Effects of Two Feeding Modes on the Culture Performance and Physiological Metabolism of Juvenile Chinese Mitten Crabs (Eriocheir sinensis) Reared in Rice Fields

The Chinese mitten crab (Eriocheir sinensis) is an important economic crab in China. Across most of China, juvenile Chinese mitten crabs are cultured in earthen ponds. Juvenile crab cultured in rice fields is mainly restricted to the northeastern region. Rice-crab coculture is a new mode of sustainable development, which has developed over the past 30 years in China with progress in field engineering, culture density of the Chinese mitten crab, and ecological benefits. However, there are no reports on appropriate feeds in rice-crab coculture. Presently, crabs reared in an earthen pond consume a traditional diet (including soybean meal, corn, bran, and wheat) or formulated feed. Due to the numerous advantages of formulated feed (including comprehensive nutrition, stable quality, and ease of feeding) the development of practical and cheap formulated feed for Chinese mitten crab is the latest focus for promoting the sustainable development of the Chinese mitten crab industry. Although researchers have developed a series of formulated feed for Chinese mitten crab, traditional feed is more commonly used in the culture of Chinese mitten crab due to farmers feeding habits and the low price of traditional products. However, numerous studies have shown that feeding Chinese mitten crabs formulated diets has many advantages, for example: it improves the survival rate, reduces the early maturity rate, improves non-specific immunity and disease resistance of juvenile crabs, improves the quality and culture performance of crab species, and economic benefits, etc. However, research were usually conducted under indoor or pond culture conditions. The culture environment of rice fields and ponds are quite different to each other. The feed sources and environmental stress on juvenile crabs may also be quite different between the culture environments. Therefore, it is important to explore the feeding mode of juvenile crabs cultured in rice fields. However, the optimum feed and the effect of different diets on the growth and physiological metabolism of Chinese mitten crab reared in rice fields remains unclear. It is important to optimize the rice-crab breeding mode to achieve a high-quality Chinese mitten crab industry. Formulated diets and traditional diets (soybean meal, bran, corn, and wheat) were fed to juvenile E. sinensis cultured in a rice field for 152 days. Each feeding group included three replicates. This study was conducted to compare formulated diets with traditional diets using culture performance, protein metabolism, digestive enzymes, and the antioxidant and immune indexes of juvenile crabs. The results showed that: (1) the body weights of female and male crabs from the formulated diet group were higher than those in the traditional diet group, but the differences were not significant (P > 0.05) (Fig.1–3); (2) in terms of culture performance, the average body weight and yield of juvenile crabs, 1-year-old precocious crabs, and the overall total yield and survival rate of juvenile crabs in the formulated diet group were slightly higher than those of the traditional diet group. The feed conversion ratio of the formulated diet group was much lower than that of the traditional diet group (P < 0.05); (3) in terms of protein metabolism, the total protein content in the hemolymph of both female and male crabs fed the formulated diet was significantly higher than that in the traditional diet group (P < 0.05). Conversely, the contents of urea nitrogen in the hemolymph of male crabs and the activity of glutamic oxaloacetic transaminase in the hepatopancreas of the traditional diet group were significantly higher than those of the formulated diet group (P < 0.05); (4) lipase in the hepatopancreas of female and male crabs in the formulated feed group was significantly higher than that in the traditional diet group (P < 0.05). Conversely, the α-amylase activity of the male crabs in the formulated diet group was significantly lower than that of the traditional diet group; (5) the total antioxidant capacity, alkaline phosphatase, and acid phosphatase activities in the hemolymph of female crabs and the acid phosphatase activity in the hepatopancreas of the formulated diet group were significantly higher than those of the traditional feed group (P < 0.05). In conclusion, feeding a formulated diet can improve the culture performance, promote protein deposition and lipid absorption, and enhance the antioxidant and immune capacity of juvenile E. sinensis reared in a rice field. These factors result in improved crab quality. This study provides a basis for optimizing the culture technology of Chinese mitten crab and the development of a formulated diet for improved rice-crab coculture.