Pollutant Loads and Partial Nutrient Budgets in Crab Ponds Operating By Two Farming Modes in the Middle Reach of Yangtze River Basin, China

Most of lakes along the Yangtze River were used into fish farms where many of them have been used for Chinese mitten crab (Eriocheir sinensis) farming. The rapid increase in cultured crab production has caused environmental problems. An experiment was conducted to estimate the pollutant loads and partial nutrient budgets of nitrogen and phosphorus in 4 lake-reclaiming ponds which were separated into 2 groups (farming mode I: pond 1 and 2; farming mode II: pond 3 and 4). The stocking densities of crab were similar, while stocking size in mode I and II were 10.00±2.86 and 5.56±1.65 g ind^-1, respectively. The results indicated that net loads of nitrogen, phosphorus and chemical oxygen demand through effluents in mode I were higher than those in mode II. According to the partial nutrient budgets, it is found that amounts of nutrients recovered by crab in mode I were lower than those in mode II. However, the quantities of nutrients discharged to environment in mode I were obviously higher than those in mode II. Therefore, in order to meet the environment-friendly production practice, the effects of crab ponds operating by different farming modes on water environment should be evaluated.