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populations prove stable or is enhanced, or other requirements for holding them have been met, the inventory of species is reduced accordingly. Pecos gambusia (Gambusia nobilis) was maintained at Dexter 1974-1981; it was removed from the hatchery after extensive field work revealed the four extant populations to be secure. Status of the fountain darter (Etheostoma fonticola) improved considerably after being brought to Dexter in 1974. A population was reestablished in the Comal River, Texas, so this species was also removed from the hatchery in 1981. Humpback and roundtail chubs were maintained at Dexter during 1981-1982 to continue Gila taxonomic studies initiated at Willow Beach NFH (Hamman 1981a). Hybridization among bonytail and humpback and roundtail chubs in the upper Colorado River Basin is occurring (Smith et al. 1979). Hopefully, research on hybrids will help clarify the Gila complex problems in the Colorado River basin and provide information useful in management and recovery of humpback chub and bonytail. Upon completion of hybridization studies, humpback and roundtail chubs were removed from the ~ station. The former was reintroduced in 1984 because of their limited natural occurrence. CULTURING Most efforts in developing culture techniques have been concentrated on four large Colorado River fishes: razorback sucker, Colorado squawfish, bonytail and humpback chub (Jensen in Rinne et al., in press). Marsh (1985) conducted studies at Dexter NFH to determine the influence of temperature on egg development, hatching success, and fry development of razorback sucker, Colorado squawfish and bonytail. He found the optimum temperate+re for egg incubation and fry devel^pment for these species to be near 70° F so hatchery well water is heated for this purpose. Razorback sucker Razorback sucker continues to decline in the Colorado River system. There has been no documented recruitment to the aging Lake Mohave razorback sucker population since t~,~ mid-1950's (Hinckley 1983) and ext::~.sive collecting in the uppR~ basin has not produced any young razorbacks (Lynn Kaeding, pers. comm.). Utilizing otoliths, Michael McCarthy (pers. comm.), Arizona State University, found razorback sucker collected from Lake Mohave, Arizona-Nevada in 1980-81 to range from 24 to 43 years of age (1937-57 year classes). If some of the 1937 year class are still living, which is highly probable, they are now 48 years old. -36-