State officials began advising boaters last March on how to prevent the spread of zebra mussels into Lake Powell — but two tests taken in July have shown that the microscopic larvae of the invasive species have shown up near the Glen Canyon Dam, wildlife officers said Thursday.

"Finding them was like finding a needle in a haystack," said Larry Dalton, aquatic nuisance species coordinator for the Utah Division of Wildlife Resources. "There probably is not an established population, because there are so few veligers (larvae)."

As of Thursday, divers had searched the submerged portions of about 150 out of over 1,000 boats that are in slips or tied to moors in the Wahweap Marina of Lake Powell. They're looking for signs of a breeding pair of adults or an actual population of mature mussels.

The big question right now, Dalton said, is whether any of the extremely fragile larvae have found a "suitable" spot to attach themselves in the lake.

"While the test results indicate the presence of individual larval quagga or zebra mussels in Lake Powell, much uncertainty remains," said David Britton of the U.S. Fish and Wildlife Service. "We do not know at this point if an established population is present."

The quagga mussel is considered a subspecies of the zebra, which was first detected in the Great Lakes area about 20 years ago, supposedly after a large oceangoing ship coming from Europe entered the waters there.

Dalton said that at Lake Powell, wildlife officers appear to have found evidence of mussels early in the game, meaning that if the mussels manage to mature and spread, state officials can give plenty of warning to fisheries and other watersheds to protect other areas.

Once established, mussels can reproduce quickly and voluminously, consume large amounts of food and even reduce the amount of oxygen in the water, all of which can contribute to a decline in the fish population of an infested area.

It's not known how long the larvae have been in Lake Powell waters, how many there might be or whether any have grown into full-size mussels. One fear is that, once matured and in large enough quantities, these freshwater mollusks, which are relatives of the clam and oyster families, can clog intake valves on reservoirs, such as Lake Powell.

Visual monitoring for mussels in Lake Powell began in 1999. On July 19 and 30 of this year, state and federal officials detected three mussel larvae at the Wahweap Marina and close to the Glen Canyon Dam.