Bumpy journey to managing common carp and restoring Eden at Hennepin & Hopper lakes

I clear my head at Hennepin & Hopper lakes.

Be it fishing (the season ended Monday), bird watching, bioblitzes, hiking or kayaking, I calm myself there.

It goes back to 2001. The Wetlands Initiative had just shut down the drainage pumps at Hennepin & Hopper, now part of the 3,000-plus acres of the Sue and Wes Dixon Waterfowl Refuge. Nearly overnight water restored an Illinois River backwater in Putnam County. Amazingly, a seed bank of aquatic plants buried in corn and bean fields more than 80 years responded just as quickly.

For three hours one evening, TWI board member Dick Schroeder and I gawked as we meandered the lakes, amazed at how quickly that seed bank had grown, especially the sago pondweed.

Multitudes of waterfowl discovered the miracle about as quickly. The fishing was absurdly good in coming years.

It was like a miracle. Yet, there was a mythical serpent in this Edenic setting.

Common carp crashed the party. They only took a few years to mess everything up, then it took years to reestablish Eden.

Now, it seems to be working as hoped the last eight years.

First the decline.

By 2006, common carp activity had degraded the lakes rapidly, by digging up macrophytes and contributing to turbidity. By 2007, the entire system had begun to collapse with loss of all the submersed species.

Dr. Gary Sullivan, TWI's senior ecologist, broke it down in a presentation at the annual Emiquon Science Symposium a couple years ago and available on YouTube.

They first tried commercial fishers to take out carp. That only dented the population. By 2008, the lakes were degraded to where only a few patches of American lotus and a few cattails remained. That rapid decline in aquatic plant abundance, quickly translated to a rapid decline in waterfowl. By 2009 only seven species were observed. Coots completely disappeared after 2006.

By 2009, TWI decided on a drawdown, followed by the Illinois Department of Natural Resources using rotenone to kill the remaining fish in 2010. Vegetation exploded in 2010, but water quality remained poor. By 2011, the dense vegetation disappeared and species rapidly dropped again.

In 2012, another rehab started, including capping a few remaining tile lines at the north end. As the contractor pulled the pipes, Sullivan noted, "Water sluicing out had a lot of carp in it."

They had found the problem. Carp found refuge in field tiles, something not assumed to be true.

With the second redo, vegetation boomed and by 2013 nearly twice as many waterfowl as the previous high were counted.

"We were thinking we were buying ourselves a few years, at least five, maybe eight, but it's 12 so far," Sullivan said. "Maybe they are stabilized. That is the thing we weren't sure could happen."

"I have hope we never have to drain or rotenone the lakes again, but we do know how to do it if we have to," said Paul Botts, TWI president and executive director.

In 2018, some large carp were found but there was no sign of degradation. What likely worked is getting rid of carp in the tiles, then having dense vegetation and piscivore populations before the carp could rebound.

"Hennepin-Hoppper is very predatory dense," said Blake Bushman, who took over for the venerable Wayne Herndon as IDNR district biologist in 2018.

They've surveyed carp of 6 pounds or more, bigger than can eaten, but Bushman noted there aren't carp enough to have any impact on vegetation.

"In my time, we never collected any young of the year carp," he said. "That predator population is managing the carp."

He doesn't think that eagles, cormorants or otters have much impact, rather, "Fish predators will be the primary tool that is working on the fish population. I'm pleased that [the carp] haven't blow up again. I would not say I am surprised. That was goal. Fish predators are your most effective management tool. They are there 24 x 7."

The piscivore side included northern pike, spotted gar, especially bowfin, a voracious native, walleye and largemouth bass. The IDNR also stocked muskie, another apex predator, and a year ago introduced alligator gar.

Those species also excite anglers.

"Clearly we have carp and they are reproducing, but so far that reproduction has not been enough to have an impact on the plant communities," Sullivan said. "The carp are there, but to date they have not degraded the system in a way that is showing up to us visually so far."

"Bottom line right now is that water quality is strong and aquatic vegetation is doing well, but we keep a sharp eye out and will," Botts said.