Origin and Biology of Grass Carp (White Amur)
The grass carp, also known as the White Amur (Ctenopharyn-godon idella), is a Chinese carp imported into this country as a means of achieving biological aquatic weed control. It is native to southeast Asia, and was brought into the United States in the early 1960’s as an experimental aquatic weed control method. Since that time, use of grass carp has become commonplace. Triploid Grass Carp are specially produced in hatcheries and possess three sets of chromosomes instead of the normal two. This abnormal condition causes sterility, so these are the only exotic fish that can be legally used for aquatic weed control in most states. A permit is usually required for possession and use of Triploid Grass Carp. Because they cannot reproduce, the number of fish present in an area can be regulated.
Until they are about 2 inches long, grass carp feed almost exclusively on microscopic animals called zooplankton. They become dedicated vegetarians, however, after they reach a length of about 4 inches. The amount of vegetation they will consume depends upon several environmental conditions, such as water temperature, water chemistry, and the kinds of plants available. Consumption rates also vary with fish size. For example, until they reach weights of about 6 pounds, grass carp may eat 100 percent of their body weight in vegetation per day. (This is equivalent to a 150-pound human eating 150 pounds of food per day.) As they grow larger, consumption decreases; up to about 13 pounds, they will eat 75 percent of their body weight per day, and above 13 pounds, they slow down to about 25 percent of body weight per day.
Grass carp are a viable and economical means of controlling the growth and spread of certain aquatic weeds. It is critical that problem weeds be properly identified as a preferred food for grass carp, or they may not provide acceptable control. When stocked at low to moderate rates, grass carp will not muddy a pond as do their cousins, the common carp. They typically will not disturb the nests of other fish (bass and bream), and they are not predatory, so there is no concern about their eating desirable sport fish.
Once carp reach 20 to 30 pounds, their effectiveness as a weed control agent is diminished, since their food consumption is reduced and they are not growing as rapidly as do the smaller fish. Thus, periodic restocking (5- to 7-year rotation) may be required for permanent weed control. This, however, still represents a substantial cost savings over the use of chemicals, in many situations.
Although grass carp sometimes take on off-flavors from their diets of aquatic plants, their flesh is firm and they do not have excessive intramuscular bones. Many consider them to be excellent table fare!