Carp roundup is a net gain for Minnesota's lakes

Talk about a great day of ice fishing.

Capitalizing on a quirk of nature -- carp congregate when it's cold -- University of Minnesota biologists pulled more than 3,000 of the unwelcome bottom feeders, some of them as old as 50 years, out of Chanhassen's Lake Susan on Monday.

Working under steadily pelting snowflakes, six commercial fishermen and 10 researchers located the cache of carp under the frozen surface by tracking radio tags placed on some of the fish during the summer.

A 2,000-foot skein net surrounded the fish under the ice as two tractors pulled the catch toward a 15-foot hole where the fish were scooped out onto a conveyer belt to be weighed, measured and counted.

"We probably caught 90 percent of the fish in that lake," said biologist Peter Sorensen of the university's Department of Fisheries, Wildlife and Conservation Biology. "This is an effort to remove the vast majority of the carp to see if we can improve the water quality of the lake."

And, he added, "A lot of these females being removed are just fat with eggs."

The netting marked a milestone in an ongoing U study searching for ways to eliminate destructive carp populations, which dirty up the waters where they are found around Minnesota. The study has focused on three west-suburban lakes: Lake Susan in Chanhassen and Lake Riley and Rice Marsh Lake, both of which straddle the Eden Prairie-Chanhassen city line.

The striking thing about Monday's catch, said Sorensen, was that "in that net there was almost nothing but carp -- 95 percent of the fish were carp. They were all large fish."

Game fish were thrown back, but the carp will be sold for food or used as compost at the university's Minnesota Landscape Arboretum.

Removing carp from lakes is nothing new. The key is keeping them from coming back, and that requires suppressing the birth of new fish and blocking the migration of adults between lakes, Sorensen said.

The researchers at lake Susan have made a key discovery, which raises hope for success: although some of the carp are 50 years old, young fish are scarce; the research team believes their numbers have been reduced by a predator, but they are not sure what that predator might be.

That means that if the older fish are removed, they may not be replaced by younger fish. (Age of the fish is determined by slicing open their ear bones to count their growth rings.)

With the carp out, Sorensen's team will track the water quality for the next few years and experiment with barriers to prevent migration -- carp tend to move from lake to lake along streams and even drainage ditches. Success will depend upon suppressing the birth of young carp and blocking the migration of fish among the lakes, he said.

Sorensen, who distinguished himself by finding a way to reduce the number of sea lampreys in the Great Lakes, is using similar techniques in the study of carp.

Two-thirds of all Minnesota lakes and all metro-area lakes are infested with the common carp, and they are a national problem as well, Sorensen said.

Carp make clear waters murky by churning up clouds of sediment in their search for food. They can dig a foot into the muck at the bottom of a lake, uprooting good plants and stirring phosphorus on the lake floor back into the water, where it fertilizes algae and weeds. Their own excretions further foul the water.

Sorensen considers them the most damaging aquatic species in Minnesota and says water quality can't be improved until carp are under control.

On Lake Riley, which is bigger and deeper than Lake Susan, carp will be pulled out this winter to be counted and tagged and then thrown back to help the research team develop a good estimate of carp numbers there.

Once they know the size of the population scientists will determine a strategy for controlling it.

The study is funded by the west-suburban Riley Purgatory Bluff Creek Watershed District and the state's Environment and Natural Resources Trust Fund, with assistance from the Department of Natural Resources.

Laurie Blake • 612-673-1711