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42 SURVIVAL POTENTIAL OF MOSQUITOFISH (GAMBUSIA AFFINIS) IN SALINE HABITATS. Peter G. Sanchez, Death Valley National Monument, Death Valley, California. Native to waters of the eastern and southwestern United States, t',j)e mosquitofish (Gambusia affinis) has been introduced worldwide for its reputed value as a biological control for mosquito larvae.]/ Mosquitofish thrive in a wide variety of habitats and are abundant in certain springs in Ash Meadows, Nevada (adjacent to Death Valley National Monument), where they appear to compete with native fishes. Within Death Valley National Monument, mosquitofish were stocked in irrigation ditches at Furnace Creek Ranch some 40 years ago. Around 1973 mosquitofish suddenly appeared in a fishless stream at Scottys Castle, 50 miles north of Furnace Creek-2/ Fish periodically (and mysteriously) appear in a concrete pond at the Death Valley Museum during holidays when visitors are numerous. One busy weekend in 1977 about 20 Gambusia were removed from the weather station evaporation pan at Furnace Creek. "Coffee can" transplants---the seemingly innocent, but .potentially damaging collection, transport and dumping of fish-- from irrigation ditches by children are believed to be the dispersal means in Death Valley. The potential for exotic fish introductions into native fish habitats within Death Valley National Monument is worthy of serious concern and a general lack of information concerning salinity tolerance of Gambusia has added to the uncertainity. Pupfish (Cyprinodon spp.) occur at five localities. The potential survival of Gambusia in the more saline of these habitats, including Salt Creek, has never been measured. To test survival potential, 15 mosquitofish were placed in a five-gallon aquarium containing Salt Creek Water (26,500 ppm TDS)3/ and 30 fish were kept in another aquarium containing tap water. The test began on April 20 and concluded June 15, 1978• Water lost through evaporation was replenished with Salt Creek water, thereby increasing salinity each time water was added. Calculated salinity at conclusion of the test was 38,100 ppm TDS. Despite two prolonged power outages which shut down air pumps early in the study and three weeks of virtual starvation toward the end of the test period, Gambusia survived for 55 days in water which, at the conclusion, contained a 30% higher dissolved solids concentration than did the Salt Creek water at the beginning of the study. Survival of Gambusia in Salt Creek, therefore, appears likely, should a transplant somehow occur.