Answer Analysis：China has excellent freshwater fish species, mainly bighead carp, silver carp, grass carp, bluegill, crucian carp and carp, of which bighead carp, silver carp, grass carp and bluegill are usually called the "four big fishes", and they are the selected species of fish for mixed culture and high yield in pond fish culture in China for more than 1,000 years. The bighead carp, also called silver carp, is active in the upper layer of the water and eats phytoplankton such as green algae. The bighead carp, which has a larger head and is commonly known as the "fathead carp", is also called silver carp, and inhabits in the middle and upper layers of the water and eats protozoa, water fleas and other phytoplankton, the grass carp lives in the middle and lower layers of the water and feeds on aquatic grasses, and the cyprinid fish inhabits in the lower layer of the water and eats mollusks such as snails, clams and mussels. , clams and mussels, and other mollusks. A mixture of these four species improves bait utilization and increases fish production.

Examination point: The question is "four big fish", the key to answering such questions is to familiarize themselves with the four big fish and their characteristics.