<div style="text-align: left;">Carroll County Conservation hired a company to net large grass carp from Swan Lake late last month as part of its effort to improve water clarity. About 300 of the sterilized carp were put in the lake seven years ago when it was restocked. The carp are used to control aquatic plant growth, but they have grown so large &mdash; most are more than 20 pounds &mdash; that they are now a nuisance.</div>
Carroll County Conservation hired a company to net large grass carp from Swan Lake late last month as part of its effort to improve water clarity. About 300 of the sterilized carp were put in the lake seven years ago when it was restocked. The carp are used to control aquatic plant growth, but they have grown so large — most are more than 20 pounds — that they are now a nuisance.
Swan Lake’s water level will rise about 3 feet in the next few months, but it will remain more than 2 feet below normal for most of next year as county conservation officials extend a project to spur aquatic plant growth in the lake’s shallow fringes.
The project began in April when Carroll County Conservation workers siphoned and pumped enough water to drop the lake’s level by 4 feet. At its deepest point, the lake, which is just southeast of Carroll, is normally about 12 feet deep.
That drop exposed an estimated 20 acres of lake bottom where plants such as smartweed and yellow nut sedge took hold. The plants will help stop soil erosion — which clouds the lake — and take up nutrients in the water — which feed sudden, massive growths of algae, conservation Director Mark River has said.
The project mimics the cycle that happens with natural lakes, which rise and fall with rainfall and drought, Ben Wallace, a fisheries biologist for the Iowa Department of Natural Resources has said. Swan Lake is manmade, and its dam keeps the lake’s level from swinging wildly.
But this year’s widespread drought — with rainfall deficits of at least 10 inches in much of the state — pushed Swan Lake’s level down 2 1/2 feet beyond what was pumped and siphoned, River said. That drop exposed an estimated 10 more acres of lake bottom and made some of the shoreline too dry for some aquatic plants to flourish.
The upside to the substantial drop in water level was that conservation workers were able to lay strips of fabric on lake-bottom on the east side of Swan Lake and cover it with tons of pea gravel to make new spawning beds for small fish.
River said his department began this month pumping water back into the lake to avoid a potential winter fish kill.
“We worried that if we’re six to 61/2 feet low, then we only have 6 feet of water left in the deepest part of the lake,” River said this week. “I’ve seen ice on the lake 18 inches to two-feet-deep. That wouldn’t leave much room for fish.”
River estimates that about 300 gallons of water are being pumped every minute into the lake from a well just south of the Carroll Country Club, a little more than a mile away. A nearby resident, Ed Tomka, worries that the move might cause the water level in his own well — about a half-mile away — to drop to the point that he can’t get water to his house.
“I wish there was more eyes on it, or more gauges, to see how it is affecting the general aquifer,” he said.
Carroll County Conservation gained a state permit to pump the water into Swan Lake. River said a state geologist who examined the issue thought the pumping would not likely cause problems for nearby wells.
“I can understand Ed’s perspective — he’s trying to protect his water supply,” River said. “But if it was going to impact his well, it should have happened already.”
Conservation workers will keep the lake 2 to 3 feet shallower than normal for much of next year to further encourage plant growth along the lake’s perimeter.
Wallace has said the wetland plants could solve another problem at Swan Lake: few fish can spawn effectively. He said the plants could provide enough shelter for young fish to survive the full-grown predatory fish, such as crappie and largemouth bass.
County conservation officials have also used the lake’s drawdown to rid the lake of some of the large grass carp that muddy its water. They hired a commercial fish harvester to catch the carp with nets late last month. They removed 40 of the fish.
About 300 carp were put in the lake to keep aquatic plants at bay, but they’ve grown so large — many of them are 20 pounds or more — that they are now a nuisance. It’s unclear how many of them remain in the lake. River has encouraged residents to catch the fish as well. He offered free camping passes to anglers who caught carp this summer, but only three did. He said the commercial harvester might return this winter to remove more of the carp.
The last time conservation workers siphoned water from the lake was in 2004, when they dropped the depth by 2 feet and poisoned river fish that found their way to the lake during flooding. They restocked the lake with fish starting in 2005.