In composite culture of fishes, various different fishes like mrigal, rohu and catla can be cultured together in a single pond because

Composite fish culture, also known as polyculture or mixed farming, is an innovative concept in fish farming. Its main objective is to grow compatible fish species with different feeding habits in a single pond to maximize fish production. By selecting fish that do not compete with each other and have distinct feeding preferences, the system efficiently utilizes all available food resources. For example, species like Rohu, Catla, Mrigal, Common Carp, Silver Carp, and Grass Carp are combined to exploit various food organisms present in the pond.  Their feeding habits are entirely different, they never compete with each other and are not predatory fishes. Rohu is a column feeder and utilises the plankton. Catla is a surface feeder and feeds on only zooplankton. Mrigal is bottom feeder and feed on the plankton which is available at the bottom, mostly benthos. Common carp is also bottom feeder, but eats the detritus only. Silver carp is a surface feeder, but feeds only on phytoplankton. Grass carp feed only on aquatic vegetation. That means they utilize most of the food organisms present in the pond. This increases the fish yield from the pond.