Nevada Department of Wildlife: Fish Rearing Facilities Operation and Maintenance Grant

The RASU is one of four endemic, large-river fish species of the Colorado River Basin currently listed as endangered under the authority of the Endangered Species Act of 1973 (ESA) (USFWS 1991). Historically abundant and widespread throughout the Colorado River Basin, RASU were predominantly found in the mainstem river and major tributaries in Arizona, California, Nevada, New Mexico, Utah, and Wyoming (USFWS 1998). The current distribution and abundance of RASU are generally reduced from historic levels, mainly due to mainstem dam construction and the introduction of nonnative game fishes in the resultant reservoir habitats (Holden and Stalnaker 1975, Joseph et al. 1977, Wick et al. 1982, Minckley et al. 1991). Populations have however persisted following the creation of reservoirs in the lower Colorado River Basin (LCRB), but this has mostly been possible due to recruitment events occurring soon after dam closures (Minckley 1983) and ongoing fish augmentation (Mueller 2006).

BONY is another one of the four endemic, large-river fish species of the Colorado River Basin currently listed as endangered under the ESA. Similar to the RASU, BONY were once common throughout the Colorado River Basin, with known distribution ranging from Mexico to Wyoming (Behnke and Benson 1980, Deacon and Minckley 1991, Mueller and Marsh 2002). BONY experienced a range-wide decline in abundance during the 1950s, presumably due to habitat alterations through mainstem dam construction and introductions of nonnative fishes. However, unlike the RASU, BONY have had difficulty persisting in reservoirs in the LCRB. A captive brood stock has been developed from 34 BONY that were captured from Lake Mohave during 1976-1988 (Minckley et al. 1991).

FLSU were historically the most abundant large fish species in the Upper Colorado River Basin (UCRB) (Vanicek et al. 1970; Holden 1973; Holden and Stalnaker 1975; McAda 1977). They were found to be declining in the LCRB, and for a period they were largely restricted to the Colorado River above Lake Mead and a short segment of the Salt River (Minckley 1973). FLSU are currently listed as species of special concern in Arizona, and are an extinct species in California. They are included in the LCR MSCP due to potential of the species being listed under the ESA or CESA, or becoming protected under Nevada or Arizona law.