The Colorado pikeminnow (Ptychocheilus lucius, formerly squawfish) is the largest cyprinid fish of North America and one of the largest in the world, with reports of individuals up to 6 ft (1.8 m) long and weighing over 100 pounds (45 kg). Native to the Colorado River Basin of the southwestern United States and adjacent Mexico, it was formerly an important food fish for both Native Americans and European settlers. Once abundant and widespread in the basin, its numbers have declined to the point where it has been extirpated from the Mexican part of its range and was listed as endangered in the US part in 1967, a fate shared by the three other large Colorado Basin endemic fish species: bonytail chub, humpback chub, and razorback sucker. The Colorado pikeminnow is currently listed as vulnerable by the IUCN.
Like the other three species of pikeminnows, it has an elongated body reminiscent of the pike. The cone-shaped and somewhat flattened head is elongated, forming nearly a quarter of the body length. Color grades from bright olive green on the back to a paler yellowish shade on the flanks, to white underneath. Young fish also have a dark spot on the caudal fin. Both the dorsal and anal fins typically have 9 rays. The pharyngeal teeth are long and hooked.
The reports of 6-ft individuals are estimates from skeletal remains, but a number of community elders, interviewed by the Salt Lake Tribune in 1994, reported that such individuals were once common. Catches in the 1960s ranged up to 60 cm for 11-year-old fish, but by the early 1990s, maximum sizes reached no more than 34 cm. Biologists now consider the typical size of an adult pikeminnow to be between 4 and 9 pounds, and reports of the fish lately exceeding 3 feet in length are now in question.
Young pikeminnows, up to 5 cm long, eat cladocerans, copepods, and chironomid larvae, then shift to insects around 10 cm long, gradually eating more fish as they mature. Once they achieve a length around 30 cm, they feed almost entirely upon fish.
This fish has an ontogenetic separation of life history stage. The altricial young emerge from whitewater canyons, enter the drift as sac-fry, and are transported downstream. Habitat for the young fish is predominately alongshore backwaters and associated shorelines of more alluvial reaches of the turbulent and turbid rivers of the Colorado system. In contrast, adults reside in more well-defined channels, where they seek eddy habitats and prey on suckers and minnows. Colorado pikeminnows are potamodromous, making freshwater spawning migrations to home in on their natal areas. These migrations can begin as upstream or downstream movements, depending on the location of home range of individuals, and may involve 100 km or more. Spawning occurs around the summer solstice, with declining flows and increasing temperatures. Breeding males are bronze-colored and heavily covered with tubercles, while females are generally larger, lighter in color and with fewer tubercles. As the fish reach the spawning location, they stage in deeper pools and eddies and make spawning runs into nearby runs and deep riffles, where the adhesive eggs are released. Upon hatching and swim-up, the small fry are entrained and carried 50–100 km downstream.
The species was once found throughout the Colorado Basin, so occurred in Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming, as well as in Mexico. Damming and habitat alterations have confined the species to the upper Colorado drainage; currently, remnant populations are known from the Green, Gunnison, White, San Juan, and Yampa Rivers. They have been transplanted to the Salt and Verde Rivers, both within their native range.
Population Reduction Efforts
Additionally, land managers in the past have attempted to reduce the native fish population of the Colorado Basin in favor of sport fishing. In the mid-1960s, the federal government poured the poison rotenone into the Green and San Juan Rivers, attempting to create an environment supportive of non-native sportfish. In September 1962, the Green River was poisoned beginning upstream of Flaming Gorge. The poison worked downstream for 3 days until it reached upstream of Dinosaur National Monument. Potassium permanganate was used to neutralize the rotenone, but concentrations were higher than expected and rotenone continued into the Dinosaur National Monument area.  
Recovery efforts are focused on operating dams to create more natural flow patterns, improving fish passage up- and downstream, and restricting stocking of non-native fish to reduce ecological interactions. Thus far, progress in recovering this pikeminnow has been limited.
- Smith, K. & Darwall, W.R.T. (2012). "Ptychocheilus lucius". IUCN Red List of Threatened Species. 2012. Retrieved 28 July 2016.CS1 maint: ref=harv (link)old-form url
- Froese, Rainer and Pauly, Daniel, eds. (2006). "Ptychocheilus lucius" in FishBase. October 2006 version.
- H.M. Tyus.2012. Ecology and conservation of fishes. Taylor and Francis Group, CRC Press, Boca Raton, London and New York
- H.M. Tyus. 1990. Potamodromy and reproduction of Colorado squawfish in the Green River basin of Utah. Transactions of the American Fisheries Society 119:1035-1047
- Lisa Kearsley (2002). Guide to The San Juan River. Flagstaff, Arizona: Shiva Press
- An Entirely Synthetic Fish (Yale University Press, 2010)
- Restoring Colorado River Ecosystems (Island Press 2007)
- William F. Sigler and John W. Sigler, Fishes of Utah (University of Utah Press, 1996), pp. 109–114
- USFWS Upper Colorado Endangered Fish Recovery Program
- USFWS Environmental Conservation Online System page for Colorado pikeminnow
- USFWS List of Fish Listed under Endangered Species Act