The Fish Professor

Are their options for carp control?

By Mark Cornwell

There seems to be a lot of interest in carp removal projects lately. Landowners with large lakes and planned communities with lake associations have been calling and asking for help to remove invasive common carp. In each case, the carp are making themselves a nuisance. The carp troublemakers are typically Eurasian carp, European carp or common carp (Cyprinus carpio), not grass carp (Ctenopharyngodon Idella) or their triploids. 

What is the difference between triploid grass carp and common carp? Grass carp are from Asia and common carp are from Germany. Grass carp are stocked to control submerged aquatic vegetation. However, they have preferences for certain plants and will not indiscriminately eat all plants unless they have no other options. 

Triploid grass carp are sterile, or unable to reproduce, and are checked at the hatchery before they are sold and stocked to make sure that they are indeed triploids. These grass carp, triploid or not, only move to less desirable plants when their preferred plants are gone or hard to find. 

Common carp are more interested in looking for and eating animal materials on the bottom but may ingest plants along the way. Common carp are benthic (bottom) omnivores. Some of the best carp fishing is in lakes that have zebra mussel beds because of the heavy invertebrate bottom production.

Should you stock common carp to control aquatic vegetation? 

No. 

Grass carp?  Maybe, if you have the plant they want to eat and have a plan to remove them when the plant is gone. 

So, what’s all the fuss and why are lake associations willing to pay to have carp removed? Carp can grow to incredible sizes, and in their search for food in shallow waters they create vast amounts of turbidity. In addition to a loss in clarity and aesthetics, the stirring of bottom waters can translocate nutrients from the bottom to the overlying waters. The addition to nutrients in the water column can cause algal blooms when conditions are right.

Recently, I visited a lake where a shallow, muddy, four-acre cove was simply writhing in spawning carp bodies. There were hundreds of carp. These fish turned an otherwise clear lake into a muddy mess. Most were large, spawning adults in the 10- to 20-pound range. The lake side residents had determined that this carp population was a real problem that needed a (probably costly) solution.

Once a carp problem is identified, what can be done to solve it? Unfortunately, the options are limited once carp are established. There are a few carp control strategies, but first check with the state DEP, DNR or local governments before doing anything! Option 1, go nuclear and drain the pond or lake to eliminate them. This option may be extreme but if the entire water body, and the water level, are within legal control it is effective. 

This could be done in concert with other pond manipulations including renovating a dam or water control structure, mucking out the bottom, or a plant control project.   

The second option is to remove the carp with a chemical piscicide, such as rotenone. Rotenone is natural fish killing agent that is produced (usually) by crushing up stems and roots of the tropical South American Barbasco plant. Rotenone is the most common piscicide used to kill nuisance fish. Again, this is an extreme option because at the concentration needed to kill the carp, rotenone is indiscriminate and will kill most other fish.  

The other fish in the pond or lake will perish as well as the carp. 

Applying an aquatic pesticide will likely require a professional that is permitted to perform the application. Several lake management companies may offer this service. Chemical treatment with rotenone may be difficult to permit and can be quite expensive depending on lake volume, which determines the quantity of rotenone to purchase. 

The rotenone strategy may be used in tandem with a draw down in the event that all the water cannot drained. After a drawdown, any remaining water that is treated to kill the carp. Do this later in the season to avoid carp spawning as eggs may be more resistant to rotenone than the actual fish.

Another control method is to mechanically harvest the carp. This means hiring a boat electrofishing crew to run the electrofishing equipment around the lake during spawning season and manually remove as many carp as possible.

Alternatively, netting the lake with a large seine can accomplish the same goal. But the seining area must be open, no obstacles…and where carp are. Seining a mud flat isn’t easy.  Selectively removing carp from the lake and letting the other species of the fish go alive and hopefully unharmed.  

This option is labor intensive and while expensive it can be successful in the short term.

Last week, our electrofishing team did a carp removal with the electrofishing boat. 1,000 pounds of carp were removed from a small impoundment in about 5 hours. Did we get all of them? Nope, not even close. But we did put a dent in the spawning adult population. If possible, do this removal at the beginning of the spawn to catch carp BEFORE they litter the lake with fertilized eggs. Another benefit of the mechanical harvest strategy is that a qualitative check of other parts of the fishery happens during the electrofishing process. As the electrofishing boat navigates around the lake removing carp, netters shout out fish species and numbers to a crewmate who records the data on a tally sheet. The stakeholders gain an additional point of fisheries data, essentially for free.

Another mechanical method is using gill nets. In the right part of the country, lake owners can contract with trappers who set large mesh gill nets for size selection and capture the bigger carp. These trappers typically have a market for harvested fish.

Biocontrol, or stocking predators to eat the carp, can also help reduce the severity of a carp problem. Once a carp reaches 10-12” in length (around age-2), not many lake predators can eat them and they are mostly invulnerable to predation. Predators typically considered for large carp control are big toothy fish like northern pike and tiger muskie. 

However, these piscivores are similar to lone apex predator, like cougars. They are relatively large, long-lived, solitary, apex predators that may not be stocked at densities high enough to control the carp once the carp have grown larger. Focusing biocontrol predation on smaller carp using largemouth bass is likely to be more effective. 

Moderate densities of largemouth bass >12 inches will have a substantial impact on smaller carp. Essentially, if bass hammer the smaller carp there will be lower densities of larger carp. This results in the carp being a small-time bit-player in the lake system. In other words, acknowledge that the carp cannot be totally eliminated make them a more mild-mannered fish seen rarely, rather than a massive, disruptive, turbulent nuisance.

Do you want to stock a large toothy predator to try to clean up some bigger carp anyway? Tiger muskies are the best bet because they are a sterile hybrid. Sterile hybrids can’t reproduce and the decision to stock them can be taken back if they did not accomplish the goal of carp clean-up. Once the tiger muskies age out, don’t stock more if they did not earn their keep. 

Will any of the above strategies work?  Yes, if you drain the pond or kill it with rotenone.  However, you need to ask “How did the carp get there in the first place?” If water is running into the lake from other surface waters, they likely came in there. In that case, the carp will repopulate relatively quickly. Mechanical harvest is a short-term sugar high that turns into an annual maintenance program. Many lakes contract annually to remove carp when fish show up in shallow coves to spawn. 

The electrofishing crew spends at least a day and a night to remove as many carp as possible to keep carp biomass and associated turbidity under control. This is similar to aquatic plant harvesting or raking nuisance plants around a dock. It’s a maintenance program not an eradication strategy. 

Biocontrol with a predator that may produce low densities of small carp that will (hopefully) behave themselves.

Success can happen, but it will likely take a combination of strategies, unless the lake is drained or fish taken out with rotenone.

It’s worth mentioning that we have also been called to remove triploid grass carp that became a problem once they over-ate the lakes plant base. Grass carp grow VERY large and can also cause problematic turbidity if they consume all the plants in a lake. Stories of grass carp eating lake shore lawns are accurate, I have seen grass carp right up on golf course lawns eating grass because they have exhausted the aquatic plants in the ponds they were stocked into. When the plants are gone pond turbidity increases rapidly, leaving a muddy undesirable mess. Once large carp (of either type) get established in a lake they are very difficult to eradicate without drastic measures.

Many times, the best a lake can hope for is to reduce carp to a low enough density to keep them from being a nuisance. 

Don’t have common carp in your lake or pond? 

Good. 

Keep it that way.