Ctenopharyngodon idella has a high potential environmental impact in the Great Lakes.
Potential:
Various authors (e.g., Shireman and Smith, 1983; Chilton and Muoneke, 1992; Bain, 1993) have reviewed the literature on Grass Carp; most also discuss actual and potential impacts caused by the species' introduction. Shireman and Smith (1983) concluded that the effects of Grass Carp introduction on a water body are complex and apparently depend on the stocking rate, macrophyte abundance, and community structure of the ecosystem. They indicated that numerous contradictory results are reported in the literature concerning Grass Carp interaction with other species. Negative effects involving Grass Carp reported in the literature and summarized by these authors included interspecific competition for food with invertebrates (e.g., crayfish) and other fishes, significant changes in the composition of macrophyte, phytoplankton, and invertebrate communities, interference with the reproduction of other fishes, decreases in refugia for other fishes, and so on. In their overview, Chilton and Muoneke (1992) reported that Grass Carp seem to affect other animal species by modifying preferred habitat, an indirect effect. However, they also indicated that Grass Carp may directly influence other animals through either predation or competition when plant food is scarce. In his review, Bain (1993) stated that Grass Carp have significantly altered the food web and trophic structure of aquatic systems by inducing changes in plant, invertebrate, and fish communities. He indicated that effects are largely secondary consequences of decreases in the density and composition of aquatic plant communities. Organisms requiring limnetic habitats and food webs based on phytoplankton tend to benefit from the presence of Grass Carp. On the other hand, Bain reported that declines have occurred in the diversity and density of organisms that require structured littoral habitats and food chains based on plant detritus, macrophytes, and attached algae. Removal of vegetation can have negative effects on native fish, such as elimination of food sources, shelter, and spawning substrates (Taylor et al. 1984). Hubert (1994) cited a study that found vegetation removal by Grass Carp lead to better growth of Rainbow Trout Oncorhynchus mykiss due to increases in phytoplankton and zooplankton production, but it also lead to higher predation on Rainbow Trout by cormorants Phalacrocorax auritus due to lack of cover, and changes in diet, densities, and growth of native fishes. Although Grass Carp are often used to control selected aquatic weeds, these fish sometimes feed on preferred rather than on target plant species (Taylor et al. 1984). Increases in phytoplankton populations is a secondary effect of Grass Carp presence. A single Grass Carp can digest only about half of the approximately 45 kg of plant material that it consumes each day. The remaining material is expelled into the water, enriching it and promoting algal blooms (Rose, 1972). These blooms can reduce water clarity and decrease oxygen levels (Bain, 1993). In addition to the above, Grass Carp may carry several parasites and diseases known to be transmissible or potentially transmissible to native fishes. For instance, it is believed that Grass Carp imported from China were the source of the introduction of the Asian tapeworm Bothriocephalus opsarichthydis (Hoffman and Schubert, 1984; Ganzhorn et al., 1992). As such, the species may have been responsible indirectly for the infection of the endangered Woundfin Plagopterus argentissimus (by way of the Red Shiner Cyprinella lutrensis) (Moyle, 1993).
Grass Carp have environmental impacts on the ecosystems they have been introduced. For instance, Grass Carp is known to be the source of major alterations to the trophic structure and food chains of aquatic systems. Many of these changes in plant, invertebrate and fish communities are largely secondary consequences of reductions in the density and composition of aquatic plant communities (Bain, 1993, Cudmore and Mandrak, 2004). When stocked at high densities, Grass Carp can eliminate all vegetation in even large aquatic systems (e.g., 8100-ha Lake Conroe) (Kiussmann et al., 1988). Declines have occurred in the diversity and density of organisms that are dependent on structured littoral habitats and food chains based on plant detritus, macrophytes, and attached algae as a consequence of reduced plant surface habitat, increased invertebrate food supplies (i.e. plant detritus), altered substrate conditions, and increased dissolved oxygen conditions (Bain, 1993, Martin and Shireman, 1976, Vinogradov and Zolotova, 1974).
Grass Carp is known to out-compete native species for both food and habitat. Research in small closed systems has demonstrated that due to Grass Carp’s preference for native aquatic plants over Watermilfoil (Myriophyllum spp.), these fish compete with waterfowl, which feed on these plants as well (Fowler and Robson, 1978; McKnight and Hepp, 1995; Pine et al., 1990; Pine and Anderson, 1991). Furthermore, direct competition for plant material may also occur between Grass Carp and other native fishes that include macrophytes in their diet, such as Gizzard Shad (Dorosoma cepedianum), Lake Sturgeon (Acipenser fulvescens), as well as several species of Buffalo (Ictobius spp.)(Cudmore and Mandrak, 2004; Coker et al., 2001). Grass Carp may compete with planktonic and benthic species, including catfishes and hybrid sunfishes for aquatic plants (Shireman and Smith, 1983), especially during Grass Carp juvenile stages and at lower water temperatures (Fedorenko and Fraser, 1978). Direct competition for habitat has been found to occur between Grass Carp and other fish species, particularly bluegill. With their schooling habit, Grass Carp invade and disturb bluegill spawning areas, greatly reducing Bluegill weight and numbers (Forester and Lawrence, 1978).
Grazing by Grass Carp has been associated with alterations of water quality. The decay of these large volumes of dead aquatic plants due to Grass Carp’s grazing and waste production elevate nutrient levels in water, induce phytoplankton blooms, reduce water clarity, and decrease oxygen levels (Bain, 1993; Boyd, 1971; Vinogradov and Zolotova, 1974).
Grass Carp, are known to be carriers of numerous parasitic organisms. Shireman and Smith (1983) thoroughly list a wide array of organisms, from viruses to protozoans to trematodes. Worth noting is Bothriocephalus acheilognathi, the Asian tapeworm. This parasite has been introduced, particularly by its native host the Grass Carp, to every continent except Antarctica (Bain, 1993; Salgado-Maldonado and Pineda-Lopez, 2003). Additionally, Grass Carp are the source of Ergacilus spp. in UK waters (Cowx, 1997). However, disease and parasitism are not as prevalent in wild populations as in fish culture (Shiremand and Smith, 1983).
Ctenopharyngodon idella has a moderate potential socio-economic impact in the Great Lakes.
Potential:
Grass Carp are not known to pose a threat to human health or infrastructure. One of the undesirable consequences of stocking Grass Carp is increased turbidity, either algal or abiotic (Bonar et al., 2002; Lembi et al., 1978; Maceina et al., 1992; Water Environmental Services Incorporated, 1994). These effects can be harmful to human health, recreation, and the perceived aesthetic of the areas they inhabit. When in excessive numbers it can destroy existing food chain relationships (Petr and Mitrofanov, 1998). Their feeding may threaten Yellow Perch (Perca flavescens) spawning by consuming emerging macrophytes where Yellow Perch lay their eggs (Kocovsky pers. com., 2019)
There is little or no evidence that Ctenopharyngodon idella has significant beneficial impacts in to the Great Lakes.
Potential:
Because of its strong preference for aquatic vegetation, ability to be cultured easily, and hardiness, Grass Carp is being widely introduced throughout the United States to control aquatic vegetation in lakes and ponds (Chilton and Muoneke, 1992; Page and Burr, 1991).
However, Grass Carp populations are below the necessary threshold to have an effect on submerged aquatic vegetation (SAV) in the Great Lakes, and at the population level necessary to control SAV they could possibly contribute to eutrophication by releasing nutrients sequestered in wetlands (Cudmore and Mandrak, 2004).
Grass Carp are fished in some areas in their native range (Shireman and Smith, 1983). However, they rarely comprise a large proportion of the catch and are taken incidentally in common or silver carp fisheries in the Amur basin (Shireman and Smith, 1983). In the United States, Grass Carp are not widely accepted as a food fish.