With ice still too thin to skate on, most of the action at Casey Lake in North St. Paul is bubbling beneath the surface.
City staff recently installed an aeration system at the lake, which is surrounded by sports courts, trails and a playground. Air is now being mechanically pumped into the water to keep up oxygen levels, helping native Minnesota fish stay alive over the winter to boost fishing and keep out the common carp.
Casey Lake used to be a breeding ground for the invasive species of scum-flinging bottom-feeders that kick up and spew out phosphorous, which fuels algal blooms that eventually decay and suck oxygen out of water bodies and smother other aquatic plants.
"That's why (Casey Lake) was so murky, and it couldn't sustain any kind of healthy, aquatic plants," said Keith Stachowski, a public works department employee.
Carp have spread throughout the watershed, mucking up the lakes, wetlands and streams in Washington and Ramsey counties, all the way down to Lake Phalen.
Common carp were eliminated from Casey Lake last winter, when the water was partially drained in November of 2012 to fix an outlet pipe, leaving virtually no spots for the fish to survive the freeze.
"We drew down Casey in the winter time, did the pipe repair and kind of a side benefit of that was the elimination of the carp in Casey," said Bill Bartodziej, a Ramsey Washington Metro Watershed District natural resources specialist. "All the carp died."
Bass and bluegills, which gobble up carp eggs, were released in the 12-acre lake this spring when it was refilled to normal depths of up to 3 1/2 feet. Casey doesn't qualify as a lake, according to the Minnesota Pollution Control Agency requirements.
"We get two benefits from stocking," Bartodziej said. "We have a nice fishery for the neighborhood. In addition, it kind of safeguards us from having that carp problem again in that wetland, which affects the entire chain of lakes."
The aeration system installed on Dec. 2 can help the fish survive in the ice-covered water, where oxygen can get fatally low.
"It enables those game fish to make it through the winter," said Bartodziej. "That's part of the puzzle."
The city has collaborated with the University of Minnesota's Sorensen Lab, the watershed district and the Minnesota Department of Natural Resources to snuff out the carp at one of the chain's sources.
Research started in 2009. U graduate student Justine Koch has seen a significant difference in Casey Lake since she started working on the project about two years ago.
"It was basically green, and there was no water clarity and there were no aquatic plants, aside from cattails," she said. "It's, I don't know how many folds better, but you can see the bottom of the lake. It's a whole different lake."
Common carp, which have large scales and whiskers, are native to Europe and Asia, according to the DNR. The species was introduced to the Midwest in the 1880s.
The species was purposefully stocked in waters with the idea that it would be a good game fish, and has since wreaked havoc on shallow lakes and wetlands.
While the participating entities put in great effort to get rid of the species locally, there are some simple things the DNR suggests to keep carp from being introduced -- or reintroduced -- into a water feature.
• Throw away unused fish bait in a trash can, rather than into the water.
• After catching carp, don't release them into new waters.
Casey Lake is key
Thousands of carp used to spawn in the spring at Casey Lake. Some would then swim from the top of the watershed through an outlet to another small wetland, wriggle through a creek and settle in Markham Pond in Maplewood.
They can then course through the watershed to Kohlmann Lake, Lake Gervais, Keller Lake and Lake Phalen, which connects to the Mississippi River.
"There's a potential of having those thousands and thousands of carp move from Casey downstream and eventually make it to the Phalen chain of lakes," Bartodziej said.
Working on her master's degree, Koch has helped capture and remove from the chain nearly 4,000 carp, according to the watershed district's count this spring, including fish as old as 60 and ones measuring nearly 3 feet long. The carp population in Casey Lake tended to be younger and smaller, due to the shallowness of the water body, according to Koch. The oldest caught was 18 years old.
Casey Lake was a nursery for carp, according to Koch. Key word: was.
"We've essentially gotten rid of one of the primary sources of carp to downstream waters," she said. "We've met our goal to reduce the carp population to where we want it."
Bartodziej said the project has helped cut down the carp population in the Phalen chain of lakes by about 50 percent.
"We reduced it to a manageable level," he said. "We don't want these young carp to re-infest the Phalen chain. That's why Casey is key in that management over time."
Clean water locally is good news for North St. Paul's neighbors connected to the watershed, including Oakdale, Maplewood, Little Canada and St. Paul.
"The cleaner water we can put downstream, the better off we are," Stachowski said. "Once it leaves our city, we'd like to have it as clean as possible."
Stachowski said that the city wouldn't have been able to do the project on its own.
"Partnerships are always the best way to attack these kinds of problems," he said.
On thin ice
The Minnesota Department of Natural Resources recently issued a warning, asking people to be cautious on lakes covered in ice, particularly water bodies with aeration systems. Aerators cause currents that weaken ice.
As of March, researchers have removed nearly 4,000 carp from the Phalen chain of lakes in the past few years. That's an estimated 50 percent of the chain's overall carp population.
Oldest carp found in Phalen chain of lakes: 60 years old
Longest carp found: Nearly 3 feet long
Oldest carp found in Casey Lake: About 18 years old