The Moapa dace has spent more than 40 years on the endangered species list. Its entire habitat has been in the protective custody of the federal government and the Southern Nevada Water Authority since 2007.
So where have all the fish gone?
Two years ago, biologists counted 1,172 dace in the spring pools and streams at the headwaters of the Muddy River, 60 miles north of Las Vegas. A count last week revealed a population of just 462, prompting fears of extinction for the tiny fish found nowhere else in the world.
No definitive cause for the decline has been determined, but one federal official said recent efforts to protect the fish might have killed them instead.
Restoration crews have been rebuilding stream channels and tearing out non-native vegetation, including some of the imported palm trees that now surround the Warm Springs area where dace are concentrated. That work may have disrupted the dace's spawning and forced the fish into areas where they can't thrive.
"These are short-term impacts for long-term gains," said Bob Williams, state supervisor for the U.S. Fish and Wildlife Service. "We believe with the restoration the numbers will rebound."
The real decline came sometime in 2007, when more than 60 percent of the population was lost, leading to a count last February of 459 dace.
Until then, the population had never been known to fall below 900.
In 1994, the area was home to some 3,825 dace, at least until a fire in the palm groves that summer dropped smoldering fronds into the water and wiped out about half the population.
When the recent drop in population was discovered last year, it triggered a review of the habitat restoration work. Officials also decided to begin counting the fish twice a year instead of once.
But some are not so quick to blame the restoration effort.
Janet Monaco, who heads the water authority's Muddy and Virgin River Division, said it is unclear what caused so many dace to disappear in such a short time.
"I would say we don't know. We don't know, but the good news is we've had three counts where the number has held steady," Monaco said.
The counts Monaco is referring to are the ones from February 2008, August 2008 and last week, all of which peg the population at between 459 and 462 fish. That suggests something happened to the dace, but it isn't happening anymore, Monaco said.
The counts aren't exact, but they are about as thorough and accurate as conditions allow, said Cailin Doyle, a biologist for the water authority.
She should know. She was part of the team that fanned out across the refuge and surrounding property late last week to count fish in two-person teams, one person facedown in the water and the other on shore with a clipboard.
Doyle was the one who got wet. In some places, she had to crawl through water barely deep enough to get her snorkeling mask halfway under the surface. In others, she had to scramble over sunken logs and search for dace among thick clumps of plants. It took her about four hours to cover less than a half mile of stream.
"You have to get into all of the nooks and crannies to count these fish," she said.
Williams said no study has been done to determine the minimum number of Moapa dace needed for the species to remain genetically viable.
This much is for sure: Those in charge of protecting these fish have only one chance to get it right. Unlike other protected species of desert fish, there is no backup population of dace being kept in a secluded tank somewhere, Williams said.
Efforts to breed dace in captivity have failed because the fish needs specific conditions that are hard to duplicate, namely a flowing stream ranging in temperature from about 90 degrees for larval fish to the low 80s preferred by the finger-length adults.
The dace's natural habitat is confined within the 117-acre Moapa Valley National Wildlife Refuge and the neighboring Warm Springs Natural Area, a 2,000-acre tract the water authority acquired in September 2007.
The primary threats to the dace are habitat loss and the invasion of tilapia, a non-native game fish that found its way up the Muddy River from Lake Mead in the early 1990s. Tilapia are known to eat both the dace and their food.
"It's a competitor in every sense of the word," Williams said.
Beavers also have posed a problem over the past year. Two of the tenacious dam builders had to be trapped and killed in recent months to halt the destruction of stream habitat.
The Moapa dace is expected to remain under federal protection until at least 75 percent of its historical habitat has been restored and its population is holding steady -- at least 6,000 adult fish.
Williams is confident the situation will improve.
"Conditions out there are better now than they were last year. We would hope to see the numbers up a couple hundred" by the next year's February count, he said. "We want this to be the low point."
Others are not optimistic.
Two days before the start of last week's annual fish count, the Tucson, Ariz.-based Center for Biological Diversity announced plans to sue the federal government over its protection of the dace.
The latest numbers only bolster the center's warnings against large-scale groundwater pumping planned in the area by the water authority and others, said Randy Serraglio, conservation advocate for the environmental group.
If the dace are this vulnerable before the pumps are even turned on, he said, "the proposed development could be the death knell for the species."
Contact reporter Henry Brean at email@example.com or 702-383-0350.