The Devils Hole pupfish (Cyprinodon diabolis) is the rarest fish in the world. Found only in a single, tiny limestone cavern in the Devils Hole geothermal pool about 100 km east of Nevada's Death Valley National Park, these fish have the smallest known geographic range of any vertebrate in the wild.

It’s thought that they made it to this 500,000-year-old limestone cavern when its roof caved in, exposing it and its water to the ground surface. Once inside, the fish got used to the oxygen-poor and super-warm, 33 degree Celsius (93 degrees Fahrenheit) water. But then a decline in food resources, defects caused by reduced genetic variation and climate change bringing the water temperature up and the dissolved oxygen levels down, the population has been struggling.

Since researchers started monitoring them in the early 1970s, sending scuba divers down into the 129-metre-deep cavern to manually count each individual, the population has decreased from 200 to just 68 to 35 fish in 2013. While more recent population counts have put the number slightly higher than this, they are still at severe danger of extinction, and conservationists have pretty much resigned themselves to the fact that if another population of Devils Hole pupfish is not successfully established elsewhere, they will almost likely disappear altogether.

But of course, relocating a number of these adorable little fish - which rarely grow to more than 2.5 cm long - is easier said than done, and if the process is not carried out properly, it could see the relocated population die off, as well as the original population, having been irreparably depleted. Which is why Steven Beissinger, a UC Berkeley professor of environmental science, policy and management, set out to find the best and safest way to take fish from the Devils Hole pupfish population by performing the first model-based risk analysis for this very special species.

"Some species are critical to conserve because they serve important functions in their ecosystems, and others are important because they might be sources of new medicines, or they may have unique biological features that could inspire human innovation," Beissinger said in a UC Berkeley press release. "We don't know if the Devils Hole pupfish has or ever will serve any of these important functions, which seems unlikely as long as the species remains restricted to its single cavernous refuge. We do know that the pupfish, like all species, has a fundamental right to exist that is independent of its value to humans."

By modelling a range of different scenarios, including removing different numbers of adults, juveniles, and eggs from the Devils Hole cavern, at different times of the year over several years, Beissinger came up with the least risky move - no more than six adults were taken from the Devils Hole cavern population during the course of a year, and only during the autumn months.

Even better would be to remove just eggs from the population and raise those in separate locations. Publishing his results in the journal PeerJ, Beissinger calculated that “removing eggs had the least impact on the pupfish in Devils Hole; RV (reproductive value) of an adult was roughly 25 times that of an egg.” If nothing is done, the wild pupfish faces a 28 to 32 percent risk of extinction over the next 20 years.

Even knowing the perfect way to populate a new group of Devils Hole pupfish doesn’t make it any easier to do it successfully. Between 1969 and 2012, researchers tried several times to expand the Devils Hole pupfish species. These attempts usually took up to 30 pupfish from their cavern at a time, and these were distributed to 14 different locations, including highly managed commercial fish farms and natural ponds and springs that were left more or less alone. Each time, the relocated population failed, lasting at maximum 20 years, but sometimes only 12 months. The reasons for failure ranged from poor equipment, predation, and failure to reproduce, to breeding with other species of pupfish and vandalism of their ponds.

But now, says Beissinger a new captive propagation facility has been built, and for the first time it's been designed specifically for this species. Located near Devils Hole at Ash Meadows in the Amargosa Valley, Nevada, the facility has already been raising 60 Devils Hole pupfish eggs since late last year.

If this separate population doesn’t work, the absolute last resort would be what Beissinger refers to as the fish's "California condor moment”. Just like in 1986 when researchers removed the entire critically endangered population of California condors (Gymnogyps californianus) from the wild in order to raise them in captivity, this could be the only way to save the Devils Hole pupfish. But once you remove animals from the wild and raise them in captivity, it becomes extremely difficult for them to learn how to survive in the wild again.

Whatever happens, this tiny fish - named after their mating behaviours that resemble playful puppies (seriously) - has a tough road ahead of it. But, as Beissinger insists, they’re entirely worth the effort:

"Somehow, this handsome little fish has heroically persisted in the harsh desert environment through thousands of years of drastic climate warming and droughts. Should the human condition ever arrive at this point after another century of increasing carbon dioxide emissions and climate warming, we may need someone to help us out of our hole."

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