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Carp roundup is a net gain for Minnesota's lakes

By LAURIE BLAKE, Star Tribune

January 13, 2009

Talk about a great day of ice fishi= ng.

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 snow= flakes, 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 t= he fish under the ice as two tractors pulled the catch toward a 15-foot hole w= here 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 population= s, which dirty up the waters where they are found around Minnesota. The study has focused on th= ree west-suburban lakes: Lake Susan in Chanhassen and Lake Riley and Rice Marsh Lake, both of which straddle the E= den Prairie-Chanhassen city line.

"We have been studying the car= p in Lake Susan now for about three years,'' Sorensen said. "We know how many there are = and have an idea about where they are coming from.''

The striking thing about Monday's catch, said Sorensen, was that "in that net there was almost nothing b= ut carp -- 95 percent of the fish were carp. They were all large fish." <= o:p>

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 th= eir numbers have been reduced by a predator, but they are not sure what that predator might be.

That means that if the older fish a= re 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.)

The netting Monday again confirmed = the scarcity of young fish, Sorensen said.

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 c= arp 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.<= /o:p>

Two-thirds of all Minnesota lakes and all metro-area lak= es 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 M= innesota and says water quality can't be improved until carp are under control.=

He focused his research on the three west suburban lakes because they were home to thousands of carp -- some of = them 2 to 4 feet long, as big as 18 inches in girth and weighing 10 to 20 pounds= -- because they are connected in the same watershed.

On Lake Riley, which is bigger and deeper than Lake Susan, carp will be pulled out this winter to be counted a= nd tagged and then thrown back to help the research team develop a good estima= te 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

© 2009 Star Tribune. All rights reserved.

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