"We're feeling like we're at least maintaining the population. There is still lots and lots of work to do," he said about the endangered Devil's Hole pupfish that exist in the wild only around a shelf in the water-filled limestone cave, 90 miles northwest of Las Vegas.

For unknown reasons the species began steadily declining in about 1995 after historically numbering about 500 since the mid-1970s. They reached a low of 38 fish in the spring of 2006.

But the population, which is in a part of Death Valley National Park within Ash Meadows National Wildlife Refuge, took a sharp turn for the worse in September 2004. That's when a flash flood sent a tub of glass fish traps tumbling into the hole, killing about one-third of the pupfish population at the time, 80 in all. The traps were being used by Southern Oregon University researchers to assess pupfish reproduction.

Fall pupfish surveys by divers in 2006 and 2007 turned up 85 and 92 fish, respectively, indicating the efforts by biologists to increase the survival rate during winter months might be working.

Williams said biologists focused on increasing nutrients in the pupfish diets during the winter using an artificial food developed at the service's Bozeman Fish Technology Center in Montana.

In addition, floating baskets were used to protect pupfish larvae from predation. Adult pupfish eat eggs and young fish.

"We'll be vigilant about monitoring the population over the winter," Williams said.

Hopefully, the methods will increase the numbers to the point that they double "in the next year or two," he said. If so, that would allow biologists to explore breeding techniques away from Devil's Hole.

"The higher the population, the more opportunity we'll have to do other things," he said.

Hybrid pupfish that were being raised at Willow Beach National Fish Hatchery on the Arizona side of Lake Mohave have been moved to Tucson for researchers there to study propagation of the fish.

Some hybrid pupfish also are kept at Mandalay Bay in Las Vegas.