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" THREATENED & ENDANGERED FISHES 275 the station and is isolated from all other surface waters in the area. Operational Role Dexter NFH was originally constructed to meet demands for warmwater. game fishes, but new laws and FWS policies have brought changes to the program. Thirteen species of imperiled fishes and four additional species of the genus Gila are presently being held at Dexter (Table 1). Holding such an array of fishes requires care to ensure isolation to prevent mixing of genomes and/or other adverse interactions. To this end, detailed operating procedures are followed to preserve genetic integrity and health of all fishes, Each species is maintaiqed in a separate pond to prevent loss due to hybridization and predation by or competition from other species. Congeneric spe- cies are maintained in widely separated ponds. Seines, dipnets, and personal gear are dried, disinfected, or changed when work proceeds from one pond to another. Taxonomic identification of incoming species is verified. They are also ex- amined for presence of parasites or disease or- ganisms that could endanger other captive-held fishes. Objectives The program at Dexter NFH was implemented to assist in the recovery of imperiled southwestern fishes, most of them federally listed. The following objectives were selected to plan and carry out the program: (1) maintain a viable protected gene pool of imperiled desert fishes, (2) develop tech- niques for culturing them, (3) study their biological requirements, (4) provide live and preserved fish to cooperating agencies and institutions, (5) pro- vide exchanges of expertise and data on . their culture and management, (6) publish findings, and (7) implement a public information program con- cerning the plight of, and recovery efforts for, imperiled fishes of the region. Holding To date, objectives have been met with varying degrees of success. Perhaps the most outstanding 1 fact is that never before have so many species of imperiled fishes from a wide diversity of native habitats been formally held and cultured at a single location. Comanche Springs pupfish, Leon Springs pupfish, and desert pupfish, Yaqui chub, . and beautiful shiner have been readily maintained. Pupfish populations are typically cyclic, with num- bers peaking in August and September and de- clining during the winter. Chub and shiner pop- ulations are usually more stable although their numbers follow the same pattern. All are multiple spawners with young fish present throughout the reproductive season, usually mid-April through early October at Dexter. Two live-bearers, Big Bend gambusia and Gila topminnow, have produced similarly large pop- ulations at Dexter, but experience extreme annual population fluctuations. It is not 'unusual to ex- perience a 90% loss during the winter. To ensure survival, several individuals are over-wintered in aquaria and restocked outdoors in the spring. The larger species over-winter successfully at Dexter. Chihuahua chub was thought to be extinct for nearly 40 years, until "rediscovered" in 1975. Following extensive flooding during December, 1978, only 10 fish could be captured in the wild; they were brought to Dexter in April 1979. Chi- huahua chubs have numbered more than 1,000. Re-introduction of hatchery-reared fish is planned during 1987. Some fish species are held at Dexter NFH to safeguard their genotypes while field investigations are under way to determine their status. The Dexter facility is not intended to be a final holding place for imperiled fishes. If the status of natural populations proves stable or is enhanced, or other requirements for holding them have been met, the inventory of species is reduced accordingly. In fact, four species were removed from the station, although two were later returned. Pecos gambusia (Gambusia "obilis) was maintained at Dexter from 1974 to 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 lomico/a) improved consid- erably after being brought to Dexter"in', 1914. A population was re-established in the Carnal River, Texas, so this species was also removed from the hatchery in 1981. Humpback and roundtail chubs were main- I tained at Dexter during 1981-1982 in order to " continue Gila taxonomic studies initiated at Willow Beach NFH (Hamman 1981a). Hybridization among bony tail and humpback and roundtail c~ubs in the upper Colorado River Basin is occurring (Smith et al 1979). Research on hybrids will help clarify the Gila complex problems In the upper basin and, hopefully, will provide information leading to recovery of the humpback chub and the bony tail. Upon completion of hybridization studies, hump- back and roundtail chubs were removed from the