|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Organic residues in aquaculture|
Fertilization is so widely practiced in aquaculture that it seems almost superfluous to point out the economic advantages that can accrue through judicious use of the inexpensive nutrients present in manure or sewage. These substances, together with other agricultural or organic industrial processing residues, can also provide substitutes for expensive feed ingredients. Obviously, there are regions where few agricultural residues are available, and the aquaculturist must resort to chemical fertilization. But where these residues are available, and where ecological and sociological factors permit the use of such wastes, handling and transportation of the wastes to the ponds are important cost factors that may limit the use of waste residues. The closer the animal pens are to the water, the more economical the fertilization task is and a premium can be placed on planning and modifying new or existing farms to promote optimal operational logistics of joint animal husbandry, market gardening, and aquaculture. We are not aware of any studies concerning the distances between residue sources and ponds, or comparisons of various farm lay-out schematics for this joint agriculture-aquaculture production system, under various edaphic, climatic, and socio-economic conditions.
Manures from pigs and birds are more frequently used than cattle manure, and even in some areas where cattle exist their droppings are often unavailable to aquafarms (e.g., India, Afghanistan) because the dung is dried and used as fuel. In many cases, animal wastes are anaerobically digested in order to allow for multiple benefits, specifically biogas for cooking, and supernatant and sludge for agricultural or aquacultural use. Although the latter two products are largely used for fertilization, their potential use as feed ingredients is high and is currently receiving considerable research attention.
It should be mentioned that plant residues until now have had little use as fertilizers for aquaculture, in contrast to their application in agriculture. This is because of the fundamental physico-chemical differences between water and soil as cultivation media. However, some fish ponds are occasionally or regularly fallowed. Then crops or crop residues are ploughed into the pond as a sort of "green manure." Rice paddies are good examples, where fish can be stocked (ecologically sound pest management permitting), and the rice straw and stubble are ploughed into the paddy soil as soil conditioners.
The use of organic residues in aquaculture is, to a certain extent, dependent on competition for these residues by agricultural production systems. Although detailed studies comparing agricultural and aquacultural use of organic residues from an energy or material-accounting viewpoint are not available, it is quite possible that manure recycling is more efficient in aquacultural animal production (including integrated land animal-cum-fish systems) than in agricultural animal husbandry. Manures produced by fish in a polyculture pond immediately enter a detritus food chain. A portion of this detritus is recycled into higher trophic levels, in this case, table fish. The system, while bearing some resemblance to terrestrial grazing systems (i.e., cattle produce manure, which fertilizes plants, which are eaten by cattle, etc.), is considerably more efficient than that found in intensive agricultural animal production systems. In the latter, manure must be collected and distributed with an attached expenditure of energy and human labour.
Sewage utilization in aquaculture is conditioned by cultural, sanitary, and economic constraints. The simplest use is the establishment of family or village privies over Asian fish ponds, and/or the use of domestic effluents from rural settlements into flowages for cage culture. In these situations, the presence of toxic substances (e.g., trace metals, carcinogens) in the wastes are minimal, and there is less chance for excessive accumulation of harmful substances in the flesh of the fish. Disease and parasite transmission, while still a consideration, is often over-rated. Adequate pond management, as discussed earlier, and careful cooking can overcome most of these potential problems. When the sewage of small or large towns is used for aquaculture (e.g., Calcutta), the presence of materials such as industrial wastes may be potentially dangerous from the standpoint of toxic substances that accumulate in the flesh of the fish. The use of the wastes of such localities for the production of food for man is entirely contingent on segregation of these substances from the normal domestic sewage. The costs of such separation systems will ultimately decide the possibility of their use in food production.
Socio-cultural objections to the use of sewage for fish culture seem to be decreasing in several societies as ecological information becomes disseminated and as fertilizer costs increase. There is, for this reason, urgent need for intensified engineering, economic, and management studies of sewage use under various conditions of light to dense urban development.
The direct use of organic residues as fish feed is highly opportunistic; as intimated in Table 3, it depends on the local availability of anything from various by-products of grain milling to cheap but otherwise unusable animal proteins. Here, as with manure, the location of the feed source in relation to the location of the animals to be fed is a prime economic consideration. Widening the pond berms in China to supply feed for grass carp, as well as locating grouper cage culture near fishing ports, are cases in point. There is urgent need, however, regardless of the nature of the feeds, to have far better characterization of their nutrient values and to incorporate such data in computerized international feed data banks.
The future prospects for the increased use of organic wastes in aquaculture (especially as fertilizers) are clearly influenced by the cost of chemical fertilizers. As the price of fertilizer increases because of increased fossil fuel costs, one can anticipate greatly increased use of organic wastes in aquaculture even without vigorous promotion. Certain key research needs to be undertaken to make such use as beneficial as possible. These investigations should relate to the big-economics of combined agri-aquacultural systems. They should place emphasis on system-wide total and energy accounting with the goal of establishing trade-off values between the use of manure in aquaculture against other agricultural/domestic purposes. The organic value of various wastes, and the cost of handling and treating them under various levels of intensity of operation and development, need to be established. Health problems related to the use of sewage also need attention, especially as they relate to cost trade-offs and permissible risks under various treatment and handling conditions. Questions of the environment need to be addressed vigorously; as the pressure on water supplies increases, fish ponds may be used increasingly to supply water for people. Multiple-use oxidation ponds supplying animal protein and furnishing domestic water will also increase, and problems of eutrophication and contamination of ground and surface waters need to be addressed. The need for interdisciplinary research is obvious if one wishes to aim for optimization in the trade-offs among the several possible goals of fish pond use stated here.
One more caveat seems necessary about aquaculture in general, but more specifically about the seemingly simple, but really complex, subject matter of the use of organic residues in aquatic animal husbandry. This warning is also a challenge embodied in the quotation from Matsuda (26) to follow, which stresses, by implication, the need for multiple-level research, with strong emphasis on pilot installations and culture-oriented extension:
"Aquaculture is not solely a matter of growing a product; it is also a part of rural development, including marketing, distribution of food and income, employment, and living conditions. Thus aquaculture should not be recommended indiscriminately to people who are not ready for it."