*Article* **Effects of Protein Level on the Production and Growth Performance of Juvenile Chinese Mitten Crab (***Eriocheir sinensis***) and Environmental Parameters in Paddy Fields**

**Yilin Yu 1, Jiwu Wan 2, Xiaochen Liang 1, Yuquan Wang 1, Xueshen Liu 3, Jie Mei 1, Na Sun <sup>4</sup> and Xiaodong Li 1,4,\***


**Abstract:** Rice–crab co-culture systems represent integrated agriculture–aquaculture systems developed in China over the last 30 years. The rice–crab co-culture area comprised approximately 1.386 <sup>×</sup> <sup>10</sup><sup>5</sup> hm2 in 2019. However, there is no specific feed designed for Chinese mitten crab (*Eriocheir sinensis*) cultured in this system until now. In this study, we investigated feed formulae for the nutritional requirements of Chinese mitten crab in this mode. The control group was not fed with any artificial feed (Co), and the experimental groups were fed with three different feeds of 15% (T15), 30% (T30), or 45% (T45) protein content, respectively. Growth performance variations in *E. sinensis* were investigated along with water quality, phytoplankton, zooplankton, aquatic vascular plants, and benthic animals in the paddy fields to determine the effect of crabs and their diet on the paddy ecosystem. Dietary protein levels had no significant effect on water quality. The biomass and species of phytoplankton, zooplankton, aquatic vascular plants, and zoobenthos in the paddy field were affected by crabs and their diet. Morphological parameters of crabs were significantly more pronounced in the high-protein group than in the other groups. However, the T45 diet negatively affected production by increasing feed costs, causing precocious puberty and inducing water eutrophication. In conclusion, adding a 15% protein compound feed can meet the nutritional needs of crabs, reduce culture costs, and improve water quality. The discharged water had low ammonia nitrogen and nitrite content and no eutrophication occurred, so the water could be recycled. These findings provide a scientific reference for supporting rice and fish co-cultivation.

**Keywords:** rice–crab co-culture; *Eriocheir sinensis*; dietary protein content; ecological environment
