Economic and nutrient discharge tradeoffs of excreta-fed aquaculture in the Mekong Delta, Vietnam

The present study quantifies the effects on production, nutrient discharge and economic return of the use of pig and human excreta in pond farming. On-farm data from various studies were integrated and analyzed applying single and multiple regression methods. Pond-dissolved oxygen concentration, water exchange and nutrient discharge interacted and were strongly affected by input level. Increased input levels coincided with farmers exchanging more water and discharging more chemical oxygen demand (COD), nitrogen (N), phosphorus (P) and total suspended solids (TSS). Fish yield and the accumulation of organic carbon, N and P in pond sediments increased with the excreta input level. Using a regression model, it was predicted that with an excreta input of 5 kg N ha−1 day−1, a fish yield of 8380 kg and an economic return of 52 million VND ha−1 year−1 can be obtained while about 2060 kg COD, 645 kg N, 210 kg P and 39,200 kg TSS ha−1 year−1 will be discharged. At this input level, it was estimated that about 9% of input-N will be recovered in harvested fish while 52% will accumulate in the pond sediment. Hence, fish culture reduces nutrient discharge from excreta by 61% while generating income for resource-poor farmers. However, in the long run such a system will become unsustainable when more farmers take up this farming practice. The challenges are to reduce nutrient discharges from ponds while maintaining high production and profitability and to use the nutrients accumulated pond sediments more efficiently.