30 MARCH 2016 • WORLD AQUACULTURE • WWW.WAS.ORG Although farmers apply such measures, problems still exist in shrimp ponds, particularly during the critical stages, when farmers will determine the concentration of Vibrio bacteria in pond water. Vibrio density is controlled by applications of Bacteriovorax, but the effectiveness of this bacteria is not ideal and it cannot always ensure a low pond concentration of Vibrio. In addition, significant changes in water quality will reduce shrimp immunity and allow rapid multiplication of pathogenic bacteria. Generally, numerous bacteria can simultaneously infect the shrimp. The use of Bacteriovorax alone cannot insure effective disease control. Normally, when disease is out of control, farmers will sell the remaining shrimp rather than attempting to improve the situation through desperate applications of veterinary drugs. Marketing and Economics After three months of production, in early June, shrimp farmers in Shanghai make the first harvest, and adult shrimp are sent to the market in batches. For example, one farmer places 25 kg of prawn larvae (2,000 individuals/g) in 15-mu (1-ha) ponds. In early June, the average size of shrimp reaches 9 g and the total amount of shrimp in the ponds is appropriately 500 kg. At this time, although shrimp are growing rapidly at optimum temperature, carrying capacity of ponds is limited. Hence, farmers will capture a batch of shrimp for delivery to market, opening more space for growth of the remaining shrimp. One farmer’s ponds are harvested three times, with an average of at least 1 t of shrimp at first harvest in early June, followed by around 1 t in midJune, and finally about 5 t in late June, reaching a total of up to 7 t. Shrimp market price fluctuates around US$ 7.80/kg and the first batch of shrimp yields gross returns of US$ 55,000. Expenses, including feed costs, were approximately US$ 17,628 (feed conversion was around1.0 for the whole cycle, with a feed price of US$ 2.50/kg); animal-protection products cost US$ 7,870; pond rent was US$ 3,148; juvenile shrimp purchasing expense was around US$ 1,574; electricity was nearly US$ 787; and equipment loss was US$ 787. Therefore, the farmer’s net income was US$ 23,610 after a three-month cycle. Usually, two production cycles of shrimp occur per year. Hence, farmers can earn at least US$ 47,220. However, farmers indicated that shrimp farming entails high risks. They are very concerned about large-scale shrimp disease outbreaks that will lead to crop failures. followed by a micro-diet for juvenile prawn and pellets for adult shrimp. Various feeds are available and farmers select suitable feed according to their needs. Farmers add probiotics to feed, such as photosynthetic bacteria, lactic acid bacteria, and Bacillus to improve the feed conversion ratio and enhance shrimp condition during feeding. Water Management and Shrimp Health Management Although larvae, water, and feed have been guaranteed, many problems still arise in farming, owning to the very high stocking density. The first step in daily management is water management, which aims to stabilize pond water quality. Generally farmers are very strict with controlling water, adding approximately10 percent of the pond water volume every two weeks. Farmers are confused with regard to aquaculture pollution. They consider the situation as a public misunderstanding about shrimp farming. The discharge from shrimp farming is very small and its pollution is far less than the pollution from the sewage of local citizens. In canals surrounding shrimp farms, sewage discharge causes water quality below normal standards during the middle and last part of the shrimp production cycle. High temperatures during summer aggravate the surrounding water eutrophication to a great extent. Generally farmers in Shanghai add groundwater to shrimp ponds during this period (Fig. 5). Farmers usually add Bacillus and Streptococcus faecalis probiotics to solve problems associated with excessive feeding. Bacillus breaks down organic matter with high efficiency, but much oxygen is consumed, requiring increased aeration. Streptococcus faecalis can degrade organic matter under anaerobic conditions but with relatively low efficiency. With reasonable use of probiotics, ammonia and nitrite in ponds is generally maintained at 0.1 mg/L or less, which not only keeps algae growth normal but also avoids threats to shrimp growth. The factors that threaten shrimp growth are related to large rainfall events that affect the stability of pond water quality, especially temperature and pH. Significant changes in water temperature also cause a strong reaction by the shrimp. The products mainly used by farmers to decrease shrimp stress include Calcii giucosum and water-soluble vitamin C. Probiotics are added to feed after the appropriate anti-stress products are added to ponds. FIGURE 3. This greenhouse has heating rods to control water temperature (Photo: Mingzhe Yuan). FIGURE 4. Shrimp post-larvae are cultured in greenhouses at a high stocking density. When juveniles reach 3-4 cm, they are transferred to grow-out ponds (Photo: Yanming Sui).
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