Title: Characteristics and environmental aspects of the improved extensive shrimp system in Cai Nuoc district, Mekong delta of Vietnam
Other Titles: Eigenschappen en milieu-impact van het 'verbeterd extensief' garnalenproductiesysteem in Cai Nuoc district, Mekong delta van Vietnam
Authors: Nguyen, Tho; S0196481
Issue Date: 26-Oct-2012
Abstract: The Mekong delta of Vietnam covers 39,000 km2 and supports a population of approximately 18 million people. It holds 88% of the shrimp aquaculture areas and accounts for more than 75% of the total shrimp production in Vietnam in 2010. Following the initial success of shrimp aquaculture practices in the late 1990’s, the central government released the resolution 09/NQ-CP in 2000 which allowed the conversion of coastal rice fields into shrimp farms. As a result, shrimp aquaculture boomed in the area. In Cai Nuoc district (Camau province, Mekong delta of Vietnam), all of the rice farmers adopted the conversion in that year or the year after. Besides the rice fields, vast areas of coconut farms, orchard areas, and small plots of land around the houses were also converted. Most of the shrimp farms are of the so called "improved extensive" shrimp system type, in which production is all year round, stocking density is low (0.8-5.5 post-larvae/m2 followed by supplementary stocking), no feeding is done, fertilizers are irregularly applied, water exchange also is irregular and sediment removal limited. The average shrimp yield is currently about 390 kg/ha/yr and is lower compared to initial yields (500 kg/ha/yr) or to potential yields (600-1,500 kg/ha/yr) in shrimp systems of similar stocking densities. This shrimp system is the most abundant (>70% of the area) in the district and has been adopted mainly by the low and middle income farmers. This study aims to identify the yield limiting factors of the improved extensive shrimp (Penaeus monodon) system in Cai Nuoc district. We hypothesize that (1) the management strategy applied to this shrimp system is not suitable, (2) the physico-chemical and biological characteristics of water and sediment in the improved extensive shrimp ponds are not optimal for shrimp production, and (3) sediment removal in the wet season during shrimp production significantly improves pond conditions of the improved extensive shrimp ponds.To explore pond characteristics of the improved extensive shrimp system and quality of water and sediment in the surrounding waters of Cai Nuoc district, a survey was carried out between 2008 and 2011. In this survey, the physico-chemical characteristics of pond water of the improved extensive shrimp system proved to be not optimal for shrimp aquaculture. Several key parameters (dissolved oxygen (DO), total suspended solids (TSS), and hydrogen sulphide (H2S)) very often showed values beyond the acceptable range for shrimp aquaculture throughout the year. DO shortage in early mornings (<3 mg/L) is a common, serious problem affecting shrimp health. In some extreme cases, DO concentrations dropped to less than 1 mg/L, which can cause shrimp mortality within a few hours of exposure. Large daily fluctuations of DO provided undesirable conditions for the cultured shrimps. In the wet season, high TSS concentrations (mean 234.1 mg/L in October) might cause a reduction in photosynthesis intensity. Total Fe concentrations were higher than recommended. H2S was found in concentrations (max. 0.02 mg/L) that can be toxic to the cultured shrimps. Large seasonal variations of salinity might reduce shrimp immunocompetence to Photobacterium damselae. Very low salinities in the wet season (5.71 g/L) may lead to acute stress which causes increased susceptibility of shrimp to white spot virus infection. The top sediment (0-5 cm) of the improved extensive shrimp ponds was found to be anaerobic (redox potential (Eh) -304 mV), which often entails the production of toxic, reduced substances. Due to the accumulation of organic materials, contents of organic matter (OM) in the sediment were high (9.84-21.96%), which may impact the cultured shrimps by favouring higher microbial activities. The pH values of the shrimp pond sediment were lower than the optimal range for shrimp aquaculture (pH 7-8). This suboptimal pH also led to a lower rate of organic matter decomposition on shrimp pond bottom. The low DO concentrations, rapid salinity changes, low salinities in the wet season, anaerobic pond bottom, presence of H2S and shallow pond water during the dry season are physico-chemically the main stress-causing factors that might lead to lower shrimp yields in the improved extensive shrimp system. Chlorophyll-a (Chl-a) concentrations in pond water showed a wide range (1.51-37.2 micrograms/L). It peaked in April, coinciding with peaks of nitrate (NO3-N), total Kjeldahl nitrogen (TKN), alkalinity of pond water, and phytoplankton counts. Chl-a values were positively correlated with pH (p<0.05) and temperature (p<0.01), indicating a higher phytoplankton photosynthesis at higher amounts of solar radiation reaching pond water. Phytoplankton was abundant (6,000-1,000,000 cells/L) with diatoms being the most dominant group (39%), followed by euglenoids (23%), green algae (17%), blue-green algae (14%), and dinoflagellates (7%). Phytoplankton species which could produce cyanotoxins were found in the ponds. Zooplankton was also abundant (7.1-517.2 ind/L) with copepods being the most dominant group. The peak of zooplankton occurred in April, coinciding with that of phytoplankton. The zoobenthos community was, however, poor (7-1,971 ind/m2), hinting at a limitation for shrimp aquaculture as zoobenthos are one of the major food sources for the cultured shrimps. The combination of fish (Asian seabass, Tilapia, Marble goby, and Grass Carp) into the shrimp ponds was probably involved in the poorness of zoobenthos as fish are known to consume zoobenthos. Total bacteria and Vibrios were abundant in the shrimp ponds. In shrimp pond water, bacterial counts showed negative correlations with salinity, chloride (Cl-), and sulphate (SO42-) (all at p<0.01) but positive correlations with biochemical oxygen demand (BOD) (p<0.001) and nitrite (NO2-N) (p<0.01), suggesting a bloom of the bacterial community in the abundance of biodegradable organic matter at low salinities. In pond sediment, bacterial counts showed positive correlations with total nitrogen and ammonium (NH4-N) (both at p<0.05), indicating that nitrogen is needed for bacterial growth under anaerobic pond bottom condition. Negative correlations between bacterial counts and pH of dry sediment (pH1/2.5) (p<0.01), and between Vibrios and pH of freshly collected sediment (pHw) (p<0.05) in pond sediment suggested that lower sediment pH can stimulate bacterial growth. Counts of total bacteria in water and in sediment were positively correlated (p<0.001), indicating a mass exchange between the two components of the aquatic system. Large numbers of total bacteria (>10000 CFU/mL) and Vibrios (>1000 CFU/mL) indicate a high potential risk for outbreaks of shrimp diseases. The low density of benthos, presence of toxin producing blue-green algae, and the high counts of Vibrios in both pond water and sediment are the main biological factors that contribute to shrimp stress and possibly lower shrimp yields.Manually removing and dumping the removed pond sediment on pond dikes in the wet season during shrimp production did not significantly improve the water and sediment quality of the improved extensive shrimp ponds. The physico-chemical and biological properties of the ponds remained almost unaltered compared to the initial conditions. The only benefit of this management activity was just a small and temporary increase in redox potential (p<0.01), which might lead to a minor decrease in harmful substances produced in the anaerobic pond bottom. Financially, the labour cost of sediment removal constitutes the main cost of shrimp pond preparation. The shrimp farmers are, therefore, strongly recommended not to undertake this task in the wet season as it is simply a waste of time, money and other resources.In the last part, environmental aspects of the shrimp productions were assessed. The waters surrounding the ponds are saline, even at the end of the wet season (salinity 17.23&plusmn;2.44 g/L). This is the direct consequence of bringing in salt water to the interior field for shrimp aquaculture since 2000. Organic loadings in the surrounding waters (BOD 4-9.6 mg/L, chemical oxygen demand (COD) 17-80 mg/L) were higher than those in the shrimp ponds (BOD 0.32-5.68 mg/L, COD 2.8-22 mg/L). Because most shrimp farms in the area are extensive and "improved extensive" shrimp system types of low intensification levels, contribution of organic loadings from shrimp pond discharge to the surrounding waters is small. Most of the organic loadings in the surrounding waters originate from domestic and industrial effluents, and discharge from the local markets other than from pond discharge. The river sediment was saline (conductivity of saturated extract (ECe) 11.79 dS/m) and anaerobic (Eh -274.1 mV). It is characterized by high contents of organic matter (OM 11.07 %). Hydrogen sulphide was present in the river sediment (mean 154.2, max. 357.5 mg/kg). Phytoplankton density in the surrounding waters was significantly lower than that in the shrimp ponds while densities of zooplankton and macrobenthos did not differ between the two environments. The surrounding waters of Cai Nuoc district can be classified as from heavily to moderately polluted.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division Soil and Water Management

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