The mystery of Death Valley's missing pupfish

 

Updated 3/12/12 3:01 p.m.

The Devil's Hole pupfish is arguably one of the cooler species around. These tiny iridescent blue fish, just a bit over an inch long, live in one place only, a deep pool in the Amargosa Valley of west Nevada, in a place called Ash Meadows, an outpost of California's Death Valley National Park. The pool, which is about 8 feet wide and 35 feet long, appears a clear and light blue where a flat rock shelf runs just below its surface, deepening to dark turquoise where the shelf drops off. No one knows how deep the pool is; divers have gone 435 down and not hit bottom. "I like to think of Devil's Hole, when you're looking at it, as a window into the aquifer," says Kevin Wilson, an ecologist and manager of Death Valley's Devil's Hole program, which is jointly run by the U.S. Fish and Wildlife Service, the Nevada Department of Wildlife, and the National Park Service.

The water body is not what biologists would consider a welcoming habitat. Its average temperature is 93 degrees, and since it is so still, lacking an outlet where moving water mixes with air, it is oxygen deprived, with about one-third as much oxygen as the average desert spring, says Wilson. Scientists have been keeping watch over these pupfish, one of the most imperiled species in the world, since before the Endangered Species Act was passed. One of the first threats was groundwater pumping from the aquifer feeding the hole in the Amargosa Valley; after a long court battle, the government secured senior water rights protecting the pupfish and their pool in 1976. When the ESA was passed in 1973, pupfish were one of the first species listed.

The Devil's Hole is protected by a chain link fence. Visitors to the site can only view the hole from inside a long steel cage, which starts inside the fencing and leads out to a viewing platform over the small canyon holding the pool.

But despite all this protection, in the 1990s, the pupfish population, which is counted each spring and fall and had hovered in the range of 200-500 individuals (there are always fewer in the spring, and more in the fall), began to dwindle. The counts kept getting slowly lower, year after year. Some years they'd bounce back up, but the general trend went down, down, down. And then, in the spring of 2006, biologists on their spring dive counted only 38 pupfish.

Naturally, people freaked out.

Wilson's graduate research in the late 1990s had looked at pupfish food availability in the Devil's Hole and found it sufficient.

But when scientists from the Devil's Hole program went and saw the fish that fall, said Wilson, who was not working in Death Valley at the time, "(they) looked emaciated. They didn't look healthy; the edges of their fins were kind of eroded. They looked thin."

In January 2007, the Park Service decided to take a fairly controversial action. They started feeding the pupfish with rations used in another program to feed an endangered species, the silvery minnow.

And as soon as researchers began adding that food, which looks a little like coffee grounds and is released daily into the pool through an automatic feeder pipe, the pupfish responded. The adults began to look healthier, and they laid more eggs.

"(That spring), when we started doing our larvae surveys, there were a bazillion!" says Wilson. "Well, not really, but 20 in a plot, as opposed to four. So we thought we found the holy grail."

By the fall, which is always when the higher numbers of fish are counted, Wilson's team was hoping for miraculous results. Instead, they counted just 92 pupfish.

The scientists realized that even though the adults were responding to the feeding and laying more eggs, the larvae were not surviving. And although the population has rebounded a bit, and hovers around 100 individuals now, Wilson and his colleagues are still working hard to determine why larvae aren't making it to adulthood.

The problems probably go back to that relatively inhospitable and highly specific habitat the pupfish occupy. It's estimated that Devil's Hole pupfish ended up in their hole sometime between 10,000 and 40,000 years ago, after a giant body of water called Lake Manly, that covered what is now Death Valley, gradually began to recede. As the climate became more arid and the lake shrank, populations of pupfish were segregated into remnant bodies of water. What's left of Lake Manly now forms an archipelago of underground water sources in the national park, where numerous springs and creeks play host to four distinct pupfish species; each evolved to fit their specific niche; one, the Salt Creek pupfish, lives in water four times as salty as ocean water.

In the case of the Devil's Hole pupfish, they've evolved to live at the edge of what is possible, but small changes to that environment might push that edge too far. Remember that shelf in the 435-foot-plus hole, covered by just a shallow bit of water? That shelf is key to pupfish survival. They lay eggs on it, the larvae hatch on it, and sediment that falls onto it adds organic matter and provides a nutrient source. But since it is so shallow, the summer sun makes the water temperatures on it rise. In recent years, the Park Service has documented a slight temperature increase on the shelf, from 93 degrees to 95. Wilson cautions that they haven't analyzed all their data yet, but this increase could be linked to global climate change, for which models generally predict the desert Southwest will become warmer and drier.

Devil's Hole pupfish on the shelf, with other pupfish below in the deep part of the pool.

Other factors the researchers are examining include changes in water chemistry, changes in algal communities that might impact the amount of dissolved oxygen available to fish (already low), or a phenomenon called "inbreeding depression," which basically means pupfish are not reproducing successfully because they lack the genetic diversity to avoid deadly mutations.

 Watch a USFWS video about the Death Valley pupfish

Last August, the Park Service began measuring these and other factors as part of a longterm ecosystem monitoring project. Wilson hopes this additional monitoring and the work other researchers are doing at the site will help the park improve its management of the species.

 "If this species was naturally going extinct, we should let it run its course," he says. "But if there are human influences or anthropogenic influences like there are, we should take some action."

Stephanie Paige Ogburn is the online editor at High Country News.

Images courtesy USFWS, National Park Service, and USGS.

Learn more about the Devil's Hole area by viewing some of the presentations from the 2011 Devil's Hole symposium.

The article has been corrected to note that Kevin Wilson was not working for the Park Service at the time of the pupfish population crash in 2006; he had left to pursue a doctorate degree by that time and later returned to work for the park, where he is now manager of the Devil's Hole program.