Sustainable use of the mangrove
 |
Alfredo Quarto describes the
silvofishery method of aquaculture, a low-input, sustainable use of
the mangrove ecosystem.
The author is Executive Director of the Mangrove Action Project and co-editor of the MAP Quarterly News. |
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 programme of conservation and utilization, such as made possible by silvofishery methods. Silvofisheries is a form of integrated mangrove tree culture with brackishwater aquaculture. It is a type of low- input, sustainable aquaculture. This integrated approach to conservation and utilization of the mangrove resource maintains a relatively high level of integrity in the mangrove area while capitalizing on the economic benefits of brackishwater aquaculture.
There are a variety of designs of silvofishery systems. Different models have been proposed for the rehabilitation or 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 some 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 a sustainable manner.
Indonesia is in the forefront of silvofisheries development. Indonesia’s mangrove forests (4.25 million hectares) represents about 25 per cent of the world’s mangroves and Indonesia is a biogeographical centre for a number of mangrove genera. Various entities from university research programmes to national programmes within the Ministry of Forestry and the Directorate General of Fisheries of the Indonesian government have been studying, demonstrating and promoting silvofisheries. Silvofisheries developments range in size from one hectare to thousands 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 per cent 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 bottom. There are variations on this basic model to increase the open water area up to 40-60 per cent. Fish, shrimp, and crabs are cultured extensively in the canal.
The productivity of the silvofisheries pond is based on 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 including the diversity of non-mangrove flora and fauna growth (such as 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 labour-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:
- mangrove area to pond water area ratio;
- the ratio of water area to pond dike length (reflects production area to capital cost investment);
- gate width ratio (cm/ha) — important in taking in wild seedstock and flushing out decayed excess mangrove debris (it should be 50 cm/ha);
- tidal flushing rate and tidal range;
- flow of water within a pond to prevent stagnation (low oxygen level) and depth of water;
- depth and width of perimeter channel; and,
- site location — soil suitability and abundance of natural stocking material, etc.
Further research is needed to support the development of silvofisheries. Studies are needed regarding ways of increasing 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 is a priority area. This needs to consider the different silvofishery models and 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 analysis of the type and amount of vegetation cover, litter production rate of the different trees and the decomposition rate of the different tree litter, important factors 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 fully 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 for 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.
Further information
Alfredo Quarto, Mangrove Action Project, PO Box 1854, Port Angeles, WA 98362-0279, USA. Fax: 1-360-4525866. Email: mangroveap@olympus.net. Web: www.earthisland.org/map/map.html.
On the Web
The mangrove ecosystem in this issue of Tiempo lists relevant sites.
Acknowledgement
This article was first published in the MAP Quarterly News.