grasshoppers, and crickets, as well as aquatic prey such as crayfish, freshwater molluscs, fish eggs, shrimp and other crustaceans, and other fish. In general, the trout adjust their diet and feeding behavior based on factors including food availability, habitat, age, and size. For example, lake-dwelling Yellowstone Rocky Mountain cutthroat eat zooplankton and bottom-dwelling insects, while Bear River cutthroat in Bear Lake have evolved a diet that depends largely on smaller fish such as the endemic Bonneville cisco. Stream-resident cutthroat are primarily drift feeders, eating insects carried along by the stream current. Rocky Mountain cutthroat trout populations are negatively affected by habitat loss and degradation caused by human activities. These harmful activities include damming, logging, mining, grazing, agriculture, water diversions, and urban development. Dams alter the natural flow of rivers and streams, fragment habitats, and change water temperatures, which can disrupt the trout's breeding and migration patterns. Logging and grazing remove streamside vegetation, leading to increased water temperatures and higher sedimentation, which reduces survival rates for trout eggs and juvenile trout. Mining activities often release pollutants into waterways, degrading water quality and harming trout populations, while urban development increases stormwater runoff that carries pollutants and sediment into streams, further reducing habitat quality. All of these activities alter water temperature, flow patterns, and the availability of food and habitat, all of which are critical for the survival and reproduction of cutthroat trout. Historically, habitat connectivity served as a buffer against extinction by linking habitats of varying quality to stream headwaters; source-sink habitat processes allowed different cutthroat strains to evolve, survive, and thrive over long periods of time. In contrast, the loss of drainage connections caused by human activities has been a major threat to cutthroat trout across the western United States.
