Population size of Devils Hole pupfish (Cyprinodon diabolis) correlates with water level

Population estimates of the Devils Hole pupfish (Cyprinodon diabolis) exten ding over a 25-year period reveal an annual population cycle that reaches a low during spring and a high during fall. The four earliest years (1972-19 76) preceded a U.S. Supreme Court water rights decision intended to protect pupfish habitat. Mean and maximum population sizes during those four years were significantly lower than during subsequent periods examined through 1 997, and annual minimum population size was significantly lower during 1972 -1976 than during 1991-1997. Annual maximum population size is positively c orrelated with water level for the month in which it occurred as well as fo r the month one year prior to the time of the estimate. The lowest historic water levels occurred during the period 1972-1976, and over the past 60,00 0 years comparable or lower water levels may have occurred only during the altithermal, about 6000 years before the present. During these two periods the Devils Hole pupfish population has probably experienced its most severe bottlenecks.