Factors driving low oxygen conditions in integrated rice-shrimp ponds

Integrated rice-shrimp ponds (IRSPs) are common in areas of Southeast Asia where saltwater intrudes into rice fields in the dry season, enabling rice production in the wet season, and shrimp farming in the dry season or throughout the year. Previous research has highlighted that IRSPs have periods of low dissolved oxygen concentrations which may have a critical effect on shrimp survival. To understand the causes of low dissolved oxygen, this study examined oxygen fluxes at two IRSPs in Ca Mau Province, Vietnam during a two-year period (two wet seasons and two dry seasons). Sediment oxygen demand (SOD) incubations and whole-system oxygen flux measurements were conducted and compared with a range of water and sediment parameters to explain drivers for low oxygen concentrations. A high percentage of oxygen demand at a whole pond scale was from the sediment; hence SOD drove low oxygen concentrations in the water column. SOD rates were significantly positively correlated with chlorophyll a concentrations in the water column. These findings suggested that algal production in the water column, rather than benthic algal production, or other organic loading, provided an organic carbon source driving SOD. Oxygen demand was much higher than oxygen production within the IRSPs, indicating high bacterial activity and low algal production. This study has shed new light on the importance of SOD in driving oxygen drawdown in IRSPs and improving shrimp survival requires new management approaches to reduce the negative effect of SOD

Does natural feed supply the nutritional needs of shrimp in extensive rice-shrimp ponds? – A stable isotope tracer approach

Rice-shrimp culture systems occur throughout Asia where there are seasonal alterations of fresh and saltwater availability. During the dry season, black tiger shrimp are grown at low densities, and in the wet season, rice, or rice and shrimp are grown together. Previous studies point to issues with suboptimal rice and shrimp production, due, in part, to climatic conditions driving changes in water quality, and pond management practices. However, the availability of natural benthic food supplies for shrimp production is less well studied. This study used a 15N‑nitrogen stable isotope tracer added to replicated enclosures within two rice-shrimp ponds to trace shrimp feeding on benthic natural biota. This experiment was conducted in both the wet and dry seasons. The 15N-ammonium rapidly enriched the 15N signature in the particulate organic matter (POM) in the water column, indicative of significant phytoplankton uptake of nitrogen (N). In contrast there was little enrichment of the benthic algae in the same time frame. After 13 d, most of the shrimp in the replicate enclosures had little or no 15N enrichment. This result occurred across both ponds and in both seasons. This suggests that enriched N was not transferred through the benthic food web to the shrimp. The results of the 15N enclosure experiments were combined with biomass estimates of benthic algae and macrobenthos, shrimp growth data, and linked to other parallel studies of water quality and sediment biogeochemistry in the same ponds. Overall, the findings point to inadequate feed stocks for shrimp, and hence the need to consider supplemental feeding with high quality formulated feeds in order to improve growth rates and survival