Razorback sucker

Species of fish

The razorback sucker (Xyrauchen texanus) is a suckerfish found in rivers and lakes in the southwestern United States and formerly northwestern Mexico. It can grow to in length and is recognisable by the keel between its head and dorsal fin. It used to inhabit much of the Colorado River Basin but commercial fishing, river damming, and habitat loss have caused great declines in populations. It is now restricted to the Colorado River upstream of the Grand Canyon and to four reservoirs, Lake Mead, Lake Mohave, Lake Havasu, and Lake Powell. It usually moves from deep water to suitable spawning grounds to breed, and research into its habits and breeding locations is ongoing. It has been a federally protected fish since 1991 and is rated as "Critically Endangered" by the International Union for Conservation of Nature, and Critically Imperiled by NatureServe. There are some signs of recovery, with fish being observed in the lower Grand Canyon in 2012 and 2013.

The razorback sucker is most notable for the sharp edged bulge on the anterior part of its back, between the head and dorsal fin, giving rise to its common name, as well as to the alternative name "humpback sucker". The fish can attain lengths of up to and weights of
Populations in the Green and Colorado rivers upstream of Glen Canyon Dam have been reestablished through a stocking program and stocked razorback suckers have consistently spawned and produced larval fish. As of 2021, the only self sustaining population of razorback sucker is found in Lake Mead. While larval fish are found in other areas, indicative of successful spawning, recruitment (survival to adulthood) is not sufficient or even absent due to predation. Non native predators of the razorback sucker include striped bass and flathead catfish in the Lower Colorado Basin and smallmouth bass, northern pike and walleye in the Upper Colorado Basin, with channel catfish present in the San Juan River subbasin. Hatching success depends on water temperature, with complete mortality at temperatures less than 10 °C (50 °F). Razorback suckers inhabit a diversity of areas from mainstream channels to backwaters of medium and large streams or rivers. They prefer to live over sand, mud, or gravel bottoms. Razorbacks feed on algae, insect larvae, plankton, and detritus. The eyes are receptive to parts of the UV spectrum, particularly that portion of the retina that receives light from below. The Razorback spends most of its life at depths where UV light cannot penetrate but they move into the shallows for breeding. In the shallows, males stake out a breeding territory and hover near the riverbed. When another male enters the breeding area, the defending male rolls his eyes downward to reveal the upper third of the eye generating a flash of reflected sunlight. The strongest reflected component of the flash lies in the UV spectrum. The intruding male, swimming overhead, can see the flash below and will shy away from it. The eye flashes are not visible from a distance underwater and can thus be used to signal intruding males without alerting predators. Females do not react to the eye flashes.

The Razorback sucker was once common throughout the Gila River watershed regions of Arizona. Commercial fishing together with dam building decimated the fish stock, which were unable to breed due to lower water temperatures in the reservoirs while dams blocked their movement into smaller channels (Nabhan 1988:553). They are now federally listed (USFWS October 23, 1991) as an endangered species with provisions for the protection of its critical habitat. Ongoing conservation efforts are taking place throughout the Upper and Lower Colorado River Basins. The largest and most genetically diverse population is found in Lake Mohave, Arizona/Nevada border. Since 2012, special spring releases from Flaming Gorge Dam have been timed to coincide with the presence of larval razorback sucker in the Green River. These flows mimic a more natural hydrograph and allow larval razorback sucker to access off channel wetlands as nursery habitat. These dam operations have proven successful with razorback sucker in wetlands regularly surviving past the larval stage and migrating back to the Green River. Subsequent encounters of fish tagged while emigrating from wetlands have documented limited recruitment to the adult population by wild spawned razorback sucker. In 2021, the Fish and Wildlife Service proposed to reclassify the razorback sucker as threatened, rather than endangered, under the Endangered Species Act.