World's Rarest Fish Population Rebounds


– Olin Feuerbacher/USFWS

The Devils Hole pupfish

The rarest fish in the world lives an hour and a half from Las Vegas, in a water-filled cavern called the Devils Hole. 


And for decades, scientists had counted about 200 Devils Hole pupfish in the spring, and more than double in the fall. 


Then in 2006, disaster: For unknown reasons, the population plummeted to a mere 38. 


But now, it’s all changed again for the tiny blue fish.

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“This last count that we’re talking about today was really a pretty momentous occasion that getting up to 187 was a lot more than we had expected,” said Michael Schwemm, senior fish biologist with the U.S. Fish and Wildlife Service.

While the increase in population is something biologists are happy about, they're still not sure why the population declined to begin with and why it rebounded.

The fish became trapped in the water-filled cavern when the climate began to dry out at the end of the Pleistocene Era. Pupfish populations became stuck in several different springs and small bodies of water in the Amargosa Valley, Schwemm said.

The Devils Hole pupfish became isolated in a particularly inhospitable pool of water. The water is about 93 degrees all the time and the cavern is small. In fact, it is one of the smallest natural ranges for any known vertebrate. 

Because of that complicated life history, breeding the pupfish in captivity has been difficult, Schwemm said.

Biologists would like to see the population return to what it was at in the 70s and 80s but they're not entirely sure that will happen.

“We would certainly like to see a population that was somewhere in that vicinity; however, that doesn’t seem like that is even a possibility in the next few years,” Schwemm said.

Schwemm would like to have pupfish at a second location as a backup in case the natural population drops again. 

“We certainly don’t want to cause the fish to go extinct on our watch,” he said.

Since they don't know what caused the decline or the recovery, Schwemm said all biologists can do is continue efforts to breed the pupfish in captivity, which is currently going on at the Ash Meadows Fish Conservation facility and study other pupfish in the area to see how they are managing to survive.


Michael Schwemm, senior fish biologist, US Fish and Wildlife Service

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