www.fao.org/docrep/field/003/...c375e/AC375E03.htm
The concept of polyculture of fish is based on the concept of total utilization of different trophic and spatial niches of a pond in order to obtain maximum fish production per unit area. Different compatible species of fish of different trophic and spatial niches are raised together in the same pond to utilize all sorts of natural food available in the pond.
In general, undrainable pond is characterized by its diversified spatio-trophic environment comprising of various natural fish food organisms (Phytoplankton, Zooplankton, Periphyton, Macrophytes, Benthos and detritus) at different strata of pond water column as well as in the bottom. Selection of species in polyculture is thus very important. There should be a compatible combination of species with diversified feeding habit that should include planktivorous surface/column feeders to benthic/detritivorous bottom feeders as well as omnivorous to macrovegetation feeding fish species.