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i ,_`al~; °+.,. Hydrobiologia 246: 165-168, 1992. 165 © 1992 Kluwer Academic Publishers. Printed in Belgium. Salinity tolerance of Colorado Squawfish, Ptychocheilus lucius (Pisces: Cyprinidae) S. Mark Nelson 1 & Stephen A. Flickinger2 1Denver Of)ice, Bureau of Reclamation, Denver, CO 80225, USA; 2Department of Fisheries and Wildlife Biology, Colorado State University, Fort Collins, CO 80523, USA Received 27 September 1991; in revised form 25 February 1992; accepted 28 April 1992 Key words: salinity, Colorado squawfish, endangered species, Cyprinidae Abstract Colorado Squawfish (Ptychocheilus lucius), were subjected to dilutions of saline water from natural springs near Glenwood Springs, Colorado, to determine salinity tolerance and aquaculture potential of Squawfish in this water. Lethal salinity to 50 percent of the fish (96 h LCSO) was 13.1 g 1-',indicating that squawfish survival in higher salinity spring water was not possible. Introduction The Colorado squawfish (Ptychocheilus lucius Gi- razd) is an endangered species, indigenous, to the Colorado River basin in the western United States. This lazgest of North American minnows, approaches lengths of 1.8 m and weights of 45 kg (Minckley, 1991). Young fish inhabit backwaters along the river, feeding on crustaceans and in- sects; adults are found throughout the river and are piscivorous. Spawning runs take place in the spring and fish return to designated spawning azeas yeazly (Minckley, 1991). A combination of factors (Behnke & Benson, 1980) such as de- pleted water volume,cooling of river water by bottom releases from dams, and introduction of non-native species into the system have caused the present endangered status of Squawfish. Along with physical and biological changes in the Colorado River, salinity has also changed. Salinity is expected to increase, from historical values of around 250 mg l-1 (Graf, 1985), up to 1100 to 1200 mg 1-1 by the year 2010, if control measures are not taken (Bureau of Reclamation, 1983). Salinity values for the Colorado River are usually measured at Imperial Dam, the last major diversion structure on the river in the United States, and salinity concentrations upstream aze less. While most of the salinity increases to this point have been from agricultural development, Cleave et al. (1979) have suggested that produc- tion of oil from shale in the western United States could also increase basin water salinity. Acciden- tal spills of saline process water or saline drain- age flows from stored shale piles are two ways in which this could occur (Selby et al., 1983). It is unknown what effect further salinity increases might have on this primary freshwater fish. Bishop & Porcella (1980) have recognized the potential for increased salinity concentrations in the Col- orado River system, and have urged that salinity effects on fishes of the basin be investigated. Along with anthropogenic sources, there aze also natural sources of salinity in the river. The