Grass Carp in the Chesapeake Bay: A Watery Gypsy Moth?

Contact: Alan Heft, Maryland DNR Inland Fisheries Management Division | 301.689.7107

Grass Carp in the Chesapeake Bay: A Watery Gypsy Moth?
Photo: USDA APHIS – Oxford, North Carolina Archives,

ANNAPOLIS, MD (March 5, 2009) – It is illegal to import grass carp (Ctenopharyngodon idella) into Maryland waters for many reasons, the most important of which is that grass carp pose a serious threat to the ecological integrity of Chesapeake Bay. Grass carp, also known as white amur, are large, salt-tolerant, herbivorous fish native to rivers in eastern China and Siberia. As their name implies, grass carp eat aquatic vegetation in huge amounts, particularly preferring submerged aquatic vegetation (SAV). Preserving and increasing SAV is a lynchpin in any plans to improve aquatic life in our great estuary, Chesapeake Bay. Now that the yellow perch are appearing in Chesapeake tributaries, and fishermen's thoughts turn to rivers, MISC has named the grass carp the March Invader of the Month.

Grass carp have chunky, torpedo-shaped bodies, and can reach a length of over 40 inches. In coloration they are typically silvery to dark olive along the back, fading to brownish-yellow on the sides, with white bellies. Identifying characteristics include firm lips with none of the barbels so obvious on catfish, the dorsal fins have three simple and seven branched rays, and individual scales have slight outlines around them.

Grass carp were first imported into the United States in 1963 to study their potential as a biological control for aquatic vegetation in freshwater ponds, lakes, and reservoirs. Research shows, however, that the filamentous algae that the fish were imported to eat is not their preferred food; instead they prefer rooted, submerged, vascular plants – submerged aquatic vegetation (SAV). The benefits to aquatic ecosystems from SAV are well documented and include water quality improvement, fish and wildlife habitat and food, and increased sportfish production. SAV presence and abundance is also considered a key indicator of the success of Bay cleanup efforts. Grass carp can consume up to ten times their body weight in vegetation daily, and add ten pounds of body weight annually. In their native rivers, they typically reach weights of 66 to 79 pounds, and can live up to 20 years. Research by Maryland DNR fisheries biologists indicates that the establishment of self-sustaining populations of grass carp in the Chesapeake Bay could eradicate all SAV in its tidal-freshwater tributaries.

As with any exotic introduced species, a major concern is whether it will escape into the wild and then be able to reproduce and establish a self-sustaining population. Grass carp have a strong desire to escape from a pond or lake to flowing waters, and have proven very successful at this in various places around the country where they have been introduced. Both supporters and opponents of grass carp agree that if you stock grass carp some will inevitably escape into the watershed. Initial assurances that grass carp would be incapable of reproducing in rivers of the United States, and thus posed no threat to the environment, have since been proven false. At present self-sustaining populations are known to occur throughout the Mississippi river system from Ohio to Louisiana and reproduction has been documented in the Galveston Bay estuarine system in Texas. Physical conditions in both the Susquehanna and Potomac rivers in Maryland provide ideal grass carp spawning conditions and successful reproduction would likely occur.

After grass carp reproduction had been found in U.S. waters, researchers turned their efforts to producing a sterile grass carp. The result was a "triploid" grass carp, so designated because the fish retains an extra chromosome set as opposed to a normal "diploid" grass carp. Triploid grass carp are similar in appearance, feeding, and growth to normal grass carp but are considered to be functionally sterile and incapable of reproducing in the wild.

Acknowledging that diploid grass carp pose a threat to the Bay, Virginia, Pennsylvania, Delaware, and New York allow pond owners to stock only triploid grass carp. However triploid grass carp use is not failsafe; production of triploids is not 100% successful and some diploids are also produced. The ploidy of each individual fish imported into neighboring states is not checked, the system relies on the honesty and accuracy of the fish importer and an inspection program of the U.S. Fish and Wildlife Service.

Another problem with triploid grass carp is that even if they can't reproduce they will still eat SAV. We estimate that 2,000 escaped adult triploid grass carp could consume 600 metric tons of SAV in one growing season. While it is recognized that nuisance aquatic vegetation has created resource use problems in some waters, grass carp use is not the solution. The majority of vegetation problems in small Maryland impoundments are caused by algal growth, typically filamentous alga that form dense mats on the bottom and surface. Grass carp will actually eat the grass and overhanging tree branches along a pond's edge before they eat algae! When they do ingest algae most of it is not digested, but just mechanically broken down into small pieces. The excretion of this material actually acts as a fertilizer and can cause a deterioration in water quality.

Maryland DNR believes that use of grass carp, diploid or triploid, in ponds and reservoirs within the Bay watershed is inconsistent with management of the Chesapeake Bay ecosystem and will continue to urge for a ban on the use of these fish

For more information about this and other Invasive Species of Concern, visit