Grass Carp Farming: A Production Guide

How grass carp (Ctenopharyngodon idella) is farmed: herbivorous biology, role in Chinese polyculture, hatchery induced spawning, growth, weed control and invasive-species caution.

Overview

Grass carp (Ctenopharyngodon idella) is a large herbivorous cyprinid native to eastern Asia, ranging from northern Vietnam to the Amur River. It is one of China's Four Great Domestic Fish and reportedly the freshwater fish with the largest farmed production in the world, over five million tonnes per year. It is farmed mainly in earthen ponds, almost always within a polyculture, and is also stocked to control aquatic weeds.

Biology and feeding

Grass carp is strictly herbivorous as an adult, feeding on aquatic macrophytes and submerged terrestrial vegetation and able to eat up to about three times its own body weight in plant material daily. It grows quickly and reaches a large size, commonly 60 to 100 cm and up to about 45 kg. In farming it is fed cut green fodder such as grasses and aquatic plants, often supplemented with formulated pellets.

Role in polyculture

In the centuries-old Chinese polyculture system, grass carp occupies the macrophyte (vegetation) feeding niche. Because its abundant droppings fertilize the water and promote plankton, grass carp is stocked together with plankton-feeding silver carp and bighead carp, which use that plankton and keep the water from becoming over-fertile. This division of feeding niches lets the pond support a much larger total harvest than any single species alone.

Reproduction and seed supply

Grass carp is a riverine spawner that, like the other major Chinese carps, does not spawn naturally in still ponds, so seed was historically gathered from rivers. Hatcheries now produce fry by induced spawning of pond-reared broodstock. FAO describes hormone induction using carp pituitary extract, human chorionic gonadotropin (HCG) or synthetic LRH-A, with spawning in an optimum temperature range of about 22 to 28 °C; natural spawning occurs at roughly 20 to 30 °C in flowing rivers.

Weed control and triploids

Because it eats aquatic plants so voraciously, grass carp is widely introduced to control nuisance vegetation in ponds, lakes and canals. To prevent the fish from breeding where it is not wanted, sterile triploid grass carp are often used for weed control, since they cannot establish reproducing populations.

Main diseases

The most serious disease of farmed grass carp is haemorrhagic disease caused by grass carp reovirus (GCRV), which can cause heavy losses in young fish. Bacterial infections also occur, particularly in crowded or stressed stock. Good water quality, controlled stocking and vaccination programs where available are the main defences.