Aquatic feed, because the digestive function of fish is weaker than that of livestock and poultry, it has a higher protein content and requires a certain amount of animal protein. According to the source of raw materials, protein can be divided into two categories: animal protein feed materials and vegetable protein feed 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 then packaged. When using, add appropriate amount of water and oil and stir well to form a mass feed with strong cohesion and elasticity, which is insoluble in water. Powdered feed is currently mostly used in eel, silver carp and other aquaculture.
 
2. Pellet compound feed: It has a certain shape, such as a short rod. The particle diameter depends on the size of the fish and shrimp being fed. The diameter of pellet feed for fish is 2~8mm, and the length is 1~2 times of the diameter. According to different processing methods and physical properties of finished products, it is divided into three types: soft pellets, hard pellets and extruded pellets.
 
(1) Soft pellet compound feed: the moisture content is 25%~30%, and the pellet density is about 1g/cm'. Soft texture. Poor stability in water.
 
(2) Hard particle compound feed: the moisture content is below 12%, and the particle density is about 1.3g/cm. The production of hard pellet feed is all continuous mechanized production from raw material crushing, mixing to molding and granulation. Steam quenching and tempering before molding, and the temperature can reach above 80℃ during granulation. High degree of mechanization, large production capacity, suitable for large-scale production. The particle structure of the hard pellet feed is fine, the stability in water is good, the nutrients are not easily dissolved and lost, and it is a sinking feed.
 
(3) Extruded compound feed: the moisture content is about 6%, the starch content is more than 30%, and the fat content is less than 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 increased by screw pressure and machine friction until 120~180℃. When the feed is extruded from the hole die, due to the sudden decrease in pressure, the volume expands at once to form foamed granules with loose structure and firm granulation. The particle density is less than 1g/cm³, which belongs to floating feed.
 
(4) Particulate compound feed: for crustacean (such as shrimp) larvae, shellfish larvae and fish larvae and juveniles. It can also be used for filter feeding fish. In the production of seed, especially the seed cultivation of shrimp, marine fish and famous, special and excellent breeding objects, all need 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 due to natural conditions, 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.