Grass carp (Ctenopharyngodon idella) are voracious feeders. Many of their introductions have been for the control of aquatic vegetation. However, they are known to completely eliminate aquatic plants in introduced habitats altering trophic structure and inflicting widespread detrimental effects on ecosystems. They may also feed selectively on softer plants thereby enhancing development of tougher plants. Grass carp remove macrophyte cover, eliminate spawning substrate, disturb sediment and muddy waters, reduce water quality, increase nutrients in waters accelerating eutrophication, decrease oxygen levels, and promote alagal bloom. They compete with native invertebrates and fish for food and other important resources. Reported impacts on native fishes include the reduction of bluegill, sunfish, smelt, bully, and pike populations. Grass carp are believed to impact waterfowl by reducing aquatic vegetation, an essential food source. Significant declines of gadwall (Anas stepera), American wigeon (Anas americana), and American coot (Fulica americana) have been reported following grass carp introductions. They carry diseases and parasites which are transmittable to other fish and are believed to be the main vector for Asian tapeworms (Bothriocephalus opsarichthydis) known to infect several fishes in Canada including common carp (Cyprinus carpio), golden shiner (Notemigonus crysoleucas), fathead minnow (Pimephales promelas), channel catfish (Ictalurus punctatus). One record cites grass carp as the vector for the infection of endangered woundfin (Plagopterus argentissimus) (Standish & Wattendorf 1987; Jordan, 2003; Jacobson & Kartalia, 1994; Nico et al. 2006; GSMFC, 2005; McKnight & Hepp, 1995; Mitchell, 1986; Elvira, 2001).
Location Specific Impacts:
Modification of natural benthic communities: Ctenopharyngodon idella is thought to cause pronounced successions of vegetation in canals by consuming large amounts of macrophytes (FishBase, 2008)
Ecosystem change: Ctenopharyngodon idella has altered trophic structure and food chains of introduced habitats of Greece (Leonardos, 2008).
Reduction in native biodiversity: The introduction of Ctenopharyngodon idella to Lake Pamvotis resulted in a significant reduction of submerged macrophytes and the near dissapearance of endemic Epirus minnow (Phoxinellus epiroticus), the native Epirus barbell (Barbus albanicus) and and Squalius pamvoticus by means of habitat reduction, egg predation, and reduction of habitat (Leonardos, 2008).
Reduction in native biodiversity: Stocking of Ctenopharyngodon idella in Parkinsons Lake, New Zealand resulted in reduced the size and abundance of native New Zealand smelt (Retropinna retropinna) and the New Zealand common bully (Gobiomorphus cotidianus) (Mitchell, 1986).
Texas (United States (USA))
Reduction in native biodiversity: A decrease in sunfish accompanied the removal of aquatic vegetation by Ctenopharyngodon idella (Jacobson, 1994).