|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Organic residues in aquaculture|
Organic waste utilization in aquaculture can either be extensive, with wastes occurring naturally or being added, with little or no further management of production (1), or it can be highly intensive under conditions where extraneous feeding and fertilization with inorganic plant nutrients play a major role in augmenting animal protein yield from the system. Aquatic animal husbandry is pursued in embayments, ponds, rivers, lakes, raceways, in brackish water, fresh water, and full-salinity ocean water. Many variants of extensive and intensive fish culture rely importantly on the growing together of a few compatible species (polyculture) to make fullest use of the various types of food present (plankton, bottom fauna) in a body of natural or managed water (Figure 1). Feeds and/or inorganic or organic fertilizers may also be added. Shellfish - molluscs and prawns or shrimp - can also be grown in polyculture, but managerial practices involving several species of fish exclusively are more advanced than those that combine the husbandry of invertebrates and fishes.
The rationale of polyculture is obviously to divert as much as possible of the attainable biomass into channels that are useful to man, compared to those that prevail in a wholly natural food web. It should be pointed out that stocking fish in a lake does not necessarily increase the productivity of the lake, only the productivity of harvestable (i.e., desirable) organisms. In a lake supporting a complex food web ending in only a few harvestable fishes, and many other non-desirable organisms (for human consumption), stocking with desirable fish fingerlings, as well as control of undesirable species, especially predators, diverts the biomass of the "non-suitable" species into production of desirable fish biomass. It should be added, though, that monoculture, especially of molluscs in brackish water and of predatory fish such as trout, salmon, or groupers, can furnish high annual yields that are commercially attractive in spite of substantial inputs: for these species, the food is extraneously supplied.
A brief comparison of materials flow in ponds versus cage culture seems appropriate here. The food webs of manured, fed, and fertilized polyculture ponds are exceedingly complex in comparison with cage monoculture systems.
The principal pathways of nitrogen in ponds are illustrated in Figure 2. Possible nitrogen inputs to intensively managed polyculture ponds may be many and varied: atmospheric nitrogen, and nitrogenous substances present in in-flowing water, in inorganic fertilizers, in organic wastes, and in feeds. Similar diversity is also present in losses of nitrogen from ponds. Measurement of nitrogen flow between the various nitrogen pools (e.g., dissolved nitrogen pool, bacterial nitrogen pool, etc.) is a difficult task, requiring sophisticated techniques. For the present, aquaculturists are utilizing information on a few such inputs, pathways, and pools, and are only very slowly evolving models allowing prediction of harvests of fish from a pond given a specified set of conditions.
In contrast, cage culture represents a simplified ecosystem, at least within the confines of the cage itself (Figure 3). Cages are the aquatic counterpart to high-density terrestrial husbandry systems, including cattle feedlots, chicken batteries, and so forth. Such production systems are fashioned to provide all of the environmental needs of the animal and maximize production per unit surface area. The cage system is quite simple, with feed or food supplied to the fish (usually monoculture, but with polyculture cage culture becoming more common), and faeces, uneaten food, and waste metabolites being swept away from the cage by the continuous flow of fresh water. Cage culture, however, is generally more capital-intensive than pond culture, and requires a considerably higher degree of knowledge of the nutritional needs of the fish, as the aquaculturist often supplies the fish with their sole source of food or feed.
The denser the stocking rate in fish culture, the more difficult the management, largely because of the accumulation of waste substances, depletion of dissolved oxygen, and other problems of sanitation. Yet it is known from the interaction of sewage with rivers that fertilization does not necessarily lead to oxygen depletion when the river flow is reasonably swift, causing diffusion of atmospheric oxygen into the water. Thus, it is not surprising that higher aquatic production is more often registered in flowing than in still waters, and that estuarine waters with their river-borne nutrients and continuous water circulation (via river flow and tides) represent the world's most fertile and productive aquatic environments. A comparison of annual productivity for several aquatic ecosystems is provided in Table 1, based on Crisp (2).
TABLE 1. Examples of the Productivity of Various Aquatic or Marine Ecosystems
|Sargasso sea (oligotrophic)||1,340|
|Peru current (eutrophic)||36,500|
|Estuarine and brackish water marsh||16,000|
|Lake (oligotrophic)||70 - 250|
|Lake (eutrophic)||750 - 2,500|
After crisp (2).
Several compilations in the literature 11, 3 - 6) permit quantitative comparisons of these various conditions under more or less intensive management regimes. The time base of a hundred days rather than a year is chosen, on the advice of Ohla of the Hungarian Aquaculture Research Institute, so that summer production in the temperate zone can be compared with the average in the tropics.
The animal protein yield to man in natural waters of the temperate zone ranges from less than 20 to several hundred kg/ha (Figure 4). There are specific sites, often with inadvertent fertilization by run-off, where yields are much higher: a dead river arm in Hungary with a slow flow that also serves a duck farm produces 1.3 tons/ha/100 days, and a portion of the lake Laguna del Bay in the Philippines, the shallow recipient of much agriculture) and domestic drainage where the wind also concentrates the plankton, is capable of producing 3 tons or more of milkfish per ha/100 days; however, the site is now plagued with pollution from highly populated shores.
A comparable level of fish productivity prevails in the more typical fish ponds of Israel or India, albeit with more material inputs. Animal wastes are applied and often there is extraneous feeding; there also, the 100-day yield reaches 3 to 4 tons/ha. Sewage oxidation ponds that are stocked with polyculture species able to make the best use of the rapid production of algae and invertebrate biota yield between 2.4 and 4 tons of fish per ha/100 days.
Still higher yields are reached in flowing waters: Indonesian carp cages in sewage-fed streams produce 8 kg/m²/100 days (cage surface area only), amounting to a practical harvest of at least 20 tons/ha in the same period. The apparent discrepancy is due to the fact that only a portion of the flowage can be obstructed with cages lest inundation become severe. Intensive feeding of the carp is not practiced because blood-worms thrive in the sediments of these streams, feeding on the bacteria-rich silt and ooze, and these worms are grazed upon by the carp. The peculiar aspect of the situation, however, is that the worms regenerate fast and easily after the fish nibble off a portion of them. The worms, incidentally, contain growth stimulators, probably hemoglobin-related compounds (7).
The relationship between fish stocking, density, and water flow rate is illustrated in Figure 5. Although these values are species-specific, the benefits of increased oxygenation and removal of toxic body metabolites by rapid water exchange hold true for most species.
Sewage oxidation ponds are also good bases for aquaculture. J. Ohla (personal communication, 1978) established that 2.5 tons/ha/100 days of severe) species of carp, mainly silver carp, can be grown if 200 m³/ha/day of primary sewage effluent is added to the water (after settling of the solids).
The stocking of fish in sewage oxidation ponds is beneficial not only to fish culture, but to the waste treatment processes within the oxidation pond itself. Tilapia and silver carp were stocked in an oxidation pond and compared to an oxidation pond not containing fish (8). Bacteria levels were lower in the pond containing fish (Table 2), perhaps due to the disinfection potential of waters with high pH and oxygen. The high pH probably results in a greater loss rate of ammonia to the atmosphere, considered to be beneficial from the waste-treatment viewpoints, and also from the viewpoint of fish health management (ammonia is toxic to fish). However, it represents a loss of valuable fixed nitrogen fertilizer, and hence can be a somewhat negative trade-off.
TABLE 2. Measured Chemical and Biological Parameters in Waste Treatment Ponds with (+) or without (-) Liquid Cow Manure, and with (+) or without (-) Polyculture Fish (Tilapia, Carp)
|Dissolved oxygen (0900 h)||0.7 9.5||9.0 - 15.9||10.0 - 13.8|
|pH||7.9 - 8.3||8.3 - 8.9||8.6 - 8.7||-|
|Bacteria (103/ml)||17 - 27||1.6 - 6.7||0.7 - 4.3|
|Phytoplankton (principally) (g dry/m³)||0.2 - 4.3||0.3 - 1.4||< 0.06 - 0.2||< 0.06|
|Zooplankton (principally) (g dry/m³)||0.3 - 42.4||0.1||- 1 0 < 0.06||< 0.06|
|Chironomides (10²/m²)||79 - 215||1 - 4||0 - 2||1 - 7|
Source: Schroeder (8).
The use of raw or primary treated sewage has been questioned for health reasons. It should be noted here that intestinal parasites and flora of man and other warm-blooded animals are anaerobic, or nearly so, and that the high oxygenation of fast flowages, of balanced sewage ponds, or of balanced fish ponds receiving organic wastes, does not permit the survival of most such pathogens (5). Finally, thorough cooking provides an additional safeguard.
The addition of sewage to pond or river water, and the use of fertilization by means of animal wastes in aquaculture, are well established and economically sound practices. Higher yields per unit surface area are reached with other extraneous inputs, mainly by supplementary feeding and/or by forced water circulation or even filtering. The main secret of success in achieving up to several hundred tons/ha annual fish production (often extrapolated from smaller surface areas) lies in the use of natural or artificial flowages in which many thousands of fish can be stocked and fed. Similarly, cage-reared fish in strong tidal flows can achieve phenomenal rates of growth and production. They are fed with compounded feed (however expensive), with trash fish, with household wastes, or with cereals, cereal wastes, or other agricultural residues. Attractive economic returns conditioned by cultural food tastes invite these practices (1; see chapter 1).
Yet another high-yield aquatic production system ought to be mentioned, this time in the sea. In certain bays near Vigo, Spain, where tidal exchange as well as fertile run-off from the land are high, mussels are grown on containment/ attachment devices suspended from rafts. Three hundred tons of mussel flesh per year have been reported (9). Under roughly comparable conditions, similar planktonfiltering bivalves (Mytilussmaraydinus) also reach such high yields (10). Tidal movement-based mollusc culture, however, is more advanced in Europe and Japan than it is in tropical Asia; nonetheless, in all locations it is threatened by pollution.
Having noted the beneficial effects of dissolved organic wastes on aquacultural systems, one must hasten to add that most fish farmers use chemical fertilizers, predominantly or entirely. Manure is just not sufficiently ubiquitous, and the use of domestic sewage, even while more widely available, has strong cultural and economic barriers against its use. While the cultural component of this barrier is perhaps understandable, though ill-advised, the economic one invites further comment.
Where sanitation is advanced, investments in rendering sewage effluents innocuous and infertile are so great that sewage aquaculture can only be envisaged when cost of invested capita) need not be considered. That is, in the temperate zone and in technologically advanced countries, one may think of planning sewage aquaculture for small to medium-sized towns with readily available cheap land or a nearby lake. Installations will be necessary to separate domestic sewage from all other liquid effluents such as industrial wastes and nonsewage domestic and stream run off components. Even then, big-accumulation of toxic substances may occur (11). The actual levels of trace metal accumulation will be species- and site-specific. For example, rainbow trout fed a diet containing activated sludge selectively accumulate certain metals, but not others (Figure 6).
In the tropics, as already indicated with reference to the cage culture for carp in Indonesia, the situation may be more favourable to the direct use of domestic sewage because there is, as yet, less admixture with the effluents of industrial or hydrocarbon chemicals. Aquaculture and agriculture in mainland China utilize both animal and human wastes. Reports suggest that, while animal wastes are normally either applied untreated, or composted and applied, anaerobic treatment and composting are the preferred methods for human wastes (12). This process greatly reduces parasites and pathogens, rendering the manure safer to use (13).