Leech Farming Techniques: What Are the Requirements for Bait?

I. Nutritional Requirements of Leeches. Like other animals, leeches require five major categories of nutrients for their growth and development: proteins, fats, carbohydrates, inorganic salts and vitamins. At different developmental stages and in different environments, leeches also have different requirements for nutrients. If the functions of each type of nutrient are correctly understood and bait is used rationally, this is of great significance for promoting the healthy growth of leeches and increasing production.

(1) Proteins. Proteins are the main component of the cells of the various organs and tissues in the leech body. The various pigments, antibodies, hormones, enzymes and so on of leeches are also composed of proteins. The structural components of proteins are amino acids, and their main functions are to build new tissues, such as for growth, development and reproduction; to maintain balance among the various systems in the body, such as excretion; to regulate cells and body fluids, such as regulating the acid-base balance of body fluids and producing growth and digestive hormones; and to produce energy, with each gram of protein producing about 16.7 kilojoules of heat. Therefore, a protein deficiency will affect the growth of the leech body. Leeches have a relatively high demand for protein, and when starving they will use protein as the main energy substance to sustain life. The protein requirement of juvenile leeches is about 30% of the total bait. As the individual grows, the amount of protein required as a proportion of the total bait also gradually increases. During the reproductive period, the protein requirement of leeches reaches about 80%.

(2) Fats. Although fats are not present in large amounts in the leech body, they are also widely distributed in the various tissues of the leech body. Especially during the reproductive period and hibernation period, leeches rely on the fat stored in adipose tissue to maintain their physiological needs. In the processes of decomposition, transformation, absorption and utilization, fats can form hormones and various substances secreted by other endocrine glands. Therefore, fats are an indispensable nutrient for the growth and reproduction of leeches. Since leeches can convert carbohydrates into fat, and ordinary bait generally contains a certain amount of fat components, the fat nutritional requirements of leeches are not difficult to meet.

(3) Carbohydrates. Carbohydrates are the general term for starch, sugars and cellulose, and are the main source of heat energy for the leech body. The functions of carbohydrates are: first, to meet the direct energy needs of the leech's tissue cells; second, to be converted into glycogen and stored in the liver and body tissues to prepare for future energy needs; third, to be converted into fat as a reserve of greater energy.

(4) Vitamins. Vitamins are indispensable nutrients for maintaining the normal physiological functions of leeches, and they are the basic components of coenzymes or prosthetic groups. If vitamins are lacking in the leech body, it will lead to disordered enzyme activity, metabolic disorders and the appearance of disease symptoms. For example, long-term lack of vitamin A may cause lesions of the leech epidermis; long-term lack of vitamin E may cause symptoms such as muscle atrophy and slow, weak crawling; long-term lack of vitamin D may affect the normal metabolism of calcium and phosphorus, and so on.

(5) Inorganic salts. Inorganic salts mainly include elements such as calcium, phosphorus, sodium, sulfur, chlorine and magnesium. They are indispensable nutrients for composing the tissues of the leech body and maintaining normal physiological functions, and are also important catalysts in enzyme systems. They can improve the utilization rate of water and nutrients and promote growth and development. A lack of inorganic salts will lead to disease.

II. Management and Sources of Bait. (1) Green and succulent bait. Green and succulent bait has a relatively high moisture content, relatively low crude protein content, and medium nitrogen-free extract content (Table 5-1). It can serve as auxiliary bait for young leeches and can also serve as a raw material source for natural bait for leeches.

(2) Energy bait. Among energy baits, the nitrogen-free extract content is relatively high, while the protein content is relatively low (Table 5-2). They are the main bait for young leeches and for adult leeches preparing to overwinter.

(3) Protein bait. Protein bait has a relatively high protein content, especially animal-based bait, which has an even higher crude protein content (Table 5-3, Table 5-4, Table 5-5). It is the main bait source for growing leeches, adult leeches and leeches during the reproductive period.