Devil's Hole Pupfish Status Remains Precarious

The Devil's Hole pupfish is a small (typical adult size = 35 mm) fish that is endemic to a single water-filled cavern in southern Nevada. The singular habitat of Devil's Hole and 40 acres of surrounding land were established as a distinct unit of Death Valley National Monument by presidential proclamation in1952. Numbers of Devil's Hole Pupfish have been in decline for about 12 years and numbers are currently near the lowest level of the entire 35 year period of record. Ongoing threats to the species existence include increasing groundwater development, potential effects of climate change, and extremely limited distribution. The causes of the recent decline remain undetermined.

The Devil's Hole pupfish is one of a number of North American members of the genus Cyprinodon that became isolated and underwent speciation when the climate became drier at the end of Pleistocene. It has a number of characteristics that distinguish it from other pupfish including small size, absence of pelvic fins, low fecundity and reduced aggression. Some of these characteristics may be developmental responses to conditions in Devil's Hole and not indicative of underlying genetic divergence.

During the late 1960s and early 1970s, groundwater pumping for irrigated agriculture caused water level in Devil's Hole to drop, exposing a shallow shelf that is critical for pupfish feeding and reproduction. An injunction filed by the U.S. Department of Justice was appealed and ultimately reached the Supreme Court, which ruled in 1976 that the National Park Service had an implicit right to sufficient water for the protection of the pupfish and its habitat. Soon after the ruling, nearby pumping was terminated, water level in Devil's Hole rose (although not to pre-pumping levels), and pupfish numbers increased.

During the 1980s and early 1990s Devil's Hole pupfish population size fluctuated annually between 200 fish in the spring and 300 to 500 fish in the fall. In 1995, or shortly thereafter, the pupfish population began to decline, a trend that has continued through 2007. Possible causes include factors related to water level, which has been dropping slowly since 1989, changes in the thermal regime, changes in energy inputs and / or aquatic community composition, altered hydrology and sediment dynamics, intrinsic genetic factors, and / or the existence of some unidentified chemical or biological contaminant. Many of these factors could be interdependent.

Additional populations of Devil's Hole pupfish were established and managed in three refuges during the 1970s, 1980s and 1990s. By the end of 2005 one population (School Springs) had become extirpated, another (Point of Rocks) had become introgressed by a related species of pupfish and the third (Hoover Dam) was in decline. In 2006, the hybridized population from Point of Rocks Refuge (n>100) and the remaining pure Devil's Hole pupfish from the Hoover Dam Refuge (n=18) were brought into captivity. These fish and a smaller number of pure fish from Devil's Hole were used in attempts at propagation of pure fish and backcrossing of pure fish with hybrids. Attempts at backcrossing were unsuccessful and, although attempts to propagate pure fish resulted in some initial success, reproduction was insufficient to maintain the population. As a result, there are currently no pure Devil's Hole pupfish in captivity or at any location outside of Devil's Hole.

Supplemental feeding has been underway at Devil's Hole since late December 2006 when it was noted that many fish were in poor condition and exhibiting symptoms that could be related to starvation. Feeding appeared to result in improved condition of the fish and may have contributed to an increase in the number of larvae observed during the spring and summer of 2007. Although feeding continued in 2008, larval production appeared to be lower than in 2007.

Although recent counts suggest that the Devil's Hole population is increasing, numbers remain low relative to the 35 year period of record. Spring counts for 2006, 2007, 2008 and 2009 were 38, 38, 45 and 70, respectively. Fall counts for the 2006 - 2008 have were 85, 92 and 126. In addition to the fact that numbers remain relatively low, there is a great deal of uncertainty associated with the current methodology.

The ability of biologists to determine the reasons for the decline of the Devil's Hole pupfish has been impeded by a lack of long term data for key environmental variables. The National Park Service is in process of establishing a long-term monitoring program and is collaborating with the U.S. Fish and Wildlife Service, Nevada Department of Wildlife and a number of academic institutions to initiate studies of those variables that are believed to have the greatest potential to be limiting pupfish population recovery. There is a growing body of circumstantial evidence to suggest that the overall productivity of the aquatic ecosystem may be declining and, in turn, affecting adult condition, reproductive success and / or recruitment. Water temperature, which has historically been near the upper critical threshold for many biological processes, is also being investigated.