Mangrove forests are valuable biologically and economically, but these have been decreasing yearly. It is usually converted into human settlement and for Brackishwater aquaculture. To arrest the massive decrease of mangrove forests, reforestation of mangroves has been done but most failed because of conflict among users. To resolve such problems, a system that combines utilization and conservation called silvofisheries has been introduced.
There are two basic silvofishery models. One consists of mangroves within the pond with a ratio of 60-80% mangroves and 20-40% pond canal for aquaculture. The second model consists of mangroves outside the pond with similar mangrove to water ratio. There are a variety of designs within these basic models.
These silvofisheries developments range in size from one hectare to thousands of hectares at each site. Empang parit (sometimes referred to as tambak tumpangsari) is the traditional application of this integrated aquaculture in the mangrove area. It represents the greatest level of reforestation or maintenance of existing forest.
Empang parit usually consists of an unexcavated central platform that alternates between being flooded and exposed and a canal that runs along the pond dikes where fish, shrimp, and crabs are cultured.
The density of mangrove trees planted in the platform ranges from 0.17 to 2.5 trees per m2. The mangrove density influences the quantity of litter production and organic load. These in turn have an impact on the diversity of non-mangrove flora and fauna growth that may form an important part of the diet of cultured species.
The mangrove tree density also influences the type of aquaculture in empang parit. Farmers may opt for less dense mangroves for fish culture (e.g., 0.2 trees per m2 for milkfish) but not for shrimp and crab culture because these species prefer the shelter afforded by mangroves.
Farmers in Indonesia earn an average gross income of US$580 per ha per year (range = $313-946). The net profit averages $356 per ha per year. The individual farmers operate between 1.5 to 10 ha of silvofiesheries ponds. The figures suggest greater production effort per unit area by farmers with smaller farms. Further research is needed to gather a fuller assessment and an evaluation of the different silvofishery models.
The empang parit model has a number of disadvantages compared to the barckishwater open pond : (a) greater construction cost per unit area, (b) greater difficulty to manage, (c) reduced water circulation and greater potential for stagnant areas with low oxygen levels, (d) limitation on species cultured (e.g., seaweed would be shaded by trees, reducing growth), (e) mangrove trees reduce the penetration of sunlight to ponds lowering the productivity of phytoplankton and benthic alagae, (f) potential toxicity of tannin from mangroves.
The application of silvofisheries should be approached with reasonable measures of caution as with any development in an environmentally sensitive area as a mangrove ecosystem. The selection of the most appropriate silvofishery model will be site-dependent and influenced by the status of the mangrove ecosystem. It should also be integrated into an area-wide integrated approach to coastal zone management. This approachedallows for maintaining relatively high level of integrity in the mangroves while capitalizing on the economic benefits of brackishwater aquaculture. (Excerpts from the review paper presented by Mr. William FitzGerald Jr., Oceania-Pacific Rim Consultants Services)
Over the years, commercial production of mudcrab, Scylla spp. has been undertaken only in bamboo or net-fenced brackishwater ponds. This technical feasibility has been demonstrated through a series of verification trials conducted in commercial ponds by SEAFDEC/AQD researchers and which is now being adopted by mudcrab farmers in several parts of the country.
Today, the use of net enclosures in mangroves or tidal zones offers a better alternative to culture of crabs in ponds. The set-up is easy to conduct, simple to operate, and requires low investment cost. Aside from being environment-friendly, the system offers a bright prospect for creating livelihood for coastal communities. The method has been done for quite some time in Indonesia, Malaysia, Vietnam, and China. The recent success of SEAFDEC verification runs in Palawan and Aklan, Philippines paved the way for the widespread interest to establish similar methods of crab culture in Philippines. This technology is extensively discussed in the manual titled Pen culture of mudcrab in mangroves.
Fishing villages in the Philippines are located in arable land plains along fringes of coastal lines with the sea as their main resources. Adjacent to or near them are large areas of tidal flats with existing mangroves which are potential areas for aqua-mangrove integrated farming. The utilization of these mangroves for the culture of mud crabs can provide fishers with alternate livelihood or as source of additional income.