7.1 Biota in aquaculture ponds
7.2 Technical aspects of fish culture
7.1.1 Food chains
7.1.2 Fish species
7.1.3 Aquatic plants
The objective in fertilizing an aquaculture pond with excreta, nightsoil or wastewater is to produce natural food for fish. Since several species of fish feed directly on faecal solids, use of raw sewage or fresh nightsoil as influent to fish ponds should be prohibited for health reasons. Edwards (1990) has represented the complex food chains in an excreta-fed fish pond as shown in Figure 17, involving ultimate decomposers or bacteria, phytoplankton, zooplankton and invertebrate detritivores. Inorganic nutrients released in the bacterial degredation of organic solids in sewage, nightsoil or excreta are taken up by phytoplankton. Zooplankton graze phytoplankton and small detritus particles coated with bacteria, the latter also serving as food for benthic invertebrate detritivores. Plankton, particularly phytoplankaton, are the major sources of natural food in a fish pond but benthic invertebrates, mainly chironomids, also serve as fish food, although they are quantitatively less important. To optimize fish production in a human waste fed pond, the majority of the fish should be filter feeders, to exploit the plankton growth.
Figure 17: Food chains in an excreta-fed aquaculture system (Edwards et al. 1988)
A wide range of fish species has been cultivated in aquaculture ponds receiving human waste, including common carp (Cyprinùs carpio), Indian major carps (Catla catlax, Cirrhina mrigala and Labeo rohita), Chinese silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), grass carp (Ctenopharyngodon idella), crucian carp (Carassius auratus), Nile carp (Osteochilus hasseltii), tilapia (Oreochromis spp.), milkfish (Chanos chanos), catfish (Pangasius spp.), kissing gouramy (Helostoma temmincki), giant gourami (Osphronemus goramy), silver barb (Puntius gonionotus) and freshwater prawn (Macrobrachium lanchesterii). The selection reflects local culture rather than fish optimally-suited to such environments. For example, Chinese carps and Indian major carps are the major species in excreta-fed systems in China and India, respectively. In some countries, a polyculture of several fish species is used. Tilapia are generally cultured to a lesser extent than carps in excreta-fed systems although, technically, they are more suitable for this environment because they are better able to tolerate adverse environmental conditions than carp species. Milkfish have been found to have poorer growth and survival statistics compared with Indian major carps and Chinese carps in ponds fed with stabilization pond effluent in India.
Edwards (1990) gives a thorough review of current knowledge on the various fish species which can be cultured in ponds fed with human waste. It would appear that considerable confusion still exists with regard to fish feeding on natural food. Although fish are generally divided into types according to their natural nutritional habits - those that feed on phytoplankton, or zooplankton or benthic animals - several species are known to feed on whatever particles are suspended in the water. There is also uncertainty about the types of phytoplankton fed upon by filter-feeding fish. For example, although blue-green algae are thought to be indigestible to fish, Tilapia have been shown to readily digest these algae and there is evidence that silver carp can do the same.
Aquatic macrophytes grow readily in ponds fed with human waste and their use in wastewater treatment has been discussed in Section 2.3.3. Some creeping aquatic macrophytes are cultivated as vegetables for human consumption in aquaculture ponds and duckweeds are also cultivated, mainly for fish feed. Among the aquatic plants grown for use as vegetables are water spinach (Ipomoea aquatica), water mimosa (Neptunia oleracea), water cress (Rorippa nasturtium-aquaticum) and Chinese water chestnut (Eleocharis dulcis). The duckweeds Lemna, Spirodela and Wolffia are cultivated in some parts of Asia in shallow ponds fertilized with excreta, mainly as feed for Chinese carps but also for chickens, ducks and edible snails (Edwards 1990).
7.2.1 Environmental factors
7.2.2 Fish yields and population management
7.2.3 Health related aspects of fish culture
In a successful aquaculture system there must be both an organismic balance, to produce an optimal supply of natural food at all levels, and a chemical balance, to ensure sufficient oxygen supply for the growth of fish and their natural food organisms and to minimize the build-up of toxic metabolic products (Colman and Edwards 1987). Chemical balance is usually achieved through organismic balance in waste-fed ponds because the most important chemical transformations are biologically mediated. It is now recognized that depletion of dissolved oxygen in fertilized fish ponds is due primarily to the high rates of respiration at night of dense concentrations of phytoplankton. Romaire et al. (1978) introduced Eq. 15 to cover the factors influencing waste-fed fish pond dissolved oxygen (DO) at dawn:
DOdn = DOdk±DOdf-DOm-DOf-DOp
DOdn = DO concentration at dawn
DOdk = DO concentration at dusk
DOdf = DO gain or loss due to diffusion
DOm = DO consumed by mud
DOf = DO consumed by fish
DOp = DO consumed by plankton
Bacterial respiration is not specifically mentioned in this equation but is included in the mud consumption of DO and in the planktonic DO consumption. In a well-managed waste-fed fish pond the DO in the morning should be only a few mg/l whereas in late afternoon the pond should be supersaturated with DO.
Mud respiration probably lowers DO by less than 1 mg/l overnight and a fish population weighing 3000 kg/ha would also lower DO by only about 1 mg/l overnight. Phytoplankton photosynthesis is the major source of oxygen during daylight hours and, during the night, the major cause of oxygen depletion is respiration. It has been estimated that respiration of plankton (bacterioplankton, phytoplankton and zooplankton) can lower pond DO by 8-10 mg/l overnight. By far the greatest proportion of the DO depletion overnight is caused by the respiration of the phytoplankton that develop as a result of the nutrients contained in the waste. Phytoplankton provide feed for the largest percentage of fish farmed in Asia (Edwards 1990). They also exhibit a positive net primary productivity on a 24-hour basis and are net oxygen contributors to a fish pond. The objective in a waste-fed fish pond should be to maintain an algal standing crop at an optimum level for net primary productivity by balancing the production of phytoplankton biomass, in response to waste fertilization, with the grazing of phytoplankton biomass by filter-feeding fish.
Fish mortality in a waste-fed pond can result from at least three possible causes. First, the depletion of oxygen due to bacterial oxygen demand caused by an increase in organic load. Second, the depletion of oxygen overnight due to the respiratory demand of too large a concentration of phytoplankton, having grown in response to an increase in inorganic nutrients, caused by an organismic imbalance. The third possible cause is high ammonia concentration in the waste feed. All three causes of fish mortality have been reported in respect of sewage-fertilized fish ponds. The sensitivity of fish to low levels of DO varies with species, life stage (eggs, larvae, adults) and life process (feeding, growth, reproduction). A minimum constant DO concentration of 5 mg/l is considered satisfactory, although an absolute minimum consistent with the presence of fish is probably less than 1 mg/l (Alabaster and Lloyd 1980). Fish cultured in waste-fed ponds appear to be able to tolerate very low DO concentrations, for at least short periods of time, with air-breathing fish (such as walking catfish (Clarias batrachus) being the most tolerant, followed in decreasing order of tolerance by tilapia, carps, channel catfish and trout. Reducing phytoplankton biomass to maintain a reasonable DO in the early morning hours might well depress fish growth more than exposure to a few hours of low DO. A wastewater fertilized aquaculture system might occasionally require a stand-by mechanical oxygenation system for use during periods when DO would otherwise be very low. However, if the system is well managed to avoid overloading, this expense can be avoided.
Unionized ammonia (NH3) is toxic to fish in the concentration range 0.2 - 2.0 mg/l (Alabaster and Lloyd 1980). However, the tolerance of different species of fish varies, with tilapa species being least affected by high ammonia levels. Bartone et al. (1985) found that satisfactory growth and survival of tilapia was possible in fish ponds fed with tertiary effluent in Lima, Peru when the average total ammonia concentration was less than 2 mg N/l and the average unionized ammonia concentration was less than 0.5 mg N/l, with the latter only exceeding 2 mg N/l for short periods. In ponds receiving large quantities of organic matter, sediments tend to accumulate and release anaerobic breakdown products, such as methane and sulphides, which can inhibit fish growth. Bottom feeding fish, such as the common carp (Cyprinus carpio), are most affected by such conditions, especially if the macrozoobenthos disappear.
A wide range of yields has been reported from waste-fed aquaculture systems, for example: 2-6 tons/ha yr in Indonesia, 2.7 - 9.3 tons/ha yr in China and 3.5 - 7.8 tons/ha yr in Taiwan. Although the majority of waste-fed fish ponds stocks carps, research in Peru and Thailand has demonstrated the potential of tilapia for such systems. Management of fish ponds can have a significant effect on fish yields but the maximum attainable yield in practice is of the order of 10 - 12 tons/ha yr (Edwards 1990).
Increase in weight of small fingerlings stocked in a pond follows a sigmoidal curve (Figure 18). The first phase of growth is slow, so a high stocking density can be adopted to better utilize the spatial and nutritional resources of the pond. Alternatively, this can be achieved by stocking with larger fish having a higher initial weight, following growth in nursery ponds. Fish yield is positively correlated with the size of the stocked fish at a given stocking density. In South China, tilapia are stocked once a year at rates of either 30g fish and 0.15/m2 or 1.3g fish at 2.3 - 3.0/m2 stocking density. An increase in weight of fish in a pond leads initially to an increase in yield or production but there is subsequently a reduction in the growth rate of individual fish because of the limitation of natural food production in the system. The third phase of slow growth in Figure 18 is because the total weight of fish in the pond is approaching the carrying capacity. Intermediate harvesting when the rapid growth ceases, at the end of phase 2, should lead to significant increases in total yield. The high yields of tilapia reported in South China sewage-fed ponds are due to high stocking density and frequent harvesting.
Figure 18: Fish growth cycle (Edwards 1990)
Clearly, the key to achieving high yields in a waste-fed pond is to determine the carrying capacity of the pond, the maximum standing stock of fish. This can be assessed by varying the waste load and determining the maximum production of natural food consistent with satisfactory water quality, sustainable through a fish culture cycle (Edwards 1990). Fish stocking density is related to carrying capacity according to the desired weight of individual fish at harvest, as follows:
Experience has shown that there is a limit to the fish yield attainable from a waste-fed fish pond. Higher yields can be achieved by addition of energy-rich supplementary feed, such as cereals, cereal brans or pelleted-feed. The highest yields are only achieved with a sufficiently high fish stocking density to benefit from the improvement in pond nutrition. There appears to be increased efficiency of utilization of supplementary feed by fish in ponds receiving sewage effluent.
Marketable weights of fish vary with species and local market preferences but, in general, desirable sizes of the following fish range from 0.25-0.6 kg for tilapia, 0.5-1.5 kg for Indian major carps (mrigal 0.5, rohu 1.0, catla 1.5 kg) and perhaps 1-2 kg for Chinese carps. Thus, for a particular carrying capacity, Chinese carps should be stocked at an intermediate density. The length of culture cycle, or frequency of harvesting, depends on the time it takes stocked fish to reach marketable size. It should be recognized that the size of individual fish is only significant if the product is to be consumed by humans. When fish are raised as high-protein feed for carnivorous fish or livestock, size is relatively unimportant. Nevertheless, it is now appreciated that sustainable yields of even high densities of small-size fish with a high specific growth rate are not significantly different from the yield of table fish for human consumption (6.2-7.8 tons/ha yr).
Although it is good practice to limit the discharge of toxic materials to sewerage systems, inevitably some of these materials gain access and heavy metals and pesticides are frequently present in municipal sewage. This gives rise to concern about bioaccumulation when sewage effluent is used in aquaculture. Algae are known to accumulate various heavy metals but, with the possible exception of mercury, fish raised in sewage-fed ponds have not been observed to accumulate high concentrations of these toxic substances. It would appear that the concentrations of heavy metals in the pond water may be accumulated at slower rates than new tissues develop in rapidly growing fish, such as tilapia. In the case of mercury, the position of fish in the food chain seems to be important in determining their mercury uptake, with carnivorous fish accumulating more than herbivores. Fish, apparently, have the ability to regulate the heavy metal content of their tissues, except for mercury, and tend to accumulate metals in parts other than muscle tissue. There is little information on the uptake of toxics other than heavy metals but a high phenol content in the sewage fed to fish ponds in Wuhon, China caused the fish flesh to become unpalatable due to the odour of phenol. Weis et al. (1989) have reported on the effects of treated municipal wastewater on the early life stages of three species of fish and indicated that moderately toxic effluent (organic fractions) caused cardiovascular and skeletal defects, depression of heart rate and poor hatching, larval and juvenile growth rates.
The health effects of aquacultural use of human wastes in respect of pathogenic organisms have been discussed in Section 2.5. Depuration was mentioned as a means to decontaminate fish grown in waste-fed aquaculture. It is generally believed that holding fish in clean-water ponds for several weeks at the end of the growing cycle will remove residual objectionable odours and pathogens and provide fish acceptable for market. However, there is a lack of data on depuration practice and experimental assessment. What little evidence there is suggests that depuration of heavily contaminated fish with bacteria in muscle tissue will not be effective. Relatively short depuration periods of one to two weeks do not appear to remove bacteria from the fish digestive tract. Considering the lack of verification of the effectiveness of depuration as a health protective measure, Edwards (1990) has not included it in his suggested strategies for safeguarding public health in aquaculture (Figure 19).
Figure 19: Aquacultural reuse strategies with different types of excreta to safeguard public health (Edwards 1990)