Farming system transformation yields significant reduction in nutrient loading: Case study of Hongze Lake, Yangtze River Basin, China

• In China, there is an increasing trend to attain environmental integrity in freshwater aquaculture.
• The present study was conducted in the reclaimed wetlands around Hongze Lake, the fourth largest freshwater lake in China.
• The gross N and P loading from mitten crab farming sector were estimated to be 7 165 t and 2 016 t, respectively.
• Mitten crab farming sector has resulted in a reduction of gross N and P loading by more than 75% and 84%, respectively.

China dominates aquaculture production accounting for 65.7% of the global total of 76.3 million tonnes. In view of increasing environment concerns resulting from intensification of aquaculture practices there is a notable shift of aquatic farming practices in China. In this study, the gross reduction of nitrogen (N) and phosphorus (P) loading into Hongze Lake, 1597 km2 (118.48–118.72° E and 33.36–33.38° N), of average water depth of 1.77 m, the fourth largest freshwater lake in China, resulting from the transformation of intensive polyculture of Chinese major carps to farming mitten crab (Eriocheir sinensis (Milne-Edwards, 1854)) is presented. This transformation occurred since the 1990s. Current mitten crab production is 59,932 tonnes, in 40,802 ha located in 17 farming areas in the reclaimed wetlands around Hongze Lake. Mass balance models were used to calculate the nutrient loading. The net N and P in effluent from different mitten crab farming areas ranged from 8.2 to 22.6 kg ha− 1 cycle− 1 (mean 14.2 kg ha− 1 cycle− 1), and from 1.3 to 3.5 kg ha− 1 cycle− 1 (mean 2.2 kg ha− 1 cycle− 1), respectively. The total net N and P in effluent from mitten crab farming sector were 579 t cycle− 1 and 90 t cycle− 1, respectively. Compared with fish polyculture prior to 1990s, the net N and P in effluent from mitten crab farming decreased by 2334 t cycle− 1 and 494 t cycle− 1, respectively. The above results are discussed in the light of mitten crab farming practices of using macrophytes that improve water quality and also provide refuge and natural food favored by mitten crab.Statement of relevanceThe present study attempts to estimate nitrogen and phosphorus loading and discharge from the mitten crab farming sector around Hongze Lake using the nutrient balance. The changes in nitrogen and phosphorus loading resulting from the transformation of intensive carp polyculture prior to 1990s to mitten crab farming in recent decades in the reclaimed wetlands around Hongze Lake were also evaluated. It is expected that the present study will provide additional information to improve environmental sustainability of aquatic farming system in China.