Despite the popularity of common carp (Cyprinus carpio) in Europe and Asia, most people in North America consider these fish pests. General ill will towards carp has not prevented the spread of these large minnows into nearly every aquatic habitat in North America. Concern over the potential harm that common carp could have on native fishes and waterfowl has led to attempts to remove these carp, using water-level control, toxicants, seining, electrofishing, angling, and barriers. However, carp are able to quickly recolonize open systems, making removal schemes a costly, high-effort, ongoing process. Despite concern over the impact of carp on native fishes, little work has been done to experimentally determine the specific environmental effects of common carp.
Our research was designed to quantify the effects of carp on aquatic systems, and to test the effectiveness of a plastic mesh substrate covering in diminishing these effects. Two sets of experiments were used to determine how carp affect turbidity, water column nutrient concentration, phytoplankton, zooplankton, aquatic macrophytes, and aquatic macroinvertebrates. In 1997, we set up four treatments within drainable one-acre ponds: carp and mesh, carp without mesh, mesh without carp, and a control with neither carp nor mesh. All treatments with carp were stocked with a high biomass of adult males. Results thus far show carp to have strong effects on turbidity, total phosphorus concentration in the water column, macroinvertebrates, and aquatic macrophyte cover. No significant differences in phytoplankton or total nitrogen concentration were found between the treatments. Mesh substrate treatments significantly reduced the effects of carp on aquatic macrophyte loss. Macroinvertebrates, such as odonates, associated with macrophyte cover responded positively to the mesh treatment.
In 1998, we ran an experiment designed to determine if the effects that we saw in 1997 could be attributed to carp specifically or to a high biomass of benthic fish in general. Once again, we used four treatments in one-acre ponds: high biomass of carp, low biomass of carp, high biomass of channel catfish (Ictalurus punctatus), and a no-fish control. Preliminary results show that carp increase turbidity and total phosphorus concentration and decrease macrophyte cover to a greater degree than channel catfish, and lower carp biomass treatments had levels intermediate between high carp and no fish. Turbidity in the carp treatments was found to consist primarily of inorganic sediments. This experiment supports the idea that carp, an introduced fish, have a more extreme effect upon aquatic systems than native benthic fish, and that the magnitude of these effects is partly related to the ability of carp populations to achieve a high biomass.
INHS researcher Joe Parkos with common carp at Max McGraw
The goals of our research were to determine the environmental effects of common carp and to evaluate plastic mesh substrate covers as a carp management strategy. Carp increased turbidity through the resuspension of bottom sediments, caused the loss of macrophyte cover due to low water clarity and uprooting, released phosphorus normally locked up within bottom sediments and aquatic macrophytes, and lowered the abundance of macroinvertebrates by predation and loss of habitat. Plastic mesh substrate covers reduced the loss of submerged vegetation and associated macroinvertebrates, but did not diminish the effects of carp on turbidity and phosphorus. Further analysis will assess potential effects on zooplankton in pond experiments. In addition, we are also making comparisons of aquatic macrophyte cover and macroinvertebrate biomass on mesh versus nonmesh plots on six field sites in northern Illinois.
Our research highlights both the effects of a benthivorous fish on an aquatic food web and the ecological traits that allow common carp to be successful invaders. Fish that are active foragers along the sediment-water interface can shift aquatic systems from a clear-water, aquatic macrophyte-dominated state to a turbid, relatively macrophyte-free condition. By reducing submerged vegetation and water clarity, common carp create conditions under which other fish, especially large predators, may suffer. Potential reduction in predator recruitment combined with high fecundity, fast growth rate, wide physiological tolerance, and omnivorous diet result in the common carp having the ability to spread into nearly any aquatic habitat. Future research will assess the effects of common carp on growth and recruitment of important sport fish, such as bluegill (Lepomis macrochirus) and largemouth bass (Micropterus salmoides).
Joe Parkos III and David Wahl, Center for Aquatic Ecology; Victor Santucci, Jr., Max McGraw Wildlife Foundation
Charlie Warwick, editor