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73 Report on Chihuahuan Desert, Isolated Basins Robert R. Miller, Coordinator November 17, 1978 The Catarina pupfish, Megupsilon aporus Miller and Walters, described in 1972 from an isolated spring fed pond (El Potosi) in Nuevo Le6n, has been re- garded as one of the most endangered fishes in Mexico. Predation from large- mouth bass, stocked in 1974, reduced its numbers drastically, almost to the point of extinction. A three-day visit to E1 Potosi in May, 1978, by R. R. Miller and grad- uate students Michael L. Smith (Michigan) and Edie Marsh (Texas) revealed a dramatic increase which indicates a temporary recovery of this genus. The increase is correlated with high-water conditions. The only other native fish, the Potosi pupfish, Cyprinodon alvarezi Miller, is much more abundant than Megupsilon. Population estimates were approximately 2,800 for Megupsilon and 12,000 for 6yprinodon. A 1976 transplant of Megupsilon to an adjacent spring by Salvador Contreras-Balderas succeeded in establishing the genus outside of the pond area that contains the bass; the species was common in the spring and outflows at the time of our visit. A long-term prediction for the continued health of these species is difficult to make. A marked increase in water consumption could pose a serious problem for their survival. No practical method for eliminating the largemouth bass has been devised; their numbers were low during our visit. Details are given in the paper pre- sented by M. L. Smith. A new species of pupfish was collected by us in an isolated spring in northern Chihuahua, where it is abundant although exotic mosquitof ish (Gambusia affinis) are common. A new crayfish was obtained from the same spring. The mosquitof ish has become established at a number of other iso- lated springs in the Chihuahuan Desert that contain endemic pupfishes, but the Cyprinodon are still holding out.