Mixed shrimp farming: Mangrove forestry models in the Mekong Delta

• Network of Aquaculture Centres in Asia Pacific, Thailand
• Research Institute for Aquaculture No.2, Vietnam
• University of Tasmania, Australia

About one-third of the annual 500,000-tonne fisheries production in the Mekong Delta region of Vietnam comes from aquaculture, and the proportion is expected to grow. Shrimp culture in mangrove areas forms a major part of the aquaculture industry, and is targeted for increased production both through expansion of the area of ponds and increases in yields from existing ponds.
However, the rapid expansion of shrimp production has led to major losses of mangrove forest in the Delta's coastal regions. For instance, the Minh Hai Province mangrove forests support coastal fisheries, provide protection from storms, and supply most of the firewood needed for domestic heating. But since the early 1980s much intertidal land has been diverted to extensive shrimp culture, with nearly all the mangrove vegetation removed. Mangrove forests have declined at a rate of about 5000 hectares a year. Less than half the forest that was present in 1982 still stands.
Provincial managers responded to this situation by establishing 22 mixed shrimp farming and mangrove forestry enterprises, with individual farmers producing both shrimp and mangrove wood on small plots. While these enterprises are considered to offer the best potential solution to the problem of conflicting land use, current management practices have led to declining yields of both shrimp and wood.

The research objective was to study existing patterns and processes to discover the factors determining the yields of ponds and forests, and then to test the effect of manipulating these factors in order to improve shrimp farming/mangrove forest practices.

This project investigated the reasons for the decline in shrimp and wood production, and evaluated alternative management practices with the aim of providing a scientific basis for maximising sustainable yields. The Australian Institute of Marine Science was well placed to lead the project, having conducted multidisciplinary research on the hydrodynamics, forestry and fisheries aspects of mangrove ecosystems in northern Australia and Southeast Asia over the previous 15 years. Most of the work was carried out in Vietnam; in Australia, comparative studies of shrimp pond ecology and nutrient dynamics were undertaken near Mossman in northern Queensland. A high level of involvement by local farmers and provincial decision-makers was maintained.
The specific project objectives were to:
investigate factors controlling the yields of shrimp and wood from existing shrimp farming/ mangrove forestry systems in Minh Hai Province;
experiment with shrimp pond and mangrove forest management to evaluate different culture options, in cooperation with selected farmers and appropriate managers;
identify improved culture methodologies for these systems and quantify where possible their expected yields and costs;
assist national and provincial authorities to transfer results of the project to the wider coastal farming community in the Mekong Delta.

The project developed improved farm designs and management strategies to increase shrimp survival and pond yields. Farm trials carried out in the second phase of the project demonstrated that it was possible to increase survival rate from about 1% (at the beginning of the project) to at least 30% by adopting the recommended pond designs and management strategies. In relation to mangrove forestry, the project developed an improved planting and thinning strategy that could potentially increase wood yields by about 20%. These wood yields are yet to be realised, however,, due to the long-term nature of forest rotations.

In Vietnam, scientists at RIA2 and SIFR have successfully taken up the project techniques, notably those for water quality and fauna assessment, in a variety of other projects, including several Mekong River Commission projects. Aquaculture practices are changing rapidly in Ca Mau and neighbouring provinces, and the rapid increase in intensification and the development of large-scale commercial shrimp farms seen in the neighbouring provinces of Soc Trang and Bac Lieu are also evident to a lesser degree in Ca Mau province.

In addition, Ca Mau Fisheries Extension Service produces its own extension material based on a wide variety of sources, including the ACIAR project. It is evident that the Provincial People's Committee sees a continuing role for project outputs in areas designated for mixed shrimp/mangrove culture. Ca Mau Department of Agriculture and Rural Development has changed mangrove forest replanting densities to 10,000 per ha, in line with project recommendations. In addition, rotation length and thinning plans have been made more flexible, to allow for market demand for end products and to optimise the economic returns from mangrove silviculture.

Nearly all of 47 farmers surveyed at the end of the project appear to have adopted some of the project recommendations. For example, all but one of the farmers surveyed now use techniques for selecting shrimp seed that were recommended by the project. Similarly, most farmers have improved their pond designs and water management strategies, at least in part along the lines recommended by the project. Nevertheless, lack of access to project extension materials is likely to have reduced the adoption rate.

Farmers are now constructing better ponds by widening them, removing internal dikes and levees, and deepening them to 0.8-1.0 minimum depth. More than 80% of farmers now use a settlement pond to improve water quality (compared to less than 10% at the beginning of the project). In addition, most farmers now use a nursery pond or nursery area to tend the post larvae for the first 30 days after stocking. Most farmers now understand how to check and select healthy post larvae and use techniques suggested by the project for post larvae selection. They regularly check the condition of shrimp in the pond, and take appropriate action, such as harvesting or changing water, when there is evidence of a health problem. More than half of the farmers feed the post larvae for the first 30 days after stocking, but feeding after 30 days is still uncommon. There is a trend towards increasing the diversity of farm products. Crab production now accounts for about 10% of farm income. Some farmers have also diversified into fish, snails, cockles and/or pig production.