AQUACULTURE-SILVOCULTURE SYSTEMS
by
A. Quarto and the Mangrove Action Project

     Two main sustainable alternatives to coastal aquaculture pond development within or associated with mangroves are silvofisheries and mariculture. Silvofisheries is a form of integrated mangrove tree culture with brackishwater aquaculture. It is a form of low input sustainable aquaculture. This integrated approach to conservation and utilization of the mangrove resource allows for maintaining a relatively high level of integrity in the mangrove area while capitalizing on the economic benefits of brackishwater aquaculture. Further extension of aquaculture to meet the needs of the rural poor may be tolerable provided it is carried out in a controlled manner outside those areas already heavily exploited and environmentally sensitive in an integrated program of conservation and utilization, such as silvofishery methods.

    There are a variety of designs of silvofishery systems that attempt to balance the conservation and utilization issues while maximizing economic opportunity. Different models have been proposed for rehabilitation/reforestation of former mangrove areas that were converted to brackishwater ponds that are not in use or of very low productivity. A number of countries are pursuing a form of silvofisheries, including Indonesia, Hong Kong, Thailand, Vietnam, Philippines, Kenya, and Jamaica. Some of the systems are traditional long-term practices and others are new approaches to utilize the mangrove resource in an environmentally sustainable manner.

    Indonesia is in the forefront of silvofisheries development. Indonesia's mangrove forests (4.25 million ha) represents approximately 25% of the world's mangroves and is a biogeographical center for a number of mangrove genera. Various entities from university research programs to national programs within the Ministry of Forestry and the Directorate General of Fisheries of the Indonesian government have been studying, demonstrating and promoting silvofisheries. These silvofisheries developments range in size from one hectare to 1,000's of hectares at each site. Empang Parit, the Indonesian form of silvofisheries, is the traditional application of this integrated aquaculture in the mangrove area.

    An Empang Parit model represents the greatest level of reforestation or maintenance of existing forest to pond area. It essentially consists of a mangrove planted raised central pond bottom (80% of total pond area) that alternates between being flooded and exposed as the water of the pond is raised or lowered. This raised pond bottom is surrounded by a canal that runs adjacent and parallel to the pond dikes. The canal is normally 3-5 m wide and 40-80 cm below the central raised pond bond bottom. There are variations on this basic model to increase the open water area up to 40-60%. Fish, shrimp, and crabs are cultured extensively in the canal.

    The productivity of the silvofisheries pond is based on essentially the use of green manure to support the complex food web. The organic enrichment of the pond is from plant material, in this case mangrove tree debris. The density of the planted mangrove trees on the platform area ranges from 0.17 trees/m2 to 2.5 trees/m2 in the Empang Parit system. The density influences the quantity of litter production and organic load in the pond along with other factors of cultivation. This would include the diversity of non-mangrove flora and fauna growth (e.g., algae) that may form an important part of the aquaculture species diet. The tree density also influences the cultured aquaculture species production, with farmers preferring a more open density (approximately 0.2 trees/m2) for milkfish. The higher openness of the forested area allows accessibility to the platform area in milkfish culture while a greater tree density can be used for shrimp and mangrove crab culture that prefer additional structural habitat and shelter afforded by the mangroves.

    Silvofishery is a labor intensive technology appropriate for an individual or family operation and can be a viable alternative to brackishwater pond culture. It diversifies products from the land and aquatic production within an environmentally benign framework and is integrated into the mangrove forest ecosystem. Some key considerations in design include the following:

    Studies should be conducted to increase the production from the various silvofishery models. This is particularly important for reforesting private land area. Research on the optimization of production within an integrated mangrove/aquaculture silvofishery system should be a priority area. This should consider the different silvofishery models. This would include optimizing use of inputs and stocking strategies for different species within a polyculture production system. The different species of mangrove trees that can be used for specific pond conditions should be determined as well as conducting an analysis on the type and amount of vegetation cover, litter production rate of the different trees and the decomposition rate of the different tree litter. This would be an important factor in the food web and supporting the maximum productivity of the pond. The food web resulting from mangrove vegetation litter into the ponds and optimization of this input should be researched to obtain a better understanding of this process so that appropriate management practices can be developed.

    Silvofisheries has the potential of capturing some of the economic benefits of the mangrove areas within an environmentally sensitive framework and a sustainable activity. Improvement in the economic return from this system will be a key factor in the wide acceptance of silvofishery methods as an economically viable activity in the mangroves. Silvofisheries can also provide an alternative economic activity to the rural poor and reduce development pressure on the mangrove forests. Therefore, it should be considered in an overall development strategy for the coastal zone and could serve a role in the transition that shifts more intensive aquaculture to areas outside the mangroves.