50 DECEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Some other culture modes include rice-prawn culture, polyculture with fish, polyculture with Pacific white shrimp Litopenaeus vannamei and polyculture with crab Eriocheir sinensis. However, these culture systems are not large and often used only in experimental research, according to local conditions, and have not been developed on a large scale. Main Issues of Commercial Freshwater Prawn Farming in China The giant freshwater prawn has been grown in China for more than 30 years. So far, no country other than China has such an extensive farming area, even in the natural range of this species in other countries of Southeast Asia. Nonetheless, there are problems in the freshwater prawn industry of China. If these problems cannot be solved, they will have a serious negative influence on the sustainable development of prawn farming in China. Genetic degradation. At present, all cultured stocks of M. rosenbergii in China are offspring of the original stock, a small group containing only one pair of broodstock and 48 postlarvae that was introduced from Japan in 1976. Since then, the giant freshwater prawn culture industry relies on non-improved stocks after more than 30 generations of reproduction. Little attention has been paid to genetic improvement in culture stocks. Broodstock are often collected directly from grow-out ponds. This phenomenon causes a high level of inbreeding, which can have a negative effect on stock productivity. Genetic diversity in cultured stocks is low as a result of inbreeding (Schneider et al. 2013). Genetic degeneration of the stock is indicated by slow growth rate, early sexual maturity, individual miniaturization and vulnerability to disease. Some female prawns become sexually mature at 6 g but fecundity is only 1,000-3,000 eggs in these small mature females (Fig. 2). Eggs obtained from these females often result in poor-quality offspring. Lack of quality seed. Healthy and otherwise good-quality seed is the basis of sustainable development of the giant freshwater prawn farming industry. In China, the annual production of the giant freshwater prawn is over 100,000 t. The total demand for goodquality seed is 20-30 billion post-larvae. The particular demand for prawn seed is greatest near Gaoyou city, Jiangsu Province, the most important M. rosenbergii culture region and where demand for seed is around 15 billion post-larvae. Before 2010, when sufficient prawn seed could be produced by hatcheries, the average price of seed was only US$ 16 per 10,000 post-larvae. However, the price of highquality seed increased to over US$ 64 per 10,000 post-larvae after the outbreak of M. rosenbergii larval syndrome, which caused many private hatchery enterprises to close, resulting in a shortage of seed. Even more serious is that post-larvae produced by some private hatcheries grew slower than normal prawn seed, even stopping growth at 5-6 cm, leading to a smaller harvest size. These male prawns had the remarkable characteristics of a larger cephalothorax, longer second chelipeds, smaller abdomen (small edible portion), all similar to old blue-claw males (Ranjeet and Kurup 2002). The presence of large quantities of small male prawns led to a reduction of output and many farmers suffered severe economic losses. Local prawn farmers called this prawn type “stone prawn” or “iron prawn.” Finally poor-quality prawn seed contributes to the abuse of antibiotics in prawn hatcheries. An inadequate supply of quality seed is a major constraint to the healthy development of freshwater prawn faming in China. New disease outbreaks. In the late 1990s, whitish muscle disease was first recorded in Guangdong Province, spreading gradually to Guangxi municipality, Zhejiang Province, Jiangsu Province and Shanghai. These disease outbreaks caused mass mortality in postlarvae ponds within a few days and serious economic losses in many prawn hatcheries and prawn farms (Qian et al. 2006 b). Later, through the work of scientific researchers, the causative agent of whitish muscle disease was determined to be a virus called MrNV (Qian et al. 2003 a). The development of SPF seed production technologies has been effective in preventing the whitish muscle epidemic. However, since February 2010, the giant freshwater prawn seed production industry in China has suffered a severe recession from outbreaks of M. rosenbergii larval syndrome in many hatcheries, leading to mass mortality (80-90 percent) in larval stages. This disease spread across the country and many hatchery enterprises have closed down. The supply of prawn seed decreased greatly, causing heavy economic losses to hatcheries and farmers and brought tremendous constraints to the development of the freshwater prawn industry. The national production of M. rosenbergii in 2010 decreased by 13 percent compared to 2009. In 2011, the farmed production decreased further to 122,923 t, which was 21,544 t less than in 2009. Macrobrachium rosenbergii dicistrovirus (MrDV), a new pathogen, was the causative agent of the nationwide infectious disease outbreak (Pan et al. 2010). Additionally a new spiroplasma was found in diseased giant freshwater prawns in grow-out ponds (Liang et al. 2011). Suggested Solutions Effective measures must be taken to insure the healthy and sustainable development of the freshwater prawn farming industry in China. Genetic improvement of giant freshwater prawn. It is widely accepted that genetic improvement is the effective method to improve economically important traits in M. rosenbergii. Now, traits such as body weight, survival rate, growth rate and disease-resistant performance are often used as important economic traits in genetic improvement programs. Since the 1990s, few scientists have carried out studies on M. rosenbergii breeding. The methods of new variety selection include crossbreeding, artificial selection, individual selection and family selection. Selection programs for new varieties of M. rosenbergii are an effective method to achieve sustainability of giant prawn farming. Improvement of quality-seed production systems. It is necessary to guarantee the quality of broodstock prawns to produce betterThe giant freshwater prawn has been grown in China for more than 30 years. So far, no country other than China has such an extensive farming area, even in the natural range of this species in other countries of Southeast Asia. Nonetheless, there are problems in the freshwater prawn industry of China.
RkJQdWJsaXNoZXIy MjExNDY=