Biology and management of grass carp (Ctenopharyngodon idella,Cyprinidae) for vegetation control: a North American perspective

Summary The grass carp is one of the largest members of the family Cyprinidae. In their native habitat, grass carp typically reach weights of 30–36 kg, but fish have been reported up to 181 kg. Successful reproduction is a function of temperature, age/size, and water conditions. Fish reach maturity when about 4 years old (4–5 kg). Spawning occurs when water temperature rises above 20°C. Because grass carp eggs are semipelagic, current during spawning is required to keep eggs in suspension while they incubate. In general, successful spawning takes place under rising water conditions in very long rivers. Fecundity is very high in normal diploid individuals; females may produce over one million eggs in a season. In triploid fish, eggs and sperm are produced, but the incidence of viable offspring (even when mated with diploids) is very low.Grass carp fry begin feeding on microscopic animals and gradually switch to plant material as they grow. Adult diploids, triploids, and hybrids ( grass carp x bighead carp (Hypophthalmichthys nobilis, Cyprinidae)) are all herbivorous. Feeding is strongly affected by temperature. Active feeding begins as temperatures rise above 7–8°C and consumption peaks at 20–26°C. Whereas triploids feed at nearly the same rate as diploids, hybrids feed at substantially lower rates. Therefore, vegetation control is most efficiently achieved with diploid or triploid fish. These fish may consume more than their own weight in plant material each day. However, feeding rate (and hence vegetation control) is affected by the forage that is available. Grass carp exhibit strong preferences for different macrophyte species depending on the aquatic system (i.e. the same plant species is not always the most preferred). Hydrilla verticillata and similar species are almost always among the most preferred species, and control or elimination is usually assured if adequate stocking densities are used. Vegetation control has been reported with stocking densities as low as six fish per vegetated ha.Grass carp currently appear to be the most effective biological control on aquatic vegetation. However, in order to avoid ecological disaster, care should be taken to limit their use to sterile forms and to prevent their invasion of ecologically sensitive areas, such as waterfowl staging areas and threatened or endangered species habitat.