RESPONSE OF ENDANGERED DESERT FISH POPULATIONS TO A CONSTRUCTED REFUGE

The U.S. Bureau of Reclamation created a shallow, 110-m channel to pro vide habitat for two endangered fishes, Cyprinodon elegans (Comanche S prings pupfish) and Gambusia nobilis (Pecos gambusia), at the site of the fishes' former natural habitat. The cienega (marsh) associated wit h Phantom Lake Spring in Jeff Davis County, Texas, was destroyed by th e creation of an irrigation canal system. In 1993, the endangered fish es were stocked into the refuge with individuals from the irrigation c anals, and in the case of C. elegans, hatchery stocks. The condition o f habitat, status of fish populations, and fish ecology within the ref uge were then monitored for two years. The abundance and density of bo th species increased in accordance with aquatic plant development. Cyp rinodon elegans abundance peaked after one year and stabilized at an a verage density of 14.7/m(2) by the end of our study. Juvenile C. elega ns were always rare, which may indicate that the population reached th e refuge's carrying capacity and that recruitment is low. Gambusia nob ilis was the most abundant fish in the refuge (average density 96/m(2) ), used the entire refuge, and outcompeted nonindigenous G. geiseri. T he two Gambusia species used similar habitats but showed almost no die tary overlap. High densities of aquatic plants reduced the amount of o pen water areas necessary for C. elegans. The refuge will sustain the two endangered fishes at this historic site of endemism while maintain ing flow to the irrigation system; however, the refuge is not equivale nt to a restored cienega.