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The lone, mysterious zebra mussel that turned up in Lake Harriet last fall now resides in a freezer belonging to the Minneapolis Park and Recreation Board.

Debra Pilger, whose career is dedicated to protecting the City of Lakes from aquatic invasive species and other environmental harm, remembers the shudder she felt when the 2½-inch-long critter was discovered Sept. 8 on a discarded, submerged sailboat cover. Similar discoveries in more than 100 other Minnesota lakes have been forerunners to dreadful infestations.

"It was at the end of the day and they brought it to the office,'' Pilger recalled. "It was a depressing Friday.''

Cleanup volunteers from a local scuba club found the boat cover, along with other trash, in shallows just east of the Lake Harriet Bandshell. A city watercraft inspector checked over the junk and sounded an alarm that's still reverberating.

In an interview last week, Pilger said her division of the Park and Recreation Board will intensify efforts to stop zebra mussels. Under a beefed-up action plan first written seven years ago after Lake Minnetonka became infested, the city will search more often and more extensively for zebra mussels. Crews also will screen the water periodically for microscopic larvae of the mussels.

Channels connect Lake Harriet to Lake Calhoun and Lake of the Isles. If zebra mussels took hold in any one of those lakes, public recreation and the overall ecology of those waters would be greatly devalued, Pilger said. The mussels multiply rapidly by millions and billions and their sharp shells cut into the feet of waders and swimmers. The striped mollusks upset the food balance in lakes by eating huge amounts of plankton otherwise available for fish to eat. Zebra mussels filter massive amounts of water, but subsequent gains in water clarity can be offset by stinky mats of algae that the mussels are capable of producing.

"There's no great solutions if they get in,'' Pilger said.

Keegan Lund, aquatic invasive species specialist for the Minnesota Department of Natural Resources, said the finding of a lone zebra mussel in Lake Harriet last year kicked off the largest-ever search for zebra mussels in Minnesota for detection purposes. Scuba divers, snorkelers and walkers turned over rocks, inspected aquatic plants, looked at boat hulls and checked all 200 mooring lines in the lake. They found zilch.

"It's a hard one to guess about,'' Lund said. "It's a mystery why we only found one."

Chances are good that the mussel entered Lake Harriet on some kind of watercraft even though the city maintains one of the tougher aquatic invasive species inspection programs in the state, Lund said. At Harriet's only public boat launch, an inspector is in place 100 percent of the time that the access is ungated. According to Park Board data, about 3,000 watercraft are inspected annually at the site. Fishing boats comprise nearly 50 percent of the traffic, while a quarter of the inspections are on sailboats.

Urban fishery

Minneapolis spends at least $180,000 to inspect watercraft entering its lakes in hopes of stopping zebra mussels, spiny waterfleas, starry stonewort algae, milfoil and other threats to water ecology. Part of what's at stake in Lake Harriet is the health of an active fishery that's managed and stocked yearly by the DNR.

Encircled by a 2.7-mile walking and biking path, Lake Harriet has a dynamic bottom with holes as deep as 87 feet and contours ranging from 10 feet deep to 60 and 70 feet deep. Harriet offers ample shore-fishing space and two fishing piers. Bluegills and other sunfish are most abundant in the lake, but DNR officials each year stock Harriet with walleye fingerlings, yearlings, adults or a combination.

According to DNR records, 18,000 walleye fingerlings, 2,800 walleye yearlings and 39 adult walleyes have been added to the lake in the past decade. A 2014 netting survey by the DNR found relatively low walleye abundance, but the lake is scheduled to be surveyed again in 2019.

Since 2008, DNR also has stocked Lake Harriet with 619 muskellunge fingerlings. The lake's naturally occurring northern pike population supports a catch rate that is better than the median for lakes of its class. Of the 55 pike caught in gill nets in the 2014 survey, 38 percent were 28 inches or longer, and the largest northern was 38 inches in length. The largest walleye in 2014's survey measured 27.6 inches.

Lund said the DNR will follow the park board's lead in its continued monitoring of Lake Harriet. Pilger said sequenced, periodic searches will be a must this spring, summer and fall. Of Minnesota's 11,842 lakes, more than 250 are now listed as infested with zebra mussels.

The voracious mussels have been confirmed in more than 125 lakes, rivers and wetlands. Another group of water bodies are listed as infested simply because they are hopelessly connected to or surrounded by confirmed populations.

For Lake Harriet, there's a chance the lake will be de-listed if no more mussels are found during intensive monitoring over five years. But Pilger said the city can never rest easy.

"These are actually great urban lakes,'' Pilger said. "Our job is to permanently protect and preserve.''

Just two months ago, the DNR confirmed zebra mussels in Medicine Lake in Hennepin County. The discovery followed six years of reports of zebra mussels in Medicine Lake that turned out to be negative.

This time, a lakeshore property owner reported the discovery of a single adult zebra mussel on a dock being removed from the lake for the season. DNR confirmed two more zebra mussels attached to docks at separate locations, indicating a lake-wide presence.

"After at least six years of reports of zebra mussels on Medicine Lake that turned out to be negative, it's disappointing to make this confirmation," Lund said in a news release on Nov. 9.