Biologist aims to cut carp supply

A study shows some of the fish are older than 50.

MINNEAPOLIS-ST. PAUL STAR TRIBUNE

MINNEAPOLIS — Having found a way to reduce sea lampreys in the Great Lakes, Peter Sorensen now is taking on the common carp.

The biologist has begun a scientific study in three Minnesota lakes — in Eden Prairie and Chanhassen — that could offer the first hope for thinning the nasty bottom diggers from lakes across the country.

“We think it’s important to show people we can do something with an invasive animal — that science can do things,” he said.

Two-thirds of all Minnesota lakes are infested with the common carp. And it’s a national problem as well, said Sorensen, who works in the University of Minnesota’s Department of Fisheries, Wildlife and Conservation Biology.

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 phosphorous 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.

“It is unlikely that we can do much to improve the water quality in most of our lakes until we control them,” he said.

He chose to do his research on the three suburban lakes — Lake Riley, Lake Susan and Rice Marsh Lake — because they are home to thousands of carp. The bottom-feeding fish make up half to two-thirds of the fish in the lakes. Some of them are 2 to 4 feet long and as big as 18 inches in girth.

During their first two summers, Sorensen and his research team discovered something surprising and significant: Although some of the fish are older than 50, young fish are scarce.

The age of the carp can be determined by slicing open their ear bones to count their growth rings. It’s like counting the rings on a tree trunk. Judging by a sampling of the age of the fish in the lake, it’s been roughly 10 years since a baby carp grew from egg to adulthood — even though each female fish produces more than 1 million eggs a year.

Why the eggs grow into carp in some years and not others, Sorensen’s team does not yet know. But one theory, according to his research assistant, Prezmyslaw Bajer, is that in most years other fish eat the carp minnows, but that the natural predatory chain is interrupted when an especially harsh winter kills off all the fish in the lakes. Then carp come back by migrating from other lakes and multiply much faster than the game fish — resulting in an over-population of carp.

The fact that in many years young carp do not 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.

In his study of sea lampreys, Sorensen and his team identified and synthesized 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.

Pheromones also will figure into the carp study, as Sorensen looks for a chemical signal that would lure females into a single locale where they can be removed from the lake.

Sorensen’s work is getting financial support from many corners. He has a $550,000 grant from the state’s Environment and Natural Resources Trust Fund, which gets money from the state lottery, and other funding 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.

By the end of the study, Sorensen hopes to leave the lakes with low carp numbers and be able to hand officials a manual for keeping them that way.