Aquatic feed, because the digestive function of fish is weaker than that of livestock and poultry, so the protein content is higher, and a certain amount of animal protein is required. According to the source of raw materials, protein can be divided into two categories: animal protein feed raw materials and vegetable protein feed raw materials.

1) Classification by physical properties

1. Powdered compound feed: After crushing various feed materials to a certain particle size, they are fully mixed according to the formula ratio and packaged. When in use, add an appropriate amount of water and oil and stir evenly to form a block feed with strong cohesion, high elasticity and insoluble in water. Powdered feed is currently mostly used in aquaculture such as eel and silver carp.

2. Granular compound feed: It has a certain shape, such as a short rod. Particle size depends on the size of the fish and shrimp being fed. The diameter of the pellet feed for fish is 2-8mm, and the length is 1-2 times of the diameter. According to the processing method and the physical properties of the finished product, it can be divided into three types: soft pellets, hard pellets and extruded pellets.

(1) Soft granule compound feed: the moisture content is 25% to 30%, and the granule density is about 1g/cm'. The texture is soft. Poor stability in water.

(2) Hard granule compound feed: the moisture content is below 12%, and the granule density is about 1.3g/cm3. The production of hard pellet feed is a continuous mechanized production from raw material crushing, mixing to molding and granulation. Steam tempering before forming, the temperature can reach above 80°C during granulation. High degree of mechanization, large production capacity, suitable for large-scale production. The hard pellet feed has a fine particle structure, good stability in water, and the nutrients are not easy to dissolve and lose, and it is a sinking feed.

(3) Extruded compound feed: the moisture content is about 6%, the starch content is above 30%, and the fat content is below 6%. After the raw materials are fully mixed, steam and water are added, and then sent to the main part of the puffing equipment, and the temperature is continuously raised to 120~180°C through screw pressure and machine friction. When the feed is extruded from the hole die, due to the sudden drop in pressure, the volume suddenly expands to form foam particles with loose structure and firm granulation. The particle density is less than 1g/cm³, which belongs to floating feed.

(4) Granular compound feed: used for crustacean (such as shrimp) larvae, shellfish larvae, and fish larvae and larvae. Can also be used for filter-feeding fish. In the seed production, especially the seed cultivation of prawns, marine fish and famous and high-quality breeding objects, it is necessary to rely on diatoms, green algae, rotifers, cladocerans, copepods and other zooplankton. However, the cultivation of these biological feeds requires large-scale equipment and labor, and is limited by natural conditions, so it is difficult to ensure the needs of seedling cultivation. Therefore, many aquaculture workers attach great importance to the research and development of pellet feed.