The spring count of Devils Hole pupfish has found the highest population in a quarter-century/USFWS file

Numbers of Devils Hole pupfish in a tiny crater that is part of Death Valley National Park have reached the highest spring count in 25 years, according to National Park Service and U.S. Fish and Wildlife Service biologists.

Scientists this month counted 191 pupfish, which are critically endangered, the park reported. That number could point to a boom in the population by the time the next count is made this fall.

"This spring count was 191. So typically, historical population dynamics with Devils Hole is you have about a doubling of the population through spring and summer, and then through the winter when it's food limited, that population declines by 50 percent," Kevin Wilson, the national park's aquatic ecologist, said Monday during a phone call. 

Back in the fall of 2022 the tally was 263 pupfish, so if historical trends hold, the population could approach 400 individuals by this fall.

Devils Hole pupfish (Cyprinodon diabolis) live in the smallest habitat of any vertebrate species on the planet. Devils Hole is a deep, water-filled cavern located near Ash Meadows National Wildlife Refuge in Nye County, Nevada. The fish live in the upper 80 feet of the 400-foot-deep cave and depend on the 11-foot-by-16-foot sun-lit shallow shelf at the cavern’s entrance for food and spawning.

The remnants of Hurricane Hilary affected Devils Hole in August 2023. Flooding can have negative effects on the fish in the short-term. However, Hilary mostly benefited the fishes’ ecosystem by adding nutrients that washed off the surrounding land surface in a fine layer of clay and silt. 

Devils Hole pupfish are counted every spring and fall, using standard protocols. This year’s spring count was done April 6-7. Scientists used SCUBA to dive as deep as 100 feet to count fish. As the same time, other scientists count fish on the shallow shelf at the water surface.

Before the mid-1990s, scientists counted about 200-250 Devils Hole pupfish each spring. For about 20 years, the population dropped to an average of around 90 fish, with an all-time low of 35 fish in 2013.

Population counts in recent years have been higher. The 191 observable pupfish counted this month is the highest spring count recorded since March 1999. 

“It’s exciting to see an increasing trend, especially in this highly variable population," said Michael Schwemm, a senior fish biologist for the U.S. Fish and Wildlife Service. "Increasing numbers allow the managing agencies to consider research that may not have been possible in the past, when even slight perturbations of habitat or fish had to be completely avoided. We’re excited about the future directions with respect managing this species.”

A return to higher numbers of pupfish this time of year could signal important changes in the ecosystem. In a park press release Wilson said that “[R]ecent high spring and fall counts show the importance of maintaining long-term data as we work to find out what’s changed.”

Brandon Senger, supervising fisheries biologist for the Nevada Department of Wildlife, has been conducting SCUBA-counts at Devils Hole since 2014. "It was really encouraging to see such a large number of young fish during these spring dives. Conditions within Devils Hole looked healthy, so we have hopes of high recruitment over the coming months that will lead to a large population in the fall," he said.

Other biologists on-site noted fish appeared in remarkable condition and were very active. Many courting and spawning pairs of pupfish were seen during the count. 

Devils Hole pupfish are jointly managed by U.S. Fish and Wildlife Service, Nevada Department of Wildlife, and National Park Service staff.