The general over-development of aquaculture had profound disturbance on the surrounding ecosystem, affecting, not only fisheries, but aquaculture itself through release of effluent loaded with nutrients into open waters leading to eutrophication and deterioration of water quality. Shrimp aquaculture is one of the fastest growing economic activities in the Asia-Pacific region, where almost 80% of the world production of farmed shrimp occurs, but has slowed down recently for a number of reasons. These include eutrophication of coastal waters, mangrove destruction, stock losses due to disease outbreaks, primarily as a result of unrestricted expansion and environmental problems from mismanagement and over intensification (Phillips et al., 1993). Estimates of N and P quantities (95% of the N and 71% of P) entering waterways from shrimp pond indicate most of the materials originate from the added feeds and fertilizer, hence, water and soil quality in pond become a balance between metabolites pond inputs, shrimp wastes and on water exchange (Briggs & Smith, 1994 (Macintosh & Phillips, 1992; Briggs & Funge-Smith, 1994). Feed input is the major factor that causes deterioration of pond bottom and water quality (Boyd, 1992). Several processes may limit eutrophication by improving shrimp feed stability through extrusion, adoption of biofilters and bioaugmentation with the use of commercially available “waste digester” and “probiotics” through bacteria mineralization.

Vietnam has about 260,000 ha devoted for shrimp culture in early 1999. The total production was estimated at about 80,000 tons in 2000, so that productivity was about 300 kg/ha only. In 1990–1997, vast areas of mangroves were destroyed by many fish farmers for conversion into shrimp culture ponds. The Vietnam Government and local authorities have been trying to mobilize and educate farmers for them to refrain from destroying mangroves for shrimp culture; and at the same time promote increased shrimp productivity/ha in order to increase profit; and generate employment without necessarily destroying the mangroves. Cathai is an island district in north Vietnam with 12 communes in two islands, Catba and Cathai. Phulong Island in Catba Island, has the largest mangrove area. Before 1990, the total mangrove area of Phulong was about 2000 ha but in 1998 only about 1200 ha remained, because about 800 ha had been converted into shrimp ponds. However, due to lack of technology in shrimp culture especially the semi–intensive pond culture, production ranged from 200 to 300 kg/ha/crop, while in extensive ponds, production was only 50–70 kg/ha/year. Local authorities and the people of Phulong have been expecting to increase productivity from shrimp culture. Using the mangrove-friendly shrimp technology modeled by the Philippines and Thailand, the objective of the semi–intensive shrimp pond culture activity in Vietnam is to obtain 1000–1500kg/ha and to develop a model for an improved shrimp culture system. In the Memorandum of Agreement signed between the Ministry of Fisheries and the Southeast Asian Fisheries Development Center for the implementation of the Project in Vietnam, the Research Institute for Marine Products (RIMP) in Haiphong was chosen to carry out the demonstration activity focusing on the semi–intensive mangrove-friendly shrimp culture in Phulong, Cathai District, Haiphong, Vietnam. The ponds used for the Project had a total area of about 6.0 ha. Construction of the pond facilities following the Project’s scheme was completed in December 1999.

The seawater irrigation system has benefited the intensive marine shrimp culture making shrimp culture sustainable and increasing shrimp production. The results obtained from this research study indicated that the adverse effects of shrimp farms on mangrove and costal environments around the irrigation system could be controlled and minimized. There are three main critical control points identified in this study that need to be considered: 1) Supply canals in the system should be cleaned and renovated every 2-3 years. 2) Wastewater treatment system should be monitored to avoid the accumulation of solid wastes while the solid wastes should be properly managed. 3) The seawater irrigation system can minimize shrimp pathogens from entering the culture ponds, however for disease prevention, farmers should adopt good aquaculture practices including pond preparation, seed quality, and prevention of disease carriers. Managing the seawater irrigation system needs a systematic teamwork. The above critical control points of the system should be monitored regularly and managed properly. The pond manager should assess the system regularly to prevent the adverse effects to the shrimp farms and to the environment.

Environmental deterioration in ponds and costal waters has been a common evidence after each intensive shrimp culture due to the accumulation of organic wastes and occurrence of eutrophication processes. Mangrove is a wetland that potentially supports the natural food web in the estuary and coastal areas. The planting of mangroves in recycling shrimp farms would serve the purpose of developing sustainable shrimp farms by utilizing the enriched organic matters and nutrients thus enhancing the natural food web in the shrimp farms. However, mangrove trees in pond dikes tend to grow small and the roots may not be able to support the tree during pond erosion resulting in the mortality of the mangrove trees. Brackishwater weeds, which cover the soil in the pond dikes, could serve the function of the young mangrove trees.

International environmental pressure groups call for shrimp importing countries to impose a trade embargo against farmed shrimps produced in a manner considered deleterious to the environment. In fact, Europe had banned in 2000 the importation of shrimps from some Asian countries. The subject of such protest actions included: (1) discharge of substances potentially harmful to marine organisms (chemicals used during pond preparation and therapeutants used during culture period); (2) discharge of excessive organic load during regular water change and harvests; and (3) chemical residues in shrimps harvested and marketed. In the Philippines, a mandate from the national government was given to SEAFDEC in 1996 to rehabilitate the shrimp industry, which almost reached the brink of virtual collapse due to diseases brought about by some environmentally destructive practices. Mangrove-friendly aquaculture or what is sometimes referred to as the environment-friendly aquaculture was launched by AQD as a five-year program starting in 1998. The focus was later changed to shrimp culture and made part of the ASEAN-SEAFDEC Fisheries Consultative Group (FCG) collaborative project with AQD as Lead Department for technology development and verification, and Thailand as Lead Country for promoting the technology within the Southeast Asian region.

The aim of the experiments is to find a new way to accomplish intermediate culture of the penaeid prawn in nursery ponds. Experiments have been carried out in prawn farms in Haiyang County, Shandong Province. Prawn fry were stocked at high density in a nursery pond. Commercial fertilizer was added to the nursery pond to fertilize the pond water as nutrients for the planktonic and benthic organisms. The prawn fry in the pond fed only on the available natural food organisms without any special feed supply and grew normally. The survival and growth rate of the prawn fry are discussed.

The effect of different stocking densities (50, 100 and 150/m2) and two feeding regimes (natural food, consisting mainly of lablab, and natural food plus artificial diet) on the growth and survival of Penaeus monodon postlarvae (PL4 to PL5) were evaluated in eighteen 40 m2 earthen brackishwater nursery ponds using tidal water exchange for a period of 45 days. Results of the experiment indicated that the effect of different stocking densities was highly significant (P<0.01) on growth but not on survival for the two feeding regimes. Likewise, no interaction effect was discerned. Shrimps given artificial feed (Treatments II, IV and VI) obtained higher mean weight gains of 1.55, 1.17 and 1.05 g, respectively, than those that were not given artificial feed (I-1.44 g, III-0.92 g, and V-0.66 g). Similarly, those reared with artificial feed attained better survival of 41.62% (II), 67.44% (V) and 52.14% (VI) compared to shrimp that were not given artificial feed (I-42.53%, III-54.61% and V-46.90%). An exploratory economic study showed that the nursery operation gave promising results in all treatments. High rate of investment (ROI) was obtained to give a safe margin for the risk involved in this kind of business. Among all treatments, treatment V had the highest ROI of 693% and shortest payback period of 0.19 years.

An estimate of seed resources in the coastline, estuarine, and backwater bodies is an important prerequisite for developing prawn farming. A one-year (1983) survey on penaeid seed, based on tide and lunar periodicity, indicates the following species: Metapenaeus dobsoni, Penaeus indicus, P. japonicus, M. monoceros, P. semisulcatus and P. monodon in order of their abundance. P. indicus and P. japonicus are predominant in February and March (77.5 and 82.06% of total seed, respectively) when the average salinity ranges from 33.6 to 35.1 ppt followed by M. dobsoni. A second peak of P. indicus is observed in June when M. dobsoni showed its highest peak (47.35%) with continued abundance up to December. During the northeast monsoon, when the average salinity fell to a lower range of 19.9 to 24.6 ppt, P. monodon and M. monoceros showed moderate abundance. As the site chosen is very near the bar mouth, most of the seed collected were postlarvae. In Penaeus and Metapenaeus genera, total size range is 7-15 mm and 3-4 mm, respectively. Afternoon collections showed greater abundance followed by forenoon and night collections. Low tide and Full Moon collections showed greater abundance than those made during high tide and New Moon. Differences in seasonality may reflect breeding intensity of the respective prawn species in the sea. Variations in hydrographic features may also significantly contribute to seasonal abundance. A strong correlation between salinity and seed abundance is seen. The seed potential of these prawns in Ennore estuary is discussed.

Prawns, like any other animals, exhibit dissimilarities in growth rates. As they grow, a wide range of body weight distribution from the same population is observed. Staggered harvesting method is commonly practised in cultured animals having this characteristic. Selective or partial harvesting is especially useful in this type of management system. In this case, the larger shrimps are caught earlier than the small ones thus giving chance for the smaller ones to grow bigger. The study was conducted in four one-ha ponds. Recommended pond preparation was followed. Partial harvesting was employed in experimental ponds by using 2-4 units of 8 knots selective pound nets once a week commencing after three months culture until final harvest. Control ponds were harvested only once at the end of the culture period. The results show a mean production value of 506 kg from control ponds and 639 kg from experimental ponds. Average survival rate for experimental ponds was higher (92.90%) than for control (77.65%). Final average body weight was higher for experimental ponds (21.8 g) than for control (20.5 g). Size-wise, production of big size group (30-35 g) is 578.0 kg compared to 434.6 kg for small size group (13.1-13.4 g) from both control ponds with over-all production of 1,012.6 kg. On the other hand, production from the two experimental ponds for big and small size groups is 872.2 and 405.8 kg, respectively. The means of the total weights of marketable size Penaeus monodon from control and experimental ponds are 289.0 and 436.1 kg, respectively. That is, 43.5% of the stock reached marketable size in ponds with staggered/partial harvest method compared to only 27.5% from control ponds.

Growth and survival of Penaeus merguiensis juveniles were measured over four years in the Norman River estuary, south-eastern Gulf of Carpentaria. Growth in carapace length for the first 8-9 weeks after settlement was essentially linear and averaged 1.2 mm/week in summer at 29.5°C and 0.45 mm/week in winter at 19.5°C. A comparison of different cohorts under varying temperatures and salinities indicated that growth was temperature- but not salinity-dependent. Survival of newly settled postlarvae varied seasonally and was highest in spring (October-November). In the laboratory, a study of moulting rate and moult increment at 15, 20, 25, 30 and 35°C demonstrated that the optimal temperature for growth was 25-30°C. Survival of juveniles was also highest at intermediate temperatures. Effects of salinity and food ration amounts are discussed.