Chesapeake Bay Introduced Species Database

Ctenopharyngodon idella

Common name(s):
Grass Carp
Grass Carp are freshwater herbivores. They eat aquatic plants and were imported to the United States, Europe, Africa, Japan, Mexico, and other areas to control unwanted aquatic plants. They are native to China and Russia and are cultivated in China for food. They were first brought to the United States in 1963 from Malaysia by researchers at the United States Fish and Wildlife Service Fish Farming Experimental station. At the time people thought that if the fish were only used in lakes and other slow moving water bodies they would not become established because they need large swift-flowing rivers to reproduce. But, they were wrong and very soon they had established breeding populations in the lower Mississippi drainage. They've now been introduced to 45 states. The fact that Grass Carp eat aquatic vegetation is useful in areas with high densities of introduced aquatic weeds. But Grass Carp don?t discriminate between aquatic weeds and valuable native submerged vegetation, they eat them all. Trying to balance the desire to control aquatic weeds and introducing a nonnative fish that eats native plants led to efforts to develop nonreproductive fish for weed control. These sterile triploids are stocked in some states, such as Virginia and banned in others, including Maryland. So far there is no evidence of reproduction by these triplod fish in the Chesapeake Bay watershed.
Image courtesy of Pavel Dvorak.

Chesapeake Bay Status:

First Record
Regular Resident

Source Region
Native Region
East Asia East Asia

Vector(s) of Introduction

History of Spread:

Ctenopharyngodon idella (Grass Carp) is native to lowland rivers of China and the Amur basin of the former USSR. This fish has been introduced worldwide for aquaculture and aquatic weed control and is now established in the wild in Africa, Russia, Japan, Mexico, and possibly in the Phllippines and Yugoslavia (Shireman and Smith 1983; Lever 1996). In North America, it was first imported from Malaysia to the United States Fish and Wildlife Service Fish Farming Experimental station in 1963. When this species was first imported, it was believed that this fish's reproductive requirement for large swift-flowing rivers would prevent its establishment in North America (Courtenay 1993). It escaped in the vicinity of the station, and was deliberately released later, for weed control (Lake Brimley AR; 1968) , and became established in the lower Mississippi drainage (Robison and Buchanan 1988). By 1978 C. idella had spread or been introduced to 35 states, although reproduction had not then been confirmed (Guillory and Gasaway 1978). They have now been found in 45 states (Florida Caribbean Science Center 2001). Larvae and juveniles were found in the Mississippi by 1980, and C. idella were apparently reproducing extensively in IL and upper Mississippi River by the 1990s (Raibley et al. 1995).

While C. idella are regarded as a threat to native submerged vegetation, they have also proved useful for controlling aquatic weeds (mostly exotics) in reservoirs and waterways, as an alternative to draining, mechanical cutting, or herbicides (Bain 1993). This dilemma led to efforts to develop nonreproductive fish for weed control. Early attempts through hybridization with other Asian carps were unsuccessful, but eventually techniques were developed to produce sterile triploids induced through cold shock or pressure. The U.S. Fish and Wildlife Service developed a certification program to test for triploidy in batches of fish to be stocked (Allen and Wattendorf 1987). State policies vary greatly with regard to this species. Ctenopharyngodon idella is now banned altogether in 18 states; stocking of triploids is permitted by United States Fish and Wildlife Service and by 14 states; 4 states allow importation of triploids only for closely regulated experimental use (Allen and Wattendorf 1987). Private, illegal stockings of sterile and fertile fish have occcurred in many states (Bain 1993; Jenkins and Burkhead 1993).

In the Chesapeake Bay watershed, diploid C. idella were stocked by private individuals in Potomac tributaries in the 1970s (Guillory and Gasaway 1978). A diploid population near Charlottesville VA was eradicated by the Virginia Department of Game and Inland Fisheries (Jenkins and Burkhead 1993). In the 1980s, triploid (sterile) fishes have been released in several VA (e.g. Lake Burke, Fairfax County) and DE reservoirs and ponds for vegetation control in 1983-1987 (Raasch and Altemus 1991; Jenkins and Burkhead 1993). At present, diploid, fertile C. idella are banned throughout the Chesapeake watershed. Stocking of certified triploid fish, with permits, is legal in VA, WV, PA and NY; only certified triploids can be imported. All C. idella are illegal in MD, DE and PA (Christmas et al. 2001).

Scattered introductions of C. idella have occurred through the watershed since the 1970s, but reproduction has not been documented (Jenkins and Burkhead 1993). The first record which we have documented from tidal waters was a fish collected by Maryland Department of Natural Resources in 1989, in the Potomac River, in Geogetown DC (Kazyak 1995). The fertility of the fish is not known.

A proposed stocking of 6300 certified triploid C. idella, to control submerged vegetation in powerplant cooling lagoons adjacent to Lake Anna VA (York River drainage) aroused concerns over the threat to submerged aquatic vegetation (SAV) in Chesapeake Bay from possible establishment of this fish (Terlizzi 1996). Modelling studies suggested that intensive stocking of C. idella in the Chesapeake watershed, would result in some inadvertent release of diploid fish (Jacobson and Kartalia 1994). In a compromise addressing these concerns, the Chesapeake Bay Commission required a more intense inspection (of each individual fish, rather than sampling) for this stocking (Terlizzi 1996). Schultz et al. (2001) examined the ploidy and fertility of 11 C. idella captured in the Chesapeake Bay watershed. Two of these fish, both female, from the Mattaponi (York) River VA and the Patuxent River MD, were diploid, indicating that illegal releases of potentially fertile C. idella are occurring in the watershed. Gonadal development was also seen in 4 triploid individuals, but these triploid fish are very unlikely to produce viable offspring (Schultz et al. 2001).

History References - Allen and Wattendorf 1987; Bain 1993; Courtenay et al. 1984; Florida Caribbean Science Center 2001; Guillory and Gasaway 1978; Jacobson and Kartalia 1996; Jenkins and Burkhead 1993; Kazyak 1995; Lever 1996; Raasch and Altemus 1991; Raibley et al. 1995; Robison and Buchanan 1988; Schultz et al. 2001; Shireman and Smith 1983; Terlizzi 1996

Invasion Comments:

Population Status - Given the putative sterility of populations in the Chesapeake drainage, this species could be treated as a either failed invader, or a potential one. Accidents or cheating are possible in the process of commercially producing 'certified' triploid fishes for distribution, although the inspection and certification process greatly reduces this risk (Allen and Wattendorf 1987). Illegal introductions of diploid fishes or dispersal from reproducing populations elsewhere are also possible.

This data was last modified on Tuesday, May 20th, 2014.
Databases Overview| Chesapeake Bay |