Controlling the common carp

The common carp is often thought of as an invasive species that can’t be stopped, but can they be controlled?

For years, methods have been tried to slow down the spread of different species of carp all over the country. Many have provided no long-term solutions for a fish that can cause serious damage to lakes where their populations become too abundant.

Carp are notorious bottom feeders that uproot aquatic vegetation. As carp biomass goes up, vegetation goes down and lakes become turbid. Lakes like Christina in Douglas County are examples of bodies of water that were greatly impacted by the infestation of carp over the years.

“Significant,” Douglas County Lakes Association (DCLA) member Steve Henry said of the problem carp pose in area lakes. “Not major. Not number one or number two, but certainly a significant issue. I think the smaller, shallower lakes are more of an issue. A lake like Oscar, I think it causes much greater turbidity and problems than some of our bigger, deeper lakes. It’s certainly affecting our lakes negatively.”

That’s why the DCLA wants to find any answers it can on how to control the carp. To get ideas, they brought in an expert on the issue in Przemek Bajer to speak at their monthly meeting last Wednesday in Alexandria.

Bajer is a research assistant professor in the Department of Fisheries, Wildlife and Conservation Biology from the University of Minnesota who specializes in sustainable control solutions for invasive species, specifically the common carp.

His message was that carp populations in most Minnesota lakes are controlled by native fish such as the bluegill and that shallow, marshy basins prone to winterkill of those native fish are the sources of young carp that continually feed into larger lakes through chains.

CARP NURSERIES

Bajer’s research started in 2006 as they set out to look at the whole cycle of the carp population in a chain of three lakes – Lake Susan (around 20 feet deep), Ice Lake Marsh (up to 10 feet) and Lake Riley (almost 60 feet deep).

The team put radio transmitters in carp throughout all the systems in April. By May, they found all the fish were traveling to the marsh to spawn at the same time before returning to the deeper lakes. Over the years, their studies through net traps have shown that those shallow basins are the only ones that held young carp.

“It turns out that the larvae and eggs cannot survive in most lakes,” Bajer said. “In a typical lake that is dominated by native fish like bluegill and crappies, perch, walleyes and bass, the carp will be spawning in those lakes but we never found a small carp. We’ve looked at a couple dozen lakes for several years now and we see them spawning every single year, but there’s no young.”

Bajer’s research showed that these small bodies of water serve as nurseries for young carp through the first couple years of their life with their ability to survive in waters with low oxygen levels.   

“GAME-CHANGING” NEWS

Henry, who is the president of the Lake L’Homme Dieu association and also of the Viking Sportsmen Club in Alexandria, called that news a “game changer” when it comes to approaching ways to control carp in the area.

“It’s huge,” Henry said. “It’s not simple. It doesn’t immediately jump out where you say, ‘That’s the silver bullet,’ but I think it’s very significant. I think it gives us a whole other approach to controlling them. If you would prevent them from reaching those shallow basins and they have to spawn in the lakes, those fry don’t survive.”

So how can one keep the adult carp from reaching those marshes? Henry pointed to Lake Mary as a body of water that has dealt with an abundance of carp for years. Barriers and traps were set up decades ago in an attempt to slow them down. Henry says they removed thousands of pounds of carp every spring, but even that didn’t seem to be making much of an impact.

“It just kind of gradually faded and really hasn’t been active for the last 10-12 years,” he said.

The question of poisoning the nurseries was posed to Bajer as a solution, but that would only put a Band-Aid on the bigger problem. “He just said you’d basically have to do it every other year,” Henry said. “He said, ‘I’m not sure how effective it would be and it’s quite expensive.’ He didn’t think that was a simple, workable solution.”

Bajer’s suggestion to the DCLA was to concentrate on keeping the adult carp from making it to those shallow basins that serve as successful spawning grounds.

“He thought the carp barriers and then the electronic gates,” Henry said. “Again, he said it’s not easy. Overall, carp barriers are complicated and difficult, but he thought in those situations it could be used pretty effectively.”

Bajer said there were ways for them to help with a study on area lakes to help pinpoint those nurseries, but that it would be up to the DCLA to find the money to support the project for a graduate student to use as his thesis.

Henry said the DCLA could apply for grants and possibly get some money through the clean water portion of the Lessard-Sams Outdoor Heritage Fund. It’s early in the process, but he felt it was at least worth looking into after hearing about Bajer’s research.

“There’s significant interest there,” Henry said. “We would likely start with a smaller project. With Lake Mary being so interested, I could see that being one.”