white amur, silver orf, Ctenopharyngodon laticeps Steindachner, 1866, Leuciscus idella Valenciennes in Cuvier and Valenciennes, 1844, Ctenopharyngodon idellus

Distinguishing characteristics were given in Berg (1949), Shireman and Smith (1983), and Page and Burr (1991). Keys that include this species and photographs or illustrations were provided in most of the more recently published state and regional fish books (e.g., Robison and Buchanan 1988; Etnier and Starnes 1993; Jenkins and Burkhead 1994; Pflieger 1997).

Genetic testing is necessary to distinguish diploid and triploid grass carp.

Oxygen consumption (per gram of body mass) increases with higher water temperature and decreases with fish age and mass (Chen and Shih 1955; Wozniewski and Opuszynski 1988). The lethal low oxygen level for juveniles was <0.5 mg/L (Negonovskaya and Rudenko 1974). The maximum pH for culture of grass carp was reported as 9.24 (Liang and Wang 1993). Egg hatching was delayed below pH 6.5 and increased mortality and deformation of larvae occurred below pH 6.0 (Li and Zhang 1992). Sensitivity to low pH decreased with age (Li and Zhang 1992). Median lethal concentration of ammonia was determined to be 1.05 mg/L (Gulyas and Fleit 1990).

Grass Carp appears to be tolerant of low levels of salinity, and may occasionally enter brackish-water areas. Fry (32-50 mm TL) survived transfer from freshwater to a salinity of 12 ppt (Chervinski 1977). Adults (2+ years) survived 10.5 ppt salinity for about 24 days and 17.5 ppt for 5 hours (Cross 1970). However, Grass Carp acclimated to 3, 5, and 7 ppt had an upper tolerance of about 14 ppt (Kilambi and Zdinak 1980). Maceina and Shireman (1980) showed that fingerlings reduce feeding at 9 ppt and stop feeding altogether at 12 ppt; thus, they predicted Grass Carpcould inhabit brackish water bodies up to 9 ppt. Maceina and Shireman (1979) reported that the species can tolerate 14 ppt for as long as 4 days, but that the upper long-term tolerance of fingerlings to saline waters was lower, about 10-14 ppt. Maceina et al. (1980) noted that oxygen consumption decreased along a salinity gradient of 0-9 ppt. Movement of Grass Carp from one river to another through a brackish-water estuary (Cross 1970) is not surprising given the species' tolerance to low levels of salinity. Avault and Merowsky (1978) reported food preference and salinity tolerance of hybrid Common Carp X Grass Carp.

Ctenopharyngodon idella has a high potential environmental impact in the Great Lakes.

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 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 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).

Ctenopharyngodon idella 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 milfoil, 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 also occurs between grass carp and other herbivorous fishes, such as forage fishes (Cudmore and Mandrak 2004). 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 constantly invade and disturb bluegill spawning areas, consequently 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).

Cyprinids, including 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, that are parasites of grass carp. Worth noting is Bothriocephalus acheilognathi, the Asian tapeworm. This parasite has been introduced by cyprinids, 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 low potential socio-economic impact in the Great Lakes.

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). When in excessive numbers it can destroy existing food chain relationships and threatening the spawning grounds of commercial fishes (Petr and Mitrofanov 1998).

Ctenopharyngodon idella has the potential for high beneficial effects if introduced to the Great Lakes.

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). Grass carp can effectively control and eliminate aquatic plants in a variety of situations. Private fish farms have been producing large numbers of sterile, triploid grass carp as interest in stocking open systems increases (Bain 1993). Grass carp also are now routinely stocked in irrigation canals of the western United States (Bain 1993) and in Saskatchewan, Canada (Cudmore and Mandrak 2004).

Despite its bony flesh, grass carp is consumed as food in many regions of the world (Opuszynski and Shireman 1995) and 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 has been harvested Mississippi River in Missouri throughout the 1990s (Pflieger 1997) and by 1996, it accounted for 8% of the total commercial fish harvest from this area (Cudmore and Mandrak 2004, USGS 2004).

Control
The Aquatic Nuisance Species Task Force and the U.S. Fish and Wildlife Service organized an Invasive Carp Working Group (Working Group) to develop a comprehensive national Invasive Carp management and control plan. The Working Group agreed that the desired endpoint of the plan is the extirpation of Invasive Carps (Bighead, Silver, Black and Grass) in the wild, except for non-reproducing Grass Carp within planned locations (Conover et al 2007).

Biological
Erickson et al., (2017) investigated the feasibility of releasing sterile male Grass Carp to decrease successful spawning in Lake Erie; this method would require releasing too many Grass Carp into Lake Erie to be practical.

Physical

Many types of physical barriers are being examined for potential to stop the dispersal of Invasive Carp including Ctenopharyngodon idella.  These include earth berms, fences, electric barriers, bubble curtains, acoustic barriers, strobe lights and high-pressure sodium lights. 

The electrical fish barrier can function either as an impassable barricade or as a fish guidance system.  In either case, the system consists of a series of metal electrodes submerged in water to create an electrical field capable of repelling fish. Electrical barriers have been evaluated for preventing the expansion of Invasive Carp populations in both the Chicago Sanitary and Ship Canal and the Upper Mississippi River System. While considered feasible for the Chicago Sanitary and Ship Canal, it was determined that electrical barriers would be less effective and less feasible on the Upper Mississippi River System. The U.S. Army Corps of Engineers (USACE) constructed a set of three electrical barriers, the first of which opened in 2002, on the Chicago Sanitary and Shipping Canal to prevent the spread of aquatic invasive species between the Great Lakes and Mississippi River basins.  Although currently in use, electric barriers are not the end-all solution to the range expansion of Invasive Carp in the United States. Electric barriers are not selective as to species affected.

Chemical
The toxicity of many chemicals to bighead, grass, and silver carps has been examined (13 chemicals, 34 studies for bighead carp; 75 chemicals, 233 studies for grass carp; 21 chemicals, 83 studies for silver carp; Pesticide Action Network 2005).  Rotenone and antimycin are the only registered piscicides available to potentially control Invasive Carps in the United States without considerable additional expense. Rotenone and antimycin are both labeled for use in lakes and running waters (i.e., streams and rivers). The American Fisheries Society has published a manual for the use of rotenone in fisheries management (Finlayson et al. 2000). Research is needed to further investigate the effectiveness of registered piscicides to control Invasive Carps, evaluate their potential use in the control of feral populations, and to determine the potential of other chemicals to control Invasive Carps.

DeVaney et al. (2009) performed ecological niche modeling to examine the invasion potential for grass carp and three other invasive cyprinids (common carp Cyprinus carpio, black carp Mylopharyngodon piceus, and tench Tinca tinca). The majority of the areas where grass carp have been collected, stocked, or have become established had a high predicted ecological suitability for this species.