The Devils Hole pupfish (Cyprinodon diabolis) is the rarest fish on Earth.
It survives in a single flooded cave in Nevada, under conditions that should have driven it to extinction centuries ago: scalding water, almost no oxygen, and barely enough room to reproduce.
Two consecutive earthquakes reduced the wild population to just 20 fish by 2025. By 2026, after the release of captive-bred fish, that number had climbed to 77.
One Cave and Nowhere Else
The Devils Hole pupfish (Cyprinodon diabolis) has a claim no other vertebrate can make: its entire natural habitat is a single location.
It lives in Devils Hole, a flooded limestone cavern within a separate, disconnected unit of Death Valley National Park, surrounded by the Ash Meadows National Wildlife Refuge.
The cave descends more than 430 feet, but the fish live only within the first 79 feet.
There, the water holds at a near-constant 91 degrees Fahrenheit, with dissolved oxygen levels far below what most freshwater fish can tolerate.
The species’ entire feeding and breeding cycle depends on a single shallow rock shelf measuring just under 11 by 16 feet, set at one end of the cavern.
Never a Large Population
Formal counts of the pupfish began in 1972 and are conducted each spring and fall, combining surface observation with diving.
Between 1972 and the mid-1990s, the population averaged around 200 fish in spring and 425 in fall, with historic peaks of up to 548 fish recorded in 1980, 1990, and 1995.
The reason is straightforward: the population is always higher in fall than in spring, because algae bloom under the summer sun and die back through winter.
From 2005 onward, most counts fell below 200. The population hit a historic low of just 35 fish in 2013, before slowly beginning to recover.
By 2022, the count had reached 263. Then, in the spring of 2024, surveyors recorded 191 individuals — the highest spring count in 25 years, a rebound driven in part by algae growth following a 2023 hurricane.
Two Earthquakes Erased a Decade of Recovery
That recovery did not last.
Earthquakes in December 2024 and February 2025 sent waves surging through the cavern, dislodging algae and fish eggs from the shallow shelf — the only available breeding surface the species has ever known.
The March 2025 count recorded just 20 wild fish. It was the lowest number ever documented.
In response, the agencies overseeing the conservation program made an unprecedented decision: they would release captive-bred fish directly into Devils Hole.
It was the first time the backup population had ever been used, more than a decade after it was established.
The First Release of Captive-Bred Fish
Since 2013, an exact replica of the upper section of Devils Hole has existed at the Ash Meadows Fish Conservation Facility: a tank holding more than 99,000 gallons, engineered as a genetic insurance policy for the species.
Until 2025, the fish held there had never been reintroduced to the wild.
Conservation teams first released 19 captive-bred fish, then approximately 50 more in the weeks that followed.
The decision was made under significant pressure. Staff reductions at federal agencies had compressed the timeline, and the first group of fish was released without genetic samples being collected — a gap that managers themselves acknowledged.
The spring 2026 count offered the first measure of results: 77 fish in the cave, nearly four times the previous year’s count, along with an unusually high number of juveniles and newly hatched fish — a clear sign that breeding had resumed.
Several hundred additional fish remain at the backup facility.
Why This Species Should Have Vanished Thousands of Years Ago
The Devils Hole pupfish was formally described as a species in 1930 and became one of the first animals listed under the U.S. Endangered Species Act in 1967.
A landmark Supreme Court ruling — Cappaert v. United States, decided in 1976 — imposed limits on groundwater extraction in the surrounding area to keep the water level above the breeding shelf, after agricultural pumping in the 1960s and ’70s had caused the water in the cave to fall.
A 2014 study estimated the species faced a 26 to 33 percent probability of extinction within 20 years, with a median time to extinction of between 360 and 2,900 years — far shorter than the 10,000 to 20,000 years the population was long assumed to have spent in isolation inside the cavern.
With a breeding population that has rarely exceeded a few hundred individuals, the species carries almost no genetic diversity.
Any change in temperature, water level, or algae supply ripples immediately through the whole population — because there is no other in the wild anywhere on Earth, much like the isolated and enigmatic Argentine legend of La Salamanca.
