A St. Croix River menace that's waged war on the healthiest native mussel population in Minnesota has retreated, bringing hope of a turning point in what had seemed a relentless invasion.

The fingernail-sized zebra mussels that crept into the river's wider, deeper pool known as Lake St. Croix more than a decade ago diminished in significant numbers at eight monitoring locations last year, said Byron Karns, a National Park Service biologist who issued a report this week detailing his findings.

"When we went out in August to do these collections, there weren't any [zebra] mussels to count anymore," said Karns, who had noticed changes since 2006 in the 25-mile stretch of river between Stillwater and Prescott, Wis. "It suggests at some point later in the summer something was happening that was wiping out a whole class of mussels."

Higher, turbulent water that persisted into August might have flushed zebra mussels out of the St. Croix last year, Karns said. A carpet of crushed shells on the river bottom also suggests that common carp have been feasting on them.

Zebra mussels represent a significant threat to the St. Croix and other Minnesota rivers and lakes because they multiply rapidly and attach themselves to native mussels, killing them and altering ecology and food chains. Their hard shells also cut swimmers' feet and fishing lines and inundate docks.

Found now throughout the Great Lakes and Mississippi regions, they have rapidly spread to a number of state lakes in recent years, their microscopic larvae transported in bait buckets and bilges.

The dozens of native species in the St. Croix -- five of them endangered -- play an important role in the river's ecology because they filter water and are eaten by fish, ducks and otters. Because of their long lifespans -- 70 to 80 years -- native mussels give scientists a critical measurement of the river's health over time.

"They're sort of the coral reefs of the healthy water systems," said Rich Baker, endangered species coordinator at the Minnesota Department of Natural Resources (DNR). More than half of the state's 50 mussel species now are classified as endangered, he said.

Zebra mussels began appearing in the St. Croix in 2000 when they were found attached to boats.

The 2011 findings were the first to show a reversal of the zebra mussel's invasion of the St. Croix, which Baker said has the healthiest natural mussel population in Minnesota.

"We still need to find out what's driving the dynamic, but anything about fewer numbers is good news," Baker said of Karns' findings.

Karns cautioned that the apparent demise of zebra mussels in the St. Croix needs further study.

"I don't want to suggest for a moment that zebra mussels have gone away," he said. "Populations of any kind are going to vacillate over time. Nothing stays constant."

Karns looks forward to seeing the monitoring results from this year, a low-water year, to see if the mussels make a comeback.

None have been found north of the Stillwater Lift Bridge but like the more troublesome Asian carp, the invasive zebra mussel finds ways to defy scientists, said Karns, who prepared the study for the U.S. Corps of Army Engineers in St. Paul. He works out of St. Croix National Scenic Riverway headquarters in St. Croix Falls, Wis.

Zebra mussels aren't the only species under study in the St. Croix River. Three more species -- sheepnose, snuffbox and spectaclecase -- were added to the federal endangered list in recent weeks. The Higgins eye and winged mapleleaf have been on the list for years.

A new survey, beginning after Memorial Day, will determine which natural mussels might lie in the path of a new St. Croix bridge just south of Stillwater.

Baker said the DNR, on behalf of the Minnesota Department of Transportation, will finish that work in June.

"We don't know what is going to be found specifically in the footprint of the bridge project," he said. "We don't move mussels unless we need to."

Jason Alcott, a natural resources program specialist at the Minnesota Department of Transportation, said survey results should be available in late June or early July.

The agency will discuss findings with the DNR and the U.S. Fish and Wildlife Service before deciding how to proceed, Alcott said.

Earlier studies of mussel populations need updating, he said, because "it's an ever-changing environment out there."

Kevin Giles • 651-925-5037 Twitter: @stribgiles