32. Fisheries and aquaculture

Contents - Previous - Next


1. Scope

2. Environmental impacts and protective measures

2.1 Artisanal small-scale fisheries
2.2 Small-scale aquaculture
2.3 Use of artificial lakes in fisheries and aquaculture
2.4 Fishery in the 200-mile exclusive economic zone
2.5 Use of mangrove swamps in fisheries and aquaculture

3. Notes on the analysis and evaluation of environmental impacts

4. Interaction with other sectors

5. Summary assessment of environmental relevance

6. References


1. Scope

Activities for the purpose of obtaining food and other products from water bodies involve catching and gathering as well as farming and raising aquatic organisms (above all fish, crustaceans, molluscs and algae). Annual worldwide production in the fishery and aquaculture sector amounts to around 95 million tonnes.

The principal forms of activity are:

- capture fisheries
- aquaculture
- stocking and ranching

All three types of activity can be carried out in seawater, brackish water and fresh water and in both coastal and inland waters. Deep-sea operations primarily involve capture fishery, with aquaculture playing only a very small role. Stocking and ranching may include use of deep-sea areas in that fish released near the coast (e.g. salmon) may spend their growth phase in the open sea.

While inland and inshore fisheries and aquaculture are predominantly artisanal, deep-sea operations are primarily on an industrial scale where capture fisheries are concerned and exclusively so in the case of aquaculture.

Capture fisheries utilise natural stocks of aquatic organisms. Such activities influence the stocks not only by catching them but also by means of conservation measures (closed seasons, protected areas, catch quotas, use of selective gear). In aquaculture measures are taken to directly influence at least the growth stage and if possible also the reproductive stage, above all by controlling water quality (through the conditions under which the organisms are kept), nutrition (through feeding and pond fertilising) and health (by means of prophylactic and therapeutic measures). The reproductive stage can be controlled by influencing maturation, egg and sperm production, hatching and larva raising. The characteristics of the organisms bred can be genetically influenced (e.g. by means of selection, crossing or genetic engineering).

Stocking and ranching combine aquaculture with fishery (culture-based capture fisheries). Natural or artificial bodies of water are stocked with young organisms which were hatched under supervision and spent the particularly critical early stages of their life cycle under controlled conditions. When the stocks created or augmented in this way reach the end of their growth stage, they are fished using normal capture-fishery techniques.

Between the "production" process - carried out under natural conditions (fisheries) or controlled conditions (aquaculture) - and consumption of the products there are a number of other stages which may likewise have environmental impacts: keeping fresh, processing, packing, transporting and marketing.

Fisheries and aquaculture can be divided into five main areas:

- artisanal small-scale fisheries
- small-scale aquaculture
- fisheries and aquaculture in artificial lakes
- fishery in the 200-mile exclusive economic zone
- fisheries and aquaculture in mangrove swamps

In the first two areas, emphasis must be on supporting low-income groups of the population and ensuring that appropriate technologies are applied. These two aspects likewise form the focus of attention in the use of artificial lakes for fisheries and aquaculture. By contrast, activities involving fishery in the 200-mile exclusive economic zone - predominantly at industrial scale - centre on preservation of resources and on managing and monitoring their use. Particular importance must be attached to environmental protection and resource conservation when the intention is to utilise mangrove swamps for fisheries and aquaculture, as measures involving the use of this fragile ecosystem should aim from the very outset to ensure that adverse environmental impacts are avoided altogether or kept to an absolute minimum.


2. Environmental impacts and protective measures

2.1 Artisanal small-scale fisheries

The actual fishing activities have the greatest bearing on the environment, as the long-term availability of the resources depends on the extent to which these activities are geared to the resource situation and to the conditions prevailing in the ecosystem fished. Through centuries of experience, traditional artisanal small-scale fisheries based in a specific location have made sure that they do not over-fish the available resources. Any attempt to increase production can jeopardise this well-established equilibrium.

It may nevertheless be possible to increase production without endangering the resources. Such an opportunity exists in cases where the stocks fished are utilised at a level below that guaranteeing optimum yield and sustainability. The same applies if fishing activities are extended to those components of the biocoenoses within the ecosystem that were previously utilised very little or not at all.

However, utilisation of additional species may be limited by the food relationships between various components of a biocoenosis. If the prey of a predatory fish starts to be utilised in addition to the fish itself, the potential yield that can be derived from the predatory fish is automatically reduced, as the food supply has been curtailed. Since many such relationships exist, it is essential that they should be carefully reflected in the management models if it is intended to simultaneously utilise a variety of different organisms within a single ecosystem.

In management of fishery resources, a key role is played by the nature of the gear used as well as by when and where it is used. Modern fishing gear can be highly efficient (i.e. may jeopardise the existence of stocks if no restrictions are imposed on its use) and highly selective. Fishing gear is considered selective if it catches only particular species or size categories of organisms. Its selectivity can be determined by net mesh size, hook size, or the depth of water or depth zone in which it is used. The most important fishery management measures include closed seasons, protected areas, minimum mesh and hook sizes, limits on the number of sets of gear, boats or ships and on the times when they may be used, and stipulation of catch quotas and size categories for the organisms to be caught.

Stock management calls for a high level of training in fishery biology and adequate knowledge of fishery economics. Stock regulating measures should be discussed, agreed upon and implemented by the local fishermen acting on a collective basis.

Apart from the need to conserve the resources themselves, it is also essential to protect their living environment against influences that could raise problems in the short or long term; to this end, the physical, chemical and biological condition of fishing areas must be monitored. Product quality depends on the chemical and biological conditions of the water and on the sanitary conditions prevailing ashore (village hygiene). The destructive effects of using wood resources for smoking fish can be curbed in two ways: by employing energy-saving kilns which permit more rational use of wood and by ensuring appropriate management of the forest resources concerned. The amount of wood required for boat-building can be reduced by replacing dugout canoes with boats made of planks and by using alternative construction materials.

Where it is likely that infrastructure for landing places used in artisanal fishery can be modified or removed only with difficulty, such facilities should not be constructed unless their necessity and expediency have been thoroughly reviewed. Concrete structures can also mar the aesthetic value of their surroundings (tourism).

2.2 Small-scale aquaculture

Aquaculture offers considerably greater options than capture fishery as regards both the type of organisms to be produced and the production sites. The natural stocks of organisms suitable for aquaculture can be most effectively conserved if aquaculture controls the entire life cycle, beginning and ending with the reproductive stage, and does so not just for one or two generations but on a long-term basis. As yet, however, this is possible only in the case of a few aquatic organisms. The only way of overcoming this problem is to promote applied basic research in the fields of reproductive physiology and reproductive ecology.

The production site should be chosen with the aim of conserving natural ecosystems and scarce water resources. The choice of the type of organism to be produced can contribute to conserving heavily used food resources if preference is given to species whose food requirements can be met by waste products or by-products from other sectors. Such products can either be fed directly to the fish or be used to fertilise the water and thereby promote the multiplication of food organisms (algae, microfauna). This could, for example, reduce demand for fish meal as a constituent of fish food. However, producers have a tendency to concentrate on expensive organisms (e.g. certain species of predatory fish) which generally require food of extremely high quality.

Water quality within and downstream of an aquaculture facility is determined by management practices. Efforts must be made to ensure that as little leftover food as possible remains in the water and that the quantities of nutrients and pollutants washed out of the installation are kept to a minimum. The amount of leftover food can be minimised by gearing the quantities of food given and the frequency of feeding to the absorption capacity and appetite of the fish. If sizeable quantities of waste are nevertheless discharged (e.g. from intensively operated through-flow ponds), they can be caught in settling ponds and thus largely prevented from entering rivers and lakes.

Drugs for preventing and treating disease and for combating parasites should not be used in running water (through-flow ponds) for reasons of effectiveness and economy and should not be used at all in open systems (cages, pens), even if the fish then have to be transferred to special containers for treatment and are thereby exposed to stress situations.

The main way of saving energy in aquaculture is to obviate the necessity of pumping for the purpose of water renewal. Introducing new water benefits the oxygen supply and helps to wash out wastes, besides compensating for evaporation and seepage losses. The extent of the necessary water replacement depends largely on the stocking density. Pumping energy can be saved wherever natural gradients can be used to create a water flow. Artesian springs are sometimes also available.

Considerable ecological advantages are offered by ponds in which wastes can be utilised by plants and microfauna which for their part are suitable as food for productive aquatic organisms. Such ponds can be fertilised by livestock (poultry, pigs) kept above or next to them. The profitability of this type of integrated aquaculture depends on the ecological appropriateness of the aquatic organisms kept, their popularity with consumers, the production costs and market prices. A role is also played by the way in which aquaculture is integrated into the overall production system, which usually involves other forms of production requiring labour. It is important, however, to know what constitutes the basis of the microfauna's food supply (there is a risk that pesticide residues could find their way into the food chain).

When setting up ponds in tropical countries, it is essential to bear in mind the risks originating from diseases whose pathogens spend at least one stage of their life cycle in water or in aquatic organisms (malaria, schistosomiasis etc.).

Cage farming not only involves high feeding costs, but also gives rise to problems in procuring the necessary materials for making the cages, as nets, support rods and floats are expensive. Only in forested regions is the use of wood unlikely to present any problems.

Elimination of potential health risks attaching to consumption of aquaculture products must be given particular attention wherever human excrement and domestic wastewater are used for fertilising ponds. In wastewater aquaculture systems, the critical factors in this respect are the number of pond stages, the degree of dilution and the period for which the water is retained before it enters the fish ponds. Accurate management, along with regular checks on sanitary conditions and water quality, are essential in such cases.

2.3 Use of artificial lakes in fisheries and aquaculture

As use of artificial lakes involves a combination of fish farming and fishing (and can thus be placed in the category of "culture-based capture fisheries"), the environmental protection measures described in both 2.1 and 2.2 above are of relevance in this connection. However, the fact that an artificial lake is a man-made entity creates a substantially different situation, both in limnological and ecological terms as well as from the sociological and economic viewpoints. Man-made lakes

differ from natural ones by virtue of their artificial nature, the fact that they are subject to continuous management to enable them to fulfil their primary purposes (drinking-water supplies, energy generation, irrigation), their initial biological "void" which - depending on the actual and to some extent random sequence of colonisation by flora and fauna - can offer scope for a variety of biological development possibilities, and last but not least the new options which this may offer in terms of fisheries and aquaculture. While an artificial lake thus allows man a considerable degree of freedom in shaping ecological conditions, it nevertheless confronts him with far-reaching social and economic problems when it comes to developing and establishing the ways in which it is to be used.

Two important principles should be observed when determining how a new artificial lake is to be used:

- Organisms that are foreign to the ecosystem and region concerned should be introduced only with strict observance of internationally recognised precautionary measures or not at all.
- No attempt should be made to regulate fishery activities until local traditions have been studied in detail; regulation measures should be realised in consultation with existing local fishermen and those willing to settle in the area.

When a new dam is being planned, consideration should be given to the various options for fisheries and aquaculture which the newly created lake will offer. Where appropriate, such aspects should be taken into account when deciding on dam design.

2.4 Fishery in the 200-mile exclusive economic zone

Optimum fishing of the 200-mile exclusive economic zone (EEZ) calls for use of advanced technology. This will inevitably lead to conflicts in the transition area between industrial deep-sea fishing and artisanal inshore fisheries unless depth conditions and coastline configuration create a natural division between the two. Such conflicts are often to the detriment of the available resources, causing them to be over-exploited or even destroyed. They may also adversely affect the economic and social position of the artisanal inshore fishermen, who usually come off worst in such conflicts if their interests are not effectively safeguarded through government intervention.

While old-established, traditional fishing communities have developed fishing practices designed to ensure that resources are preserved in the long term, the technical potential of modern deep-sea fishing - which can totally exhaust resources within a short time - means that use of resources must be strictly limited and monitored. Minimum mesh and hook sizes must be laid down to make sure that the gear does not catch young organisms which are not yet mature enough to reproduce and thereby preserve the existence of the fish stocks. Such regulations can also reduce the pointless destruction of small food organisms caught in the nets together with the fish.

The only way to prevent trawls with "ploughing" structures from causing serious damage to entire communities of sea-bed organisms is to ban the use of such gear. Depending on local conditions (sea-bed conditions, reproductive cycle and migration of fish or other organisms), use of such nets must be banned either completely, or in specific areas or at certain times of year.

Complete bans must be imposed on catching certain types of organism while they are still going through their development phase in the "nursery areas". As such bans are often impossible to enforce, efforts are being made in many places to create artificial refuges - in the form of submerged concrete blocks, for example - to which fish and other aquatic organisms can retreat and from which they can repopulate areas which have been subject to adverse influences or whose stocks have been exhausted. However, the effectiveness and cost-benefit ratio of these "artificial reefs" are still the subject of considerable debate.

The death of numerous fish and large marine fauna (dolphins, turtles, birds etc.) in lost drift nets made of plastic that does not decompose in water can be prevented by using degradable thread to attach the net sections to the floats. The net sections would then collapse after a time and sink to the bottom. However, this method appears to be too complicated for general use and it is not known what damage the nets might cause on the sea bed.

Considerable problems are still posed by the question of what to do with the "by-catch" (of non-target species), in other words the organisms with little financial value that are caught together with the highly lucrative species (e.g. prawns or shrimps) constituting the intended catch. These organisms are large or bulky enough to be retained by the net together with the main catch even if the minimum mesh sizes are adhered to. However, their market value is so low by comparison with that of the main catch that it is not worthwhile landing them, despite the fact that a considerable proportion of this "by-catch" would often be suitable for human consumption. If a worldwide solution to this problem could be found, for example by having the by-catch continuously collected by special boats at sea or by means of other methods, several million additional tonnes of fish would become available as food each year.

As is generally the case with motorised seagoing shipping, deep-sea fishing vessels' high consumption of fossil fuel necessitates special measures to dispose of residues on land. Environmental problems on land as a result of fishing stem primarily from industrial processing of the catch. Mandatory standards regarding disposal of solid wastes and wastewater must be observed; in some places such standards have still to be introduced. Some of the solid wastes can be made into fish meal, while valuable constituents of liquid waste can be recovered in the form of extracts and used as feed additives (cf. environmental briefs Inland Ports, Shipping on Inland Waterways, Wastewater Disposal and Solid Waste Disposal).

2.5 Use of mangrove swamps in fisheries and aquaculture

The traditional ways of using the flora and fauna in mangrove swamps can be viewed in the same light as artisanal small-scale fisheries in other areas: they take into account the regeneration capacity of the resources and are thus ecologically sound. However, this is not true of modern aquaculture on large fish farms whose construction necessitates complete clearance of the mangrove vegetation. One example of this type of operation is the large-scale raising of brackish-water prawns. As production of these much sought-after crustaceans can yield high profits, the potential suitability of mangrove swamps as sites for brackish-water ponds has given rise to dangerous pressure on these areas. Since mangrove areas are subject to the daily ebb and flow of the tide, the water has the necessary salt content and water replacement can be achieved at relatively little expense because the tidal cycle can be used to minimise the amount of energy required for pumping.

Efforts should be made to counter the pressure on the mangrove swamps in as realistic and flexible a manner as possible. The paramount principle should be that no form of use is to be permitted without thorough advance planning. The principal purpose of such planning is to completely rule out non-traditional use of areas which are irreplaceable as nature reserves, genetic resources, nursery areas for important aquatic organisms or protective belts guarding against coastal erosion. Clearance of mangrove swamps for the purpose of aquaculture can also be prevented by making areas immediately upstream of the mangrove belt available for the creation of ponds. Provided that the installations are well-managed, the necessary pumping costs could be offset by earnings.

Where use of mangrove swamps appears unavoidable for economic reasons, activities should be concentrated in areas with clayey soils. In such areas the mangrove vegetation can easily re-establish itself if the ponds (or swamp-rice fields) are abandoned at a later date, whereas areas with sandy and peat soils will be nothing more than wasteland for a long time afterwards. Continuing efforts should also be made to find ways of utilising the natural productivity of suitable mangrove areas for semi-intensive small-scale aquaculture, without clearing them of all vegetation

and without major additional expenditure on feed or fertiliser. The success of such experiments will depend on whether or not it proves possible to keep costs down to a level ensuring that even low yields per unit of area offer attractive economic prospects.


3. Notes on the analysis and evaluation of environmental impacts

The environmental aspects of fisheries and aquaculture fall into five categories:

- impacts on the natural environment which have adverse effects on aquatic organisms but which do not stem from either fisheries or aquaculture (pollution of water through disposal of wastes from industry, agriculture and households or caused by nutrients, pesticides and residues being washed out of soil on land; water-resources management measures); such impacts may affect both fisheries and aquaculture.
- influences on the existence and renewal of fish resources resulting from their use (such influences relate only to natural stocks and not to those maintained and controlled by man, i.e. aquaculture is affected only where it is dependent on young organisms from natural stocks).
- environmental impacts caused by fisheries and aquaculture (disturbance of ecological equilibrium, impairment of water quality etc.).
- influences on use of resources (and thus on the resources themselves) caused by changes in the social and socio-economic situation of producers and consumers (e.g. as a result of population growth).
- effects of fishery and aquaculture activities on the social and socio-economic situation of producers and consumers (e.g. in the event of local overproduction without sufficient access to more distant markets).

Computer-aided simulations of both the ecological and economic situation, using a standard model, can help to ensure that natural fish resources are optimally utilised in a manner which preserves their capacity for renewal. Such models are essential for developing a reliable long-term utilisation strategy which takes into account the economic interests of both the fishermen and the country concerned without jeopardising in the long run the natural resources on which fishing depends.

There as yet exist no summarising overviews or evaluations of the serious impacts which various modern techniques can have on resources (use of explosives and pesticides, bottom trawling, use of drift nets etc.).

Considerable efforts are currently being devoted to studying and evaluating the environmental impacts of aquaculture activities. In September 1990 the International Centre for Living Aquatic Resources Management (ICLARM) held a symposium on environment and aquaculture (results to be published in 1992).


4. Interaction with other sectors

Activities in the fisheries and aquaculture sector can be combined with agricultural production and with water resources development. The following are examples of the ways in which fisheries and aquaculture can be integrated with agricultural production:

- combining fish farming (or artisanal fisheries) with plant production and animal husbandry in an agricultural production system without physical integration of the individual components
- combining fish farming in ponds with keeping of poultry, pigs or other livestock above the ponds
- fish farming in swamp-rice fields

The following are examples of the ways in which fisheries and aquaculture can be combined with water resources development:

- fishing in artificial lakes of all kinds (including those designed to provide drinking-water supplies)
- fish farming in small, shallow irrigation reservoirs
- fish fattening in large irrigation canals
- cage fish farming in adequately large and deep artificial lakes not used to provide drinking-water supplies

(cf. environmental briefs Large-scale Hydraulic Engineering, Irrigation and Rural Hydraulic Engineering).

Fisheries and aquaculture also have extensive links with agriculture through the use of waste products, by-products and (in exceptional cases) main products of agriculture as food or fertiliser in aquaculture and through the use of fish meal in the production of livestock fodder (cf. environmental brief Livestock Farming).

Links with the forestry sector exist by virtue of the wood required for making boats and fishing gear, for preserving and processing fish by means of smoking and for making cages. The close ecological links between forests and waters are of particularly far-reaching significance and must be taken into account in both forestry and fishery activities.

Fisheries and aquaculture also have links with the energy sector through the operation of boats, ships and sophisticated fishing gear, fishing ports, refrigeration plants and industrial processing facilities, technically complex aquaculture installations and vehicles for transporting people, equipment, supplies and products.

Attention has already been drawn in the text to links with other sectors.


5. Summary assessment of environmental relevance

Fisheries and aquaculture are dependent on the existence of an environment which is intact or at least not permanently damaged, but can themselves have negative effects on the environment and on resources. As fisheries rely on continuous natural renewal of fish resources, activities in this field must treat these resources and their habitats with care.

Wherever resources have already been overfished and their habitats adversely affected by environmental changes, they must be rehabilitated if possible. Once fishing reaches a certain level of intensity, improved utilisation of catches is the only way of raising production. To this end, particular efforts should be made in the future to promote consumption of types of fish which are currently still unattractive from the commercial viewpoint and are simply used for making fish meal, and to ensure that fewer fish are lost as a result of spoilage.

Aquaculture is for the most part still a relatively new field of activity in the fishery sector. To promote its future development it requires tailor-made strategies which take particular account of the fact that most of the natural resources employed in aquaculture (water, land, feedstuffs; spawn in the case of most cultivated species) are already used for other purposes and can thus become sources of conflict. One of the most important strategic principles must therefore be to avoid such conflicts or resolve them with a minimum of adverse consequences in the ecological, economic and social spheres. This means, for example, that

- the impacts of aquaculture must initially be compared with those of other ways of utilising resources (e.g. mangrove forests as a means of preventing coastal erosion, tourism), with aquaculture activities then being designed as far as possible such that they complement use of water resources for other purposes;
- by-products or waste products that cannot be put to beneficial use elsewhere should be used as far as possible for feeding aquaculture organisms and fertilising the water. It is vital, however, that such products should be free of contamination (e.g. by pesticides).

Observance of the basic principles can be encouraged if the long-term advantages are demonstrated by effective examples and appropriate political and ecological conditions create a balanced combination of incentives and restrictions.

When involved in the development strategy and given appropriate training, women can play a key role in helping to prevent, reduce and eliminate environmental and health risks. Particular importance must be attached to awareness-raising measures that take religious considerations and cultural aspects into account.


6. References

Alabaster J.S. & Loyd R., 1980: Water quality criteria for freshwater fish. FAO/Butterworths.

Beveridge M.C.M., 1984: Cage and pen fish farming: carrying capacity models and environmental impact. FAO Fishery Technical Paper 255.

DANIDA, 1989: Environmental issues in fisheries development. Copenhagen.

Deutsche Forschungsgemeinschaft, 1980: Forschungsbericht: Methoden der Toxizitätsprüfung an Fischen; Situation und Beurteilung. H. Boldt Verlag.

Dwippongo A., 1987: Impacts of trawl ban on fisheries and demersal resources in the Java Sea. Ph.D. Thesis, Nihon University, Tokyo.

FAO, 1975-1983: Manual of methods in aquatic environment research. Part 1 - 9.

FAO, 1981: Conservation of the genetic resources of fish; problems and recommendations. FAO Fish. Tech. Paper 217, 43 pp.

FAO, 1981: The prevention of losses in cured fish. FIIU/T219.

FAO, 1987: The economic and social effects of the fishing industry - a comparative study. FIP/C314/Rev. 1.

ICLARM, 1982: Mismanagement of inland fisheries and some corrective measures. Contribution 110.

Johannes R. E., 1981: Working with fishermen to improve coastal tropical fisheries and resource management. Bulletin of Marine Science 31.

Lasserre & Ruddle, 1983: Traditional knowledge and management of marine coastal systems. Biology International, Special Issue 4.

Metzner G., 1983: Fischtests im Rahmen nationaler und internationaler Regelungen. In: Untersuchungsmethoden in der Wasserchemie und
-biologie unter besonderer Berücksichtigung des wasserrechtlichen Vollzugs. Münchner Beiträge zur Abwasser-, Fischerei- und Flußbiologie 27, 47-69.

Mienno J. L. & Polovinci J. J., 1984: Artificial Reef Project - Thailand. ADB.

Nauen C. E., 1983: Compilation of legal limits for hazardous substances in fish and fishery products. FAO Fishery Circular 764.

Pauly D. & Tsukayama I., 1987: The Peruvian anchoveta and its upwelling ecosystem: three decades of change. ICLARM Studies and Reviews 15. (IMARPE/GTZ/ICLARM).

Pauly D., Muck P., Mende J. & Tsukayama I., 1989: The Peruvian upwelling ecosystem: dynamics and interactions. ICLARM Conference Proceedings 18. (IMARPE/GTZ/ICLARM).

Rosenthal H., Weston D., Gowen R. & Black E. (Ed. committee), 1988: Report of the ad hoc Study Group on "Environmental impact of mariculture". ICES Cooperative Research Report 154, Copenhagen.

Ruddle K. & Johannes R. E., 1985: The traditional knowledge and management of coastal systems in Asia and the Pacific. UNESCO/ROSTSEA Regional Seminar. UNESCO Regional Office, Jakarta, Indonesia.

UNDP/FAO, 1982: Fish quarantine and fish diseases in South-East Asia. UNDP/FAO South China Sea Fisheries Development and Coordination Programme and IDRC.

UNESCO, 1984: Coastal zone resource development and conservation in S. E. Asia with special reference to Indonesia.

UNIDO, 1987: Environmental assessment and management of the fish processing industry. Sectoral Studies Series 28.

World Bank, 1981: Socio-cultural aspects of developing small-scale fisheries: delivering services to the poor. World Bank Staff Working Paper 490.

World Bank, 1984: Harvesting the waters - a review of Bank experience with fisheries development. Report 4984.

World Bank, 1985: Integrated resource recovery, aquaculture: a component of low coast sanitation technology. Technical Paper 36.

Contents - Previous - Next