he rationale for the culture and management of aquatic animals and plants has remained unchanged for centuries. Fish farming has been a traditional means of producing food intended for immediate consumption at the village level. This requirement for locally-produced animal protein still persists in landlocked countries, or rural areas which do not have ready access to fish supplies, and there is still considerable scope for further development of simple non-industrial fish farming. Several countries (particularly in Africa and Latin America) have policies which give high priority to the promotion of improved food supplies for the poorest segment of the rural population. The reasons for the recent and rapid development of aquaculture as a modern technology in the last 40 years are numerous. Fish and other aquatic animals remain the cheapest sources of animal protein. They do not necessarily provide the calories of meats but their high protein content and nutritional efficiency make them in popular demand for general health. They are widely recommended for basic nutrition, preventative medicine, and recuperative diets. Fish in particular have the highest feed conversion rates of the animal groups. Weight for weight, fish require fewer units of energy, demand less gross space, and have a higher productivity than all domestic farm animals, including poultry. However, that being said, comparisons of aquaculture with agriculture are not particularly relevant as these ecological, biological and technical efficiencies are not closely related to economic efficiencies or advantages. Modern aquaculture practices are now more advanced and have been progressively developed to produce cash-crop surpluses for sale in nearby villages or accessible urban areas. Others have been integrated with agricultural activities. In recent years, through the attention of specific biological research and development, some technologically involved operations have been developed in response to the increasing demand for high-value species in both domestic and international markets. Many of these enterprises are highly engineered and sophisticated. Thus, parts of modern aquaculture - like modern terrestrial farming - have become very industrial requiring substantial capital investment and risk management; but other parts remain little evolved from their traditional simple origins. 1.2 The Role of Aquaculture in National Policies 1.2.1 Food security and nutritional improvements 1.2.2 Increase trade and foreign exchange 1.2.3 Increase producers' incomes 1.2.4 Increase employment objectives The activities of aquaculture production usefully fulfill many governmental development policies (which frequently embrace capture fisheries as well as agriculture). Most policies embody the diverse objectives of securing supplies of fish for domestic consumption; increasing foreign exchange earnings; increase producers' incomes; and providing employment opportunities. These development objectives do not necessarily compete, but the options for different aquaculture systems of production and product are many and the inherent nature of each favours the realization of some objectives more than others. 1.2.1 Food security and nutritional improvements Most governments give priority to aquaculture for food products for domestic consumption in their development plans for food security and improved nutrition. This is equally important for land-locked countries with little or no access to fish, and countries with productive marine and inland fisheries which are perhaps already exploited at a high level. Countries which may not (or need not) give priority to aquaculture are generally those with natural fish resources which are as yet relatively underexploited. 1.2.2 Increase trade and foreign exchange Governments of some developing countries do not have nutrition or food security problems, and do not anticipate them arising as populations increase. Many give priority in their policy objectives to the export of cultured fish and fish products for the purpose of earning foreign exchange. Others which do experience animal protein shortages also give priority (albeit at the same level as that given to domestic food supplies) to export earnings. This is not necessarily incongruous motivation as the products of certain aquaculture systems (e.g., intensive shrimp farming) could not, with odd exceptions, provide a solution to domestic food security problems to any extent, and markets are more lucrative abroad. 1.2.3 Increase producers' incomes Many countries give a measure of priority in their policies to increasing or maximizing fishermen's and fish farmers' incomes. This is an eminently sensible objective because the more profitable a particular aquaculture system is, the more likely it is to develop and establish itself on a sustainable basis. For this reason it is perhaps the most important objective. All worthwhile aquaculture systems are based on practices which are profitable to the producer (i.e., they provide a real rate of return, whether in kind or in cash), and therefore not only provide but hopefully increase incomes. 1.2.4 Increase employment objectives Many governments make reference in their policy statements to increasing employment opportunities. This is the most difficult goal of aquaculture and cannot be considered a very realizable objective. In general terms, fish farming is not labour-intensive. In summary, aquaculture does not play an individual or independent role in the achievement of national socio-economic goals but is an important component of broader sectors which do, such as fisheries and agriculture. As a producer of animal protein food and an earner of foreign exchange, its contribution is small in many countries when compared with those from capture fisheries. As it is not labour-intensive it provides few employment opportunities. However, it has the vital attribute of being able to produce fresh animal protein within communities where it is greatly in demand. Furthermore, it must be recognized that aquaculture is a relatively new and growing sector, the importance of which will undoubtedly increase as production from the capture fisheries sector decelerates or declines. Such importance may, indeed, already be apparent in the inland and rural regions of Africa, Latin America and Asia, where perhaps aquaculture represents the only means whereby people may obtain fish. 1.3 The Options of Aquaculture and their Constraints Notwithstanding its present relatively minor political role, most governments frequently refer to one or more of these four objectives in their development policies. However, in executing their policies to achieve or give priority to one objective more than another, they do not always focus their institutional development capacity and support services on the most appropriate aquaculture activities which will achieve the desired goals. The range of options for aquaculture development are complex and inter-related, being dependent on the economic and social settings for the systems of production and the biological environments for the species to be produced. Aquaculture is defined as the husbandry of aquatic animals and plants at numbers which are greater than those found under natural conditions. This distinguishes the practice from capture fishing operations. Whilst the reference to aquatic animals relates almost exclusively to fish (including finfish, molluscs and crustaceans), it includes amphibians (frogs) and reptiles (turtles, alligators and crocodiles) farmed for meat and other animal products, and also includes aquatic plants. Present-day aquaculture methods can be broadly categorized into three basic systems based on the intensity of production. In accordance with the following interpretations, these are: - extensive aquaculture, for organisms cultured in low densities, dependent on natural productivity for food but possibly assisted by fertilization of the substrate; - semi-intensive aquaculture, for cultured organisms at higher densities (than would be the case in extensive culture), and dependent on both increased productivity, using fertilizers or other waste resources, and receiving supplementary artificial feed; - intensive aquaculture, for cultured organisms in high densities, and dependent on artificial feed for their nutritional requirements. The principles of the three basic methods for aquaculture production can be simply defined, but the biological and engineering techniques which can be used to apply them are manifold. Furthermore, the techniques can overlap each other creating a vast matrix of options and alternatives, and all acted out within a three-dimensional medium (water) which can be freshwater, saltwater, or something in between (brackishwater). One main purpose of aquaculture in the developed world is to make a financial profit by the production of a marketable commodity. The emphasis has therefore been on the technologically-advanced intensive systems, requiring large yields per volume of water in terms of either space or flow, or multi-species production in well-managed natural environments. Because of the economic advantages of these intensive systems, specifically the large volumes of product from small areas of land, silos, raceways, cages, and net pens are appropriate technology in the developed countries and similar urban areas of Asia, etc. In the densely populated developed countries there is strong competition for land between several industrial sectors, and there is always a shortage of suitable sites for aquaculture, particularly around the coasts. There is also the need to protect the high investment from the many man-made problems in industrial regions, especially water-borne and air-borne pollution. On the other hand, the investment and operational costs are high and so is the risk, and therefore management and operational efficiencies must be constantly monitored. Such farms are ideal for the business acumen of owners/non-operators, and best restricted to the purview of corporations to invest in such large profit/high risk ventures. The alternative for avoiding some of the high risks and high investments of the intensive system is the semi-intensive system. The evolution into semi-intensive farming has been both by orderly advanced progression based on trial and error, but also by the retreat of intensive operators who have preferred to spread the many risks which still exist. Semi-intensive systems have the advantage in that artificial feed is only a supplement to a diet of natural food organisms encouraged to grow in the systems, and these live foods can supply the stock with missing amino acids, vitamins, and trace elements. Semi-intensive systems are typically based on the use of large regularly-shaped ponds, or pen structures which enclose a body of water. There are also few restrictions on size. Many farmers endeavour to increase the yields from these semi-intensive ponds by increasing the natural productivity through the addition of nutrients. These may be in the form of inorganic fertilizers, organic composts, dry and wet human sewage, or the waste products from other animal and poultry production units. Integrated aquaculture/agriculture farming has proved to be advantageous for both the large- and the small-scale investor. Because of the need for close management of highly inter-dependent operations, the fish ponds are usually small, often 0.5 ha or less. The most advanced integrated systems can be found in China and Thailand. Such integrated farms are now in many African counties, including Zambia, Central African Republic, Ivory Coast and Egypt. A primitive practice of integration is fish with culture of rice. Although this type of farming is very seasonal, it is well suited to the smaller fish species which are prolific breeders. Fish-cum-rice farming, as it is called, remains one of the most useful farming systems for remote rural areas. The same advantages of enclosures also apply to another semi-intensive production system, namely that of farming fish in permanent or temporary barrages. Use of these barrages for the farming of tilapia is being made in many African countries such as the Ivory Coast, Nigeria, and Cameroon. Not all such barrages are inland, and several coastal barrages have been created as parts of irrigation and water conservation projects, as in India and China (used for carps). Barrages have proved to be useful communal farms requiring minimal attention other than stocking of fingerlings and regulated harvesting. All semi-intensive systems require varying amounts of investment capital for construction, the annual supply of seed for stocking, and the purchase of quantities of suitable feed to supplement the productivity induced by compounded fertilizers. As many farmers are unable to purchase all or even one of these important inputs, there is still a significant place in aquaculture for the simple extensive system. Simple owner/operator practices, which may be one man with one pond no more than 1 are in size, exist throughout most African countries, Latin America, Central America and the Caribbean. They have changed little through the centuries and are exposed to all the vagaries of floods and drought, or predation and theft. It is fortunate that they are ideally suited to the African tilapia species which require little attention, and survive a wide range of natural conditions and breed prolifically. The continuous demands on these rural fish ponds and the harvesting of undersized individuals makes typical yields hardly enough for a single-pond owner and his family. Fortunately, the cost is little more than his time to dig the pond and to seed it with some breeding adults. There are, however, good possibilities for moderate improvement, especially with extension assistance, both through composting and similar means of increasing productivity, and by more careful attention to culling and harvesting techniques. It is evident that there has been a large number of technical and biological options for aquaculture development, each with its constraints. However, the growth of aquaculture has not been dependent on the correct selection of a specific permutation with the least number of constraints but rather on the two main beneficiaries. These are: (a) the consumers of aquatic products, and (b) the individuals who farm. More than anything else, these two groups continue to determine the future of aquaculture in any country, yet their interests and involvement in aquaculture development activities have been widely overlooked.