Yet a biologist's version of a red alert remains in place, officials said Wednesday.

That's because samples of the lake taken from July 19 to Aug. 6 found enough larval quaggas to convince state scientists that at least one breeding pair exists in the lake - and that could trigger an infestation serious enough to choke the Glen Canyon Dam's hydroelectric turbines, kill boat recreation and shut down agricultural irrigation.

The latest negative results have created a quandary: the National Park Service's Glen Canyon National Recreation Area and Arizona and Utah wildlife agencies must keep the public informed about the real threat quaggas pose without giving the impression that it's too late to do anything to stop them.

"My message is, there's a potential for [quagga] mussels in Lake Powell, and that potential could exist for the rest of the state," said Wayne Gustaveson, a Utah state biologist in Page, Ariz. "We can't ever let the guard down. We have to be ever-vigilant."

On Aug. 9, the Glen Canyon National Recreation Area, the U.S. Fish and Wildlife Service, the Arizona Fish and Game Department and the Utah Division of Wildlife Resources announced they found microscopic quagga larvae in water samples taken from Wahweap Marina and near the Glen Canyon Dam.

The finding shocked the agencies but didn't surprise them. On Jan. 6, Nevada officials said adult quaggas had infested Lake Mead, the first confirmation of the invasive non-native species in the West.

In June, quaggas turned up on a boat at Lake Powell's Wahweap Marina. The boat had transported the mussels from infested waters, but was decontaminated before it was allowed to launch, a procedure that is now standard for all boats that have been in water recently.

Government scientists in Page who examined 20 water samples taken in July and August in different sections of the lake found 53 quagga larvae, called veligers. They sent the samples to the U.S. Bureau of Reclamation in Denver, where lab tests using DNA fingerprint technology confirmed the positive findings.

The samples were "barely at the level of detection," which brings some hope, Gustaveson said. If a breeding couple were actively spawning, tests would have shown 5,000 to 10,000 veligers per sample, he said.

The Park Service also decided to send their samples to a lab at Portland State University in Oregon for re-testing. Those results aren't expected for at least another two weeks, said Larry Dalton, aquatic nuisance species coordinator for the Utah Division of Wildlife Resources.

Kevin Schneider, spokesman for the Glen Canyon National Recreation Area, said it's possible the microscopic larvae taken from Lake Powell could have been dead before they got into the water.

But Dalton said testing indicated the veligers were two to three weeks old.

"When you find veligers, they didn't blow there on the wind. Whether they were alive when we collected them is probably irrelevant," he said. "We believe a boat with live mussels launched into Powell."

If so, they've probably dropped off the boat into deeper, cooler water. In Wahweap, that means they could be hiding out 240 feet below the surface - beyond the reach of divers and the testing equipment available to the Park Service, Gustaveson said.

Sampling is ongoing two or three times a week with a plankton net that can reach the bottom and a robot camera that the Park Service uses to search for drowning victims is trolling a 100-square-foot area of Wahweap Marina. Since Aug. 14, the samples have all been negative.

"We still haven't seen any adult mussels," Gustaveson said. "It's hopeful. Basically, we're still searching and we don't know."