|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Organic residues in aquaculture|
On a polyculture fish farm in Israel, for instance, where yields are reasonably high (4,150 kg/ha/yr.), nitrogen and phosphorus fertilizers, applied as liquid ammonia and superphosphate, amount to 18 per cent and feed to 31 per cent of the annual calorie inputs (14). Obviously, savings could accrue when manure is used instead of inorganic fertilizer, depending, of course, on nearby production and low-cost handling of manure. Just how economical - albeit site- and condition-specific - sewage-fed aquaculture can be, is exemplified by a comparison of the energy inputs of the above-mentioned Israeli operation with that in the Indonesian cage culture for carp in sewage-fed streams, and pond-grown carp near Munich that derive their sustenance from a mixture of the sewage of Munich and the water of the Isar River. In Israeli pond polyculture, 65 keels (representing fixed and variable production inputs, excluding labour) produce 1 9 of protein; this could be reduced to about 50 kcals/g if the industrial fertilizer were replaced with manure. In contrast, the sewagebased carp culture in flowing water in Indonesia, as wed) as in Bavaria, requires only 4 - 10 kcals/g of protein. Channel catfish and chickens are less efficient, requiring approximately two times the calorie input per gram of protein required for Israeli poly culture, and fourteen to thirty-seven times that of pure sewage-based carp culture (15). Another advantage in polyculture is that nutrients are re-used as they pass through the digestive tracts of the various component species.
The use of manure and domestic sewage, however, represents a saving for the fish farmer only when these materials are available, and because of their inherent bulk, need not be transported even over medium distances.
Their use prevails in many parts of tropical Asia, in India, in communes of China, and in kibbutzes of Israel, where land animal husbandry and aquaculture are practiced conjointly (5, 12, 16). Examples of this are provided by a hog-cum-fish polyculture experiment (17), and a study comparing biomass harvest of an oyster-only culture system, and an oyster-cumdetritus feeder system (18).
In the hog-cum-fish system (Figure 7), overall productivity of biomass was increased by 67 per cent - with no additional feeding or fertilizer - by providing swine wastes (uneaten food, faeces, urine) to a polyculture pond. Food conversion efficiency increased from 3.8:1 in the hog-only system to 2.2:1 in the hog-cum-fish system. Conversion of feed nitrogen (our extrapolation) was 58 per cent in the hog-only system versus 70 per cent in the hog-cumfish system.
The addition of a detritus feeder (a polychaete worm) to an algae-oyster culture system (Figure 8) increased biomass conversion efficiency from 9.8 per cent (oysters only) to an overall 11.2 per cent efficiency for an oyster-cum-worm polyculture system (18).
The use of domestic sewage, as practiced in certain sites in Asia, is also favoured by site as well as ecological conditions, and, above all, by the availability of abundant water. Here, as in the case of animal manure, the aquacultural planner may have to weigh the advantage of biogas via anaerobic digestion of organic wastes and sludge production against direct use of sewage. The former practice generally has benefits applicable to other portions of agricultural production systems, including cooking, heating, and lighting, and production of fertilizer and soil conditioner (19).
Anaerobic digestion has been reported to increase the manure's ammonia content, a preferred species of agricultural nitrogen fertilizer, from 26 per cent in raw, unprocessed manure, to 50 per cent following treatment (19). However, the extent to which anaerobically-treated manure sludge and supernatant can be substituted for untreated manure in a fish pond while maintaining high yields of fish has not been demonstrated. This is a very important consideration, because organic substances added to a pond can be consumed directly by heterotrophic organisms and bypass the photosynthetic production level. Thus, production of fish using organic manures can greatly exceed levels predicted for a pond based entirely on an ecosystem starting with light-limited plants utilizing inorganic nutrients.
Certain special conditions of combined aquaculture and land animal husbandry have been devised that reduce to a minimum the difficulties and costs of handling organic manures, and that benefit both husbandry components. Ducks are natural manure carriers, as it were, and in duckcum-fish culture in Hungary, ducks are stocked in ponds after the fish have reached fingerling size. The presence of ducks leads to an increase in fish biomass of 0.3 to 0.4 tons/ha over conventional ponds without ducks (F. Mueller, personal communication, 1978). The ducks in this case are selectively bred to reach their market size of 2.5 kg in 45 days on copious feeding with a special pellet diet; sheds and runways for ducks are necessary, as is the skill of poultry-keeping in addition to that of aquaculture. However, intensive use of the fish ponds by ducks can threaten the pond walls. Care must also be exercised not to overload the aquatic ecosystem with excrete and bring about highly anaerobic conditions in the bottom mud, which is the nursery ground of many invertebrates that serve as important food components for the fish.
Despite these possible problems, mutual benefits to fish and bird-rearing are many, including: lower capital investment than for intensive chicken culture; shortened growing time for ducks; better utilization of feeds; ducks eat organisms not ordinarily eaten by fish (e.g. aquatic weeds, frogs, etc.); ducks distribute manure evenly throughout the pond; and fish pond ducks are healthier, leaner, and have cleaner feathers than do ducks raised in other conventional production systems (20).
Another pattern for land and water animal-rearing presents itself through the placing of pigsties partially over the ponds, in such fashion that wastes from the pig platform can be sloshed down into the ponds. The ecological basis for this practice was discussed earlier. The technique has been pioneered in Malaysia, with water hyacinths grown on part of the pond surfaces being incorporated into the pigfeed, and with the fertile pond water also being used to water market garden crops (Figure 9) (21). Such integrated farming systems that can amortize themselves and bear profit after three years depend, of course, on year-round availability of water and on having a suitably sloped terrain.