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 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.

"We have been studying the carp 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 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.)

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 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.

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 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

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