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Biologist Studies Lakes' Carp Population

Posted on: Tuesday, 30 October 2007, 18:00 CDT

MINNEAPOLIS - A University of Minnesota biologist is studying three lakes in the Twin Cities area hoping to find clues to keeping down the numbers of common carp that churn up sediment and make lakes murky.

Peter Sorensen is heading the study in three lakes in Eden Prairie and Chanhassen - Lake Riley, Lake Susan and Rice Marsh Lake - where the bottom feeders make up half to two thirds of the fish.

"We think it's important to show people we can do something with an invasive animal - that science can do things," Sorensen said. "It is unlikely that we can do much to improve the water quality in most of our lakes until we control them."

All lakes in the Twin Cities metropolitan area and two-thirds of all Minnesota lakes are infested with common carp. Sorensen, who works in the university's Department of Fisheries, Wildlife and Conservation Biology, said it's also a national problem.

Carp can dig a foot into the muck at the bottom of a lake, uprooting good plants and stirring phosphorous on the lake floor back into the water, where it fertilizes algae and weeds. Sorensen considers them the most damaging aquatic species in Minnesota.

The researcher and his team - who have been conducting the study for two summers - already have discovered that while some of the carp are older than 50, young fish are scarce. A carp's age is determined by slicing open its ear bones to count the growth rings.

A sampling of fish in the lakes indicates it's been about 10 years since a baby carp grew from egg to adulthood, even though each female fish produces more than 1 million eggs a year, the researchers said.

The team doesn't know why the eggs grow into carp in some years and not others.

Research assistant Prezmyslaw Bajer said one theory is that in most years other fish eat the carp minnows, but when an especially harsh winter kills off all the fish in the lakes the natural predatory chain is interrupted. Then carp migrate from other lakes and multiply much faster than the game fish, resulting in an overpopulation of carp.

The fact that in many years young carp don't mature raises hope that if the older fish are removed, they may not be replaced by younger fish, Sorensen said.

The biologists will work to suppress young fish, remove adults, and prevent adult fish from coming from other lakes and laying more eggs.

Sorensen and his team found a way to reduce sea lampreys in the Great Lakes by synthesizing a chemical signal, known as a pheromone, that can be used to trick the lampreys into traps. The lampreys prey on lake trout, whitefish and other fish.

Sorensen said pheromones also will figure into the carp study. The team will look for a chemical signal that would lure females into a single locale where they can be removed from the lake, he said.

Sorensen's work is being partly funded by a $550,000 grant from the state's Environment and Natural Resources Trust Fund, which gets money from the state lottery. Other money is coming from the Department of Natural Resources, the government of Australia - where carp is considered a national problem - and residents who live around the lakes he is studying.

David Florenzano, who has a home on Lake Riley, said the water quality seems to get a little worse every year.

The Lake Riley Improvement Association kicked in $2,000 to support the study because residents want to do what they can to help clean up the water, Florenzano said.

The west-suburban Riley Purgatory Bluff Creek Watershed District also has promised to kick in $2.7 million from 2008 through 2017.

"This has never been done before and it's a very important thing," said board Vice Chair Ken Wencl of Chanhassen. Until carp are under control, there is no way to improve the water quality, reduce the weeds and stop soil erosion, Wencl said.

Watershed District coordinator Paul Haik said no one expects the carp to disappear completely. "You will never eliminate the carp. The question is how can you manage the carp at a level that will allow fishing, boating and wakeboarding."

By the end of the study, Sorensen hopes to leave the lakes with low carp numbers and be able to tell the Watershed District how to keep them that way.

"I think we are smarter than carp, and I think we can figure this out," Sorensen said.

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Information from: Star Tribune, http://www.startribune.com

Source: Associated Press/AP Online

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