NEW ULM, Minn. (AP) -- The odor of rotting fish on lakes and ponds around Minnesota this spring might be the smell of success.
Spawned by a long, cold winter and heavy snowfall, many of the state's shallow lakes, ponds and wetlands have experienced winterkill, meaning mountains of carp, bullheads and other undesirable rough fish and minnows have died.
But in several cases, nature's wrath has been boosted by a new technique used by the Minnesota Department of Natural Resources this winter for the first time to promote winterkill. It could be an important new weapon in what has been an almost futile battle against carp and other undesirable fish.
The rough fish demise is great news for waterfowl and game fish enthusiasts and DNR managers. Rough fish -- particularly carp -- have overrun many of the state's shallow lakes, causing turbid waters, poor vegetation and degraded habitat for game fish and waterfowl.
The winterkill could dramatically improve water conditions and waterfowl habitat in many shallow lakes and ponds, including some of the state's key waterfowl lakes, such as Christina and Heron.
''We should see a substantial increase in habitat quality,'' said Ray Norgaard, DNR wildlife wetland program coordinator. ''This winter has been very tough for fish in those shallow basins. We haven't had this much snow on the ground this late since 1965.''
Heavy rains this spring could negate the expected improvements. And even without high water, officials say the improved conditions won't last forever. Carp are pervasive, and their introduction to the U.S. at the turn of the last century has proven to be an environmental disaster.
The fish uproot vegetation on lake bottoms, stirring up sediment. That prevents sunlight from penetrating the water, killing plants such as cattails, bullrushes and sago pondweed -- key habitat and food for waterfowl. The lack of vegetation then allows wind and waves to further stir up sediments, compounding the problem.
The end result: a lake clouded with algae, full of rough fish with little vegetation. Game fish and waterfowl are sparse. That describes Lake Hanska, a 1,800-acre shallow prairie lake southwest of New Ulm. Once known for its waterfowl hunting as well as northern, walleye and bluegill fishing, the lake has been on a downhill slide for years, exacerbated by high numbers of carp and bullheads.
The same thing happened in the 1980s. The lake was treated with the chemical rotenone at a cost of $125,000 to kill all the fish, which allowed officials to start over and restock it with game fish. Water quality immediately improved, as did vegetation, said Norm Haukos, DNR fisheries official in New Ulm.
''Angling became phenomenal,'' he said. And waterfowl conditions were excellent. ''Water quality is the key to both fisheries and wildlife,'' Haukos said.
But as is often the case, the carp and bullheads eventually regained their stranglehold. The DNR recently began considering another rotenone treatment, estimated to cost $300,000 to $500,000. But the cold and snow this season allowed officials to try something new at a fraction of that cost without the use of sometimes controversial chemicals: reverse aeration.
The DNR sometimes uses aeration pumps to prevent lakes from suffering winterkill. But they also knew that those aerators, if incorrectly used, could cause a dieoff. Haukos and Norgaard decided to experiment this winter by using aerators to deliberately boost winterkill in Lake Hanska.
''We felt we had nothing to lose,'' said Haukos.
In winter, ice and snow prevent sunlight from reaching oxygen-producing plants in lakes, which reduces the amount of oxygen in the water. When oxygen levels get too low, fish die. Some shallow lakes frequently suffer partial die-offs. But the oxygen levels generally vary throughout a lake: deep holes retain oxygen, so fish congregate in those ''refuges.''
The idea was to use aerators to stir up the water, mixing oxygen-depleted water with those pockets that still contained enough oxygen to sustain fish. Chemical reactions would cause overall oxygen levels to drop, killing off the remaining fish. The DNR lowered the water level of Hanska, and the early cold and snow greatly reduced oxygen levels. In January, workers punched holes in the ice and used two specially built aerators, costing about $2,000 each, to give Mother Nature a boost.
Almost immediately, oxygen levels plunged and dead fish -- lots of dead fish -- surfaced in a small patch of open water created by the water movement.
''We have a pretty good kill out there,'' Haukos said. ''It's pretty exciting. It shows a lot of promise.''
He expects to see many more dead fish when ice leaves the lake. Haukos said the fish should quickly decompose this spring. The lake will be restocked with northerns, walleyes, bass and panfish.
While the initial results appear impressive, Haukos said he won't know how successful the experiment was until later this summer when nets are placed in the lake to determine how many fish, if any, survived.
But the method is so promising it's already been repeated this winter on several smaller ponds, including some the DNR uses to raise walleyes.
If it works, Norgaard and Haukos said more aerators likely will be purchased and reverse aeration could become commonplace in lakes and wetlands around the state to help control undesirable fish and improve habitat for both fish and wildlife.
''We still have a lot to learn,'' said Haukos. ''But it's definitely promising.''
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