Three thousand channel catfish will be introduced to Lake Nokomis in south Minneapolis starting in the spring in a "bio-manipulation'' experiment aimed at cleaning up the water by altering the lake's food chain.

The channel catfish are expected to eat black bullheads, whose feeding habits are fouling the lake.

"There is a high density of black bullheads in Lake Nokomis -- we are estimating between 200 and 400 pounds of bullheads per lake acre,'' said Steve McComas, owner of Blue Water Science in St. Paul. "We kind of ignored them over the years, but they can have a huge impact on water quality.''

Blue Water Science will conduct the food chain experiment with the help of the Minnesota Department of Natural Resources under a $61,000 contract with the Minnehaha Creek Watershed District.

Nokomis has had persistently poor water quality despite several past efforts to improve it. Three wetlands were restored to filter storm water that drains into the lake. Grit catchers were added to storm sewers to keep sand and silt out of the lake. Carp, which are considered harmful bottom feeders, were removed by commercial fishermen. And Minnehaha Creek was diverted from Lake Nokomis because the creek has high levels of phosphorous.

With these strategies in place, "What is keeping the lake from getting better has been a bit of a mystery,'' McComas said. "The question was why isn't the water getting better?''

The algae and scarcity of aquatic plants implicates bottom-feeding fish, he said.

"They go into the roots to nose around and gobble up the aquatic insects, and in the process the plants get uprooted and it kills them.

"For the last 15 to 20 years we were pointing the finger at carp. We knew there were a lot of carp,'' McComas said. But after the carp were removed, "the most recent fish survey shows the carp are present but at low density.''

That has shifted the focus to bullheads.

"We know from other lakes that high density of bullheads can ravage aquatic root beds,'' he said.

And when they eat, bullheads "ingest but don't assimilate very much of the sediment and then they excrete it," freeing phosphorous from the lake bottom into the water and promoting algae growth, McComas said.

Sunfish have the same effect when there are too many of them in a lake, he said.

This will be the Watershed District's first use of bio-manipulation, and if it works it may be used on other lakes, said district administrator Eric Evenson.

"This is a natural system, and natural systems aren't predictable. There is no guarantee it will work,'' Evenson said. But at worst, "We are going to introduce a nice game fish into the lake."

The channel catfish will be introduced at a rate of 1,000 a year for three years. The fish will be 8 to 10 inches long when they first arrive -- big enough to eat hatchling bullheads and sunfish and carp the first year.

Channel catfish have a small, narrow head, a forked tail, skin instead of scales, and body coloring ranging from blue gray to silver, with scattered black spots on their backs and sides, according to Catfishangling.com.

Like bass and walleyes, they are predator game fish considered good for human consumption.

Channel catfish are considered native to the area because they live now in the Mississippi River. They prefer to spawn in a river and are not expected to over-populate Nokomis, McComas said. Those introduced should live 10 or 12 years in the lake. "We are hoping that by that time, the fish community will be in balance and it will be self-sustaining,'' he said.

Commercial fishermen will set hoop nets on the bottom of the lake this summer to remove the bigger bullheads. "The bullheads swim into them, and fortunately they are not smart enough to swim out.'' McComas said. He hopes anglers will continue to take out the bigger sunfish.

"These lakes can have nice communities of bullheads and blue gills. It's just a little out of balance right now. So what we are going to try to do is rebalance the fish community,'' McComas said. "We will be looking for improved water clarity and an increase in aquatic plants."

Laurie Blake • 612-673-1711