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IT O Maei{ler F15-q <br />Reprinted from THE SOUTHWESTERN NATURALIST <br />Vol. 34, No. 1, March 1989 <br />Made in United States ofAmerica <br />OBSERVATIONS OF SPAWNING RAZORBACK SUCKER <br />(XYRAUCHEN TEXANUS) UTILIZING RIVERINE <br />HABITAT IN THE LOWER COLORADO RIVER, <br />ARIZONA-NEVADA <br />Once common throughout the Colorado River drainage, the razorback sucker (Xyrauchen texanus) <br />is now rare (Holden and Stalnaker, 1975; Minckley, 1983). Attempts are being made by the U.S. Fish <br />and Wildlife Service and the Arizona Game and Fish Department to recover the species in the lower <br />Colorado River Basin by reintroduction of hatchery fish (1981 Cooperative Agreement). Although <br />generally considered a riverine species, spawning observations and studies in the lower basin have <br />primarily focused on relict reservoir populations (Ulmer, 1980; Minckley, 1983; Marsh and Langhorst, <br />in press). While riverine spawning has been suspected (Minckley and Carothers, 1979; Minckley, <br />1983), observations have not been substantiated by collection of eggs or larvae. <br />Species scarcity, logistical problems associated with underwater investigations, and poor water vis- <br />ibility have contributed to the lack o£ information concerning this species' use of river habitats. However, <br />downstream from Hoover Dam, Arizona-Nevada (Fig. 1), visibility often exceeds 15 m, and razorback <br />suckers are common. <br />In 1983 (Minckley, 1983) and 1984, razorback suckers were observed exhibiting spawning behavior <br />in the Colorado River 4.8 km below Hoover Dam. Since then, evidence of spawning razorback sucker <br />has also been discovered (pers. obser.) in the river downstream of Davis Dam, which impounds Lake <br />Mohave. <br />On 28 January 1984, divers snorkeled several areas along a 6-km section of river downstream of <br />Hoover Dam which were thought to be likely spawning areas. These included coves, hot spring areas, <br />and portions of shoreline. Razorback suckers were observed in several areas; however, actual spawning <br />or any indication that the substrate had been disturbed by spawning activities was limited to one