In mixed farming systems, one type of sustainable agriculture is combined with livestock, fish farming or forestry. In a closed mixed-farming system, the "waste" from one activity is the input for the other, and different nutritious products are produced in this way. For example, in China's Zhejiang Province, the waters of a river that flows into rice paddies are stocked with carp, a fish that serves as a major source of protein and income for farmers and their families.