Because most of the bacteria, phytoplankton, and aquatic animals in the ricefield cannot be used by the rice, they are lost with the irrigation water. Moreover, insects, pests, and mosquitoes can reproduce rapidly and adversely affect both rice and human health. When fish are introduced into ricefield ecosystem, the population and composition of aquatic organisms, and the relationships among them, change. Population numbers change. Fish, the largest consumers, eat weeds, phytoplankton, zooplankton, aquatic insects, and other animals.
Fish have the greatest effect on population density and mortality.
Because they are primary consumers, grass carp, common carp, and crucian carp feed heavily on weeds. In China, more than 100 varieties of weeds grow in ricefields. Of these, Hydrilla verticillata , Ptamogeton crispus , Vallisneria spiralis , Potamogeton matainus , and Lemna spp. are considered to be good feed for grass carp. Therefore, rice fish culture can effectively eradicate weeds and control the loss of energy from ricefields. Rice fish culture can change the direction of energy flow in the ecosystem.