One of the world's rarest fish species just got a lot rarer. The latest twice-annual count of tiny Devils Hole pupfish (Cyprinodon diabolis) at their sole habitat in Nevada has revealed just 35 of the critically endangered fish remain, down from 75 this past fall. This is the lowest count since the species was federally protected back in 1967.

Devils Hole pupfish live exclusively in a deep geothermal cavern in Ash Meadows National Wildlife Refuge, where their population has undergone precipitous declines over the past several decades for reasons that remain unclear. The 25-millimeter fish appear to be adapted by both high temperatures (33 degrees Celsius) and the water's low oxygen content, but little else about them is understood by science. Efforts to breed them outside of Devils Hole have not been fruitful, and two captive populations died out in 2006. According to the U.S. Fish and Wildlife Service (FWS), there are now too few of them to risk transferring any more of the fish from their natural environment.

Biologist Jim Deacon, who has been visiting Devils Hole for more than 50 years, told the Las Vegas Review–Journal that he thinks the species might be approaching "genetic meltdown." Back in the 1970s Deacon testified that local groundwater use threatened the geothermal cavern, and therefore the fate of the species. The case, Cappaert v. United States, went all the way to the Supreme Court in 1976, resulting in additional protections for the pupfish.

Local groundwater use was the first strike against the species, but even with protection the recent decades have not been kind. A flood in 2004 pushed some fish traps and several cubic meters of debris into Devils Hole, killing as many as a quarter of the fish. More recently the region has experienced three earthquakes and one major rain runoff event, which the FWS says may have influenced this year's decline, although they do not know for certain.

FWS biologists continue to study the fish and supplement their food in hopes of keeping the population alive. They have also tried hybridizing C. diabolis with another pupfish species in the hopes of jump-starting its genetics, a controversial effort recounted in Wired last year.

Even with this year's low count, all hope is not lost for the Devils Hole pupfish. In 2007 there were just 38 fish, but their number had rebounded to 126 by the fall of 2008. It fell again to 104 for the spring count two years ago, and then dropped to 63 in the spring of 2012. The species normally has a natural population fluctuation, with breeding in the spring and higher populations typical in the fall, but the last few years have not followed that trend. At this point we'll have to wait to see what the October count reveals. If the population enjoys its normal autumn boom, then the fish just may have a fighting chance at survival.

Photo: Olin Feuerbacher/U.S. Fish and Wildlife Service