Institute of Advanced Studies
University of Malaya
Kuala Lumpur, Malaysia
Algae from the major primary producers in livestock-fish integrated ponds. Photosynthetic oxygenation of the water is achieved while nutrients released from the bacterial breakdown of the organic matter are converted to algal cells. The three important factors governing fish productivity, being dissolved oxygen, pH and food, are intimately linked to the presence of the algae.
In the integrated system, the objective is two-fold. Firstly, to increase the productivity of the fish, and, secondly, to use the pond system, for the treatment of the animal manure. Fish productivity is enhanced through the increase in natural food materials in the form of plankton biomass resulting from the nutrients, as well as the direct consumption of the spilled food and possibly organic manure. The manure is broken down through bacterial activity and the products converted to algal cells, releasing oxygen in the process, thereby relieving the oxygen demand the manure exerts in the pond water. This process has to be balanced to prevent over-eutrophication and anaerobiosis which would result in the system breakdown and fish mortality. Information linking animal stocking rates (both livestock and fish) and manure input rates to plankton (especially algae) production and water quality are not sufficiently known to allow establishment of system models. Research towards this objective is of prime importance and is strongly recommended.
A successful integrated farming system leaves no wastes or undesired by-products. Ideally the products of one sub-system is utilized by another sub-system, generating useful materials like protein biomass in the process. In a livestock-fish integrated system, where the livestock(goats, pigs, chickens or ducks) are housed in pens constructed over the pond surface, space utilization is improved (Edwards et al., 1988). This often leads to increased productivity per unit area. The centre of biological activity is thus the pond. Wastes generated by the livestock are channeled directly into the water to be consumed directly by the fish, or converted into microbial biomass the latter process fulfilling breakdown and thus treatment of the wastes. The microbial biomass comprising of bacteria, phytoplankton (algae) and zooplankton, serves as natural food for the fish.
More than fifty percent of total operating costs in a fish farm is attributed to the cost of feed (Edwards et al., 1988). Aquaculture systems may be classified according to the feeding practice. An extensive system uses only natural feed without fertilization of the water. A semi-intensive system utilizes fertilization to increase natural feed. An intensive system uses nutritionally-complete artificial feed. A livestock-fish integrated system falls into the semi-intensive category. Fish yields from extensive systems range from 0 to 1 t/ha/yr, while 1 to 5 t/ha/yr are obtained from semi-intensive systems with low quality manure (Edwards et al., 1988). With highquality manure, semi-intensive systems may produce up to 15 t/ha/yr, while the addition of pelleted feeds and aeration may increase the yields to 20 t/ha/yr. Intensive systems may produce fish yields from 20 to 1000 t/ha/yr with pelleted feed and water change. Yields as high as 10 to 12 t/ha/yr have been obtained from livestock-fish systems with high-quality manure (duck or pig). The duck-fish integrated pond at the Experimental Farm of the University of Malaya, has fish yields averaging 5 t/ha/yr (Geeta et al., 1988, 1990; Rohani, 1991).
The objective in a livestock-fish integrated system is two-fold; to increase productivity of the fish by increasing the natural feed; as well as to use the pond to treat the livestock wastes. The biological system in the pond, which normally comes into equilibrium through a natural algal-bacterial symbiotic process, cannot be over-fertilised by very high manure input.
ROLE OF ALGAE IN THE MANURE-FED POND
Fish productivity is governed by pond water quality with dissolved oxygen, pH and feed being intimately linked to the algae.
Fish respond differently to low oxygen levels, the sensitivity depending on species, life-stage and life process (Alabaster & Lloyd, 1980). In a manure-fed pond, the oxygen demand exerted by the organic matter may be met by the oxygen evolved by the algae. This process of photo-oxygenation of the water by algal photosynthesis may be simply described as;
Light captured by the pigments in the thylakoid membranes of the choroplasts is converted into chemical energy through the photosystems and electron transport chains as shown in Figure 1. This chemical energy (NADPH and ATP) is then utilized in the Photosynthetic Carbon Reduction Cycle to convert CO2 to carbohydrate. In the synthesis of glucose from carbon dioxide, with the evolution of 6 molecules of oxygen, 48 light quanta must be used.
In the manure-fed pond, the organic matter is primarily broken down by aerobic bacteria into simple compounds including carbon dioxide and ammonia. The carbon dioxide is utilized by the algae for growth, evolving oxygen for the bacterial processes for its own respiration needs, and oxygenation of the water for the fish and other aquatic life. In a flow-through system, oxygen is supplied by the water flowing in, while in a manure-fed pond with little or no water change, photo-oxygenation is a major source.
In general, it is recommended that a dissolved oxygen (DO) level of 5 mg/1 be maintained for a productive fish pond (Alabaster & Lloyd, 1980). The work at the Institute for Advanced Studies (IAS). shows that in the manure-fed ponds, DO ranges from 1.2 to 3.4 mg/l for the goat-fish pond and 0.5 to 3.7 mg/l in the duck-fish pond, in the early morning (0600 hr); the DO levels rise to 10.8 mg/l in the goatfish pond, while it reaches supersaturation in the duck-fish pond at 1400 hr. The loading rates in the goat-fish and duck-fish ponds were 5.7 g dry weight of manure/m2/day and 1.1 g dry weight of manure/m2/day respectively. There was no mortality of fish recorded due to the fluctuating diurnal DO levels. This was probably because the low levels in early morning were present only for short periods of time. Fish mortality is often caused by sudden deoxygenation and prolonged oxygen depletion. Also the fish species cultured were of a hardly variety tolerable to the regular low DO levels.
Supersaturation of pond water with oxygen is generally non-lethal except at very high pH. DO concentration in water in inversely proportional to temperature, and high concentrations of oxygen at lethal temperatures have in fact been shown to improve survival of some fish species. DO concentration affects fish activities, mainly at (i) juvenile growth; food consumption and growth of fish may be depressed at low DO levels, and (ii) swimming behavior; swimming declines at levels below 5 mg/l DO.
The oxygen consumption of Oreochromis mossambicus
Peter was defined by Melard and Philippart
(Balarin & Haller, 1982) as;
Oxygen consumption (O2 mg/kg/hr) = 2.115 W-0.61 where W = weight of tilapia (g).
In semi-intensive culture systems, fish stocking is recommended according to the species' oxygen demand. The input of organic wastes into the pond results in increase in Biochemical Oxygen Demand (BOD3 or BOD5) at 20°C. In a study on sewage-fed ponds used for fish culture in India (Chattopadhyay et al., 1988), it was found that while DO decreased with increasing BOD levels, the net primary productivity and calculated net energy inflow increased (Table 1).
Table 1. Some characteristics of a sewage-fed fish pond.
|≤10||10 – 20||20 – 30|
|BOD (mg/l)||4.5 – 8.0||12.0 – 18.5||21.0 – 30.0|
|DO (mg/l)||4.4 – 12.6||5.0 – 8.4||2.4 – 3.8|
|(mgC m-2 h-1)|
There was no significant difference in primary productivity between conditions of 10 – 20 mg/l and 20 – 30 mg/1 BOD, and the authors recommended that to ensure a good fish production, a BOD level from 10 – 20 mg/l should be maintained.
A positive correlation was found between algal numbers and oxygen content in the IAS duck-fish pond (Geeta et al., 1990). The algal biomass in the pond averaged 44 mg dry weight/l.
Fish are found in waters having pH ranging from 4 to 10, although the safe level is 5 – 9 and for maximum productivity the pH values range from 6.5 to 8.5 (Alabaster & Lloyd, 1980). Hydrogen-ions are toxic to fish through (i) precipitation of mucus or protein on gill epithelia resulting in suffocation; (ii) inability to osmoregulate; (iii) acidosis of blood; and (iv) increased susceptibility to disease.
The lethal effect of acidic conditions is directly related to concentration of free carbon dioxide and total hardness, but is inversely related to concentrations of calcium and sodium. Juveniles are also more sensitive to low pH. Fish kills are often linked to sudden drops in pH. In acidic waters, the growth rate of fish is reduced. While feeding rate is not affected, the reduced growth rate may be due to reduction in amount of available food. Low pH reduces decomposition rate of organic matter and inhibits nitrogen fixation. Primary productivity may be decreased resulting in reduced food. Many chemical compounds dissociate into toxic elements in acidic conditions while the toxicity of many elements (like zinc) increases with low pH.
Hydroxyl-ions are toxic to fish through (i) destruction of gill and skin epithelia; and (ii) damage to eye lens and cornea.
As stated earlier high pH is lethal to fish at high DO levels, especially the juveniles. The toxicity of ammonia increases with increasing pH, due to the increase in the unionized (NH3) form at high pH. Blue tilapia Oreochromis aureus Steindachner can tolerate ammonia concentrations of 2.35 mg/l and by acclimation to a level of 3.4 mg/l (Balarin & Haller, 1982).
High pH levels are linked to periods of active photosynthesis, where the removal of carbon dioxide for synthesis of carbohydrates results in increase in hydroxyl ions. In the goat-fish and duck-fish integrated ponds at the IAS, pH was positively correlated to algal biomass. pH fluctuated from 6.2 in the early morning to a maximum of 8.4 at 1600 hr, for the goat-fish pond. The algal biomass was 11 mg/l. In the duck-fish pond where a higher algal biomass existed (44 mg/l), pH rose from 7 in the early morning to a maximum of 10 at 1400 hr. No fish death was noted during active photosynthesis, probably because the fish migrated to the pond bottoms where more acidic conditions resulting from anaerobic activities. In spite of the bigger diurnal fluctuations in pH in the duck-fish pond, fish productivity was higher, due probably to the higher amount of natural food.
In a manure-fed pond with no artificial feed supplementation, natural food for the fish includes bacterial biomass, phytoplankton (microalgae), zooplankton, larval forms of many insects, and other invertebrates. Some fish species fed directly on the manure. Information as to the relative importance of the various food sources to fish production is scarce, but to fully exploit these various food resources, a polyculture of omnivores, carnivores and phytophagous species is necessary.
There is differing opinion on the importance of algal species composition in fish production (Pullin, 1987). Blue-green algae like Microcystis sp. have been shown to be more digestible than the green algae, especially to tilapia. (Colman & Edwards, 1987). The role of algae as the basis of a food chain may also be dependent on the type of species found. Different algal species have different nutritional values in addition to different digestibility. The organic manure may promote selectively specific algal species, especially species which are more tolerant of high organic content like members of the Euglenophyta and Chlorophyta. The Cyanophyta or blue-green algae, are usually less tolerant of highly organic conditions. The organic loading regime of the pond would therefore affect the algal species composition. In a natural system, a succession of algal species would occur, with the Euglenophytes dominating at the start of manure influx, followed by an assemblage of Euglenophytes and Chlorophytes and coming to equilibrium with a mixture of the two Cyanophytes. This succession may be controlled to a certain extent by altering the loading rate or by selection of the type of manure. It has been postulated that pulsed inputs of high quality manure produce “r” type species, while inputs of low quality manure select for “k” type species (Pullin 1987). The “r” type species are those “boom and bust” species which are opportunistic and have high intrinsic growth rate. Algae which cause blooms when nutrient levels are suddenly increased, belong to this “r” class. The “k” type species e.g., blue green algae, on the other hand have low reproductive rates and invest in long-term presence.
In feed formulation for intensive fish farming, it is necessary to understand nutritional requirements of fish. The following is a short review of the nutritional requirements of fish, including protein, lipids, carbohydrates, vitamins, minerals and nutritional values of algae.
Protein: The quality of a protein, the most expensive part of the feed, is determined by its amino-acid profile and availability. Ten essential amino-acids have been identified; arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (Jauncey, 1982). Many algae are deficient in sulphur-containing amino-acids, like cysteine and methionine. Protein is required for tissue construction and repair.
The efficiency with which fish utilize protein is determined by several parameters including:
Protein Efficiency Ratio (PER)
Apparent Protein Utilization (NPU) %
= body N-content at end of test
where endogenous nitrogen losses are not considered;
Lipids and carbohydrates are incorporated into fish feed to “spare dietary protein” for energy requirement. In the diet of mirror carp, the protein content may be reduced from 45 to 29% at a lipid level of 18% with no change in growth (Jauncey, 1982). The addition of 14 % starch reduced the protein requirement from 49 to 33 %.
The main sources of protein in fish feed are fishmeal and soybean meal. Other non-conventional sources include algae, bacteria and yeast. Of these, algae have the most potential.
Lipids: Lipids are required as an energy source and for maintenance of structure and integrity of cellular membranes in the form of phospholipids. Fatty acids of the w3 series are the predominant type present in fish tissue, with 20:5 w3 being the major fatty acid. Linolenic acid (18:3 w3) has been shown to be essential for some fish species.
Carbohydrates: Required as an energy source, carbohydrates can be supplied as cellulose in plant material. Cellulase activity have been shown in many fish species.
Vitamins: Vitamins are required in the various metabolic processes and function as coenzymes. The vitamins required include riboflavin (B2), paraamino-benzoic acid (PABA), inositol, niacin, thiamine (B1), folic acid, pyridoxine (B6), pantothenic acid, cyanocobalamin (B12), biotin, choline, ascorbic acid (vitamin C), vitamin A and a-tocopherol (vitamin E).
Minerals: Minerals may be obtained from the surrounding medium. Minerals required include calcium, phosphorus, magnesium, sodium, potassium, sulphur, chloride, iron, copper, cobalt, iodine, manganese, zinc, molybdenum, selenium and flourine.
Nutritional Quality of Algae: Of the unorthodox feed sources, algae appear to have most potential for development as an alternative to fish-meal and soybean meal. Table 2 gives the gross chemical composition of some algal species.
Algae is a nutritionally-good fish food. Besides the high levels of protein, lipids and carbohydrates, it contains appreciable amounts of valuable vitamins. The amino-acid profile of selected algae in given in Table 3.
Table 2. Chemical composition (% of dry matter) of selected algae.
|Spirulina platensis||46 – 50||4 – 9||8 – 14|
|Spirulina maxima||60 – 71||6 – 7||13 – 16|
|Chlorella vulgaris||51 – 58||14 – 22||12 – 17|
|Scenedesmus obliquus||50 – 56||12 – 14||10 – 17|
|Euglena gracilis||39 – 61||14 – 20||14 – 18|
Table 3. Amino-acid profile of selected algae.
Algal lipids are usually esters of glycerol and fatty acids having C12 to C20. While different algal groups contain different lipids, the major components include triglycerides, sulphoquinovosyl diglyceride, monogalactosyl diglyceride, digalactosyl diglyceride, lecithin, phosphatidyl glycerol, and phosphatidyl inositol. The total lipid content in algae range from 1 to 40% of dry weight. Cyanophyta contains large amounts of polyunsaturated lipids, while other groups of algae contain saturated and monounsaturated fatty acids abundantly.
Besides these, algae contain pigments like chlorophylls and carotenoids, which make up to 5 % dry weight. B-carotene is a precursor of vitamin A and is commercially valuable as a colour enhancer for many species of fish.
The limitation to the use of algae as feed is the digestibility of the cell wall. For incorporation into artificial diet, processing of the algal biomass by drum-drying or freeze-drying can achieve digestibilities up to 90 % (Jauncey, 1982). Studies on the use of algal meals in artificial diets show that algae is the only vegetable protein source that can replace fish meal (Becker 1986).
The rate of photosynthesis in tropical fishponds is about 4gC/m2/day or 30t dry weight algae/ha/yr (Colman & Edwards, 1987). Assuming a feed conversion ration of 2:1 (dry algae to wet fish), the maximum fish yield is about 15t/ha/yr (Pullin, 1988). If the C:N:P ratio of algal cells is 50:10:1 by weight (Goldman, 1979), then to maintain the photosynthetic rate of 4 g C/m2/day in a 1 m deep pond would require daily inputs of 4 g C, 0.8 g N and 0.08 g P per m2 pond area/day.
ALGAL PRODUCTION IN A MANURE-FED POND
A manure-fed pond with a depth not exceeding 1 to 1.5 m functions as an oxidation pond or an algal pond if a high algal biomass (300 mg dry wt/1) exists. The role of the algae in the pond is recycling of the nutrients into algal biomass, thereby purifying the water, and releasing oxygen for use by the bacteria and other aquatic life. If fish production is an objective in the ponds, then the algae represent a food source and may be used to provide an optimal aquatic environment for fish growth.
The abundance of the algae is measured by its biomass or productivity. The productivity of the algae is dependent on 3 major factors:
(i) nutrients: The nutrients may be classified as major or macronutrients if required in large amounts and minor or micronutrients if required in lower concentrations. The macronutrients include carbon, nitrogen, oxygen, hydrogen and phosphorus, as well as calcium, magnesium, sulphur and potassium. In manure-fed ponds, the carbon comes from carbon dioxide released by the bacteria, as well as simple carbohydrates or organic acids which are used by heterotrophic algae. Often carbon is not a limiting factor. In water CO2 exists as H2CO3, HCO3 or CO3 -2 depending on the pH. This forms the basis of a buffer system. Algal growth is inhibited at pH 10.
Nitrogen in the form of nitrates are preferred although ammonium ions are also used. Depending on pH, the NH4 <---> NH3 equilibrium may be shifted to the right causing toxicity to fish. When ammonium is utilized the pH falls, and in a manurefed pond this functions to counter the increase in pH when CO2 is utilized. Urea and amino-acids are also metabolized by algae.
Oxygen comes from the air as well as from algal photosynthesis. Supersaturation resulting from active photosynthesis can cause “photooxidative death” of the algae, which is the lethal response of algae to light in the presence of oxygen. Very high oxygen concentration also inhibits photosynthesis.
Micronutrients include many of the minerals, vitamins and growth regulators.
(ii) light: Light may be a limiting factor where the total solids of the waste input into the ponds is high. As the algae grow, mutual shading may also result in light reduction. In general the light conversion efficiency of algae is between 6 to 8 %; and
(iii) pH and temperature: Different algae have different pH optima. Blue-green algae and desmids prefer alkaline conditions while green algae like Chlamydomonas sp. may prefer more acidic conditions. Chlorella sp. can tolerate pH as low as 4. pH affects the metabolic and other physiological processes as well as the availability of nutrients. Temperature interacts with light to influence the growth rate and productivity of algae.
Measurement of algal growth and productivity
Microalgal growth occurs in a logarithmic manner and the growth curve should be presented on a semilogarithmic scale.
In the pond, algal biomass may be determined as cell numbers, chlorophyll content, amount of protein, DNA or dry weight. At equilibrium or balanced growth,
dx = udt
where dx/dt is the population growth rate and u is the specific growth rate. Upon integration, we get,
In x/x0 = ut
where x0 is the initial weight. When x = 2x,
In 2 = ut2
and t2 = In 2/u = 0.693/u, where t2 is the doubling time.
Some useful equations for calculating algal concentrations, yields and productivity are:
i) chlorophyll-a content (mg/l) × 67
= dry weight of algae mg/l,
assuming that chlorophyll-a makes up 1.49 of dry weight of algae;
Assuming visible solar radiation = 0.45 total solar radiation, efficiency of solar use = 0.1, and unit heat of combustion of algae = 5.5 k cal/g;
where P = algal productivity (g dry wt/m2/day),
Is = saturation light intensity for algal growth (cal/cm2/day), range 0.02 – 0.06 cal/cm2/min.
and It = total solar radiation (cal/cm2/day);
(v) Algal productivity can also be calculated based on nutrients. Based on the stoichiometry of algal production;
122 CO2 + 16 NH4+ + PO33- + 58 H2O ------> C122H179O44N16P + 131 O2 + H+
algae are made up of 56.3% carbon, 8.6% nitrogen and 1.2% phosphorus by weight;
where C = dry weight of algae (mg/l), V = pond volume (litre), T = detention time (day), and A = surface area of pond (m2); and
(vii) The synthesis of 1 g of algae on waste produces 1.6 g of oxygen.
CASE STUDY: GOAT-FISH AND DUCK-FISH INTEGRATED PONDS AT THE IAS
Features of the goat-fish (Pond 1) and duck-fish (Pond 2) ponds at the IAS, University of Malaya are presented in Table 4.
The average algal yield in Pond 2 (43.55 mg/l) was higher that in Pond 1 (11.39 mg/l) (Geeta et al., 1991). Fish production in Pond 2 (4.54 t/ha for 9 months) was also higher than Pond 1(2. 12 t/ha for 9 months). The higher fish production in the duckfish pond may be attributed to the higher algal biomass as natural food and better water quality (Table 5).
In Pond 2, positive correlations were found between ammonia-N level and abundance of blue-green algae (r = 0.8510, P ≤ 0.0001) and orthophosphate level and abundance of euglenophytes (r < 0.7873, P ≤ 0.0005). This suggest an influence of chemical factors on the algal species composition, which may in turn affect fish growth.
In examining the growth potential of algae in the ponds based on the C, N and P contents, it was found that the algal productivity in Pond 1 was much below the theoretical based on the nutrients (Table 6). There was better conversion of nutrient from Pond 2 into algal biomass.
Table 4. Characteristics of integrated ponds at IAS, University of Malaya.
|Surface area, m2||1605.8||1625.8|
|Livestock stocking density, number||18||160|
|Fish stocking density, fingerlings/ha||3750||3750|
|Manure production, g dry wt/animal/day||420.3||15.5|
|%N in manure||1.00||1.21|
|%P in manure||0.47||0.52|
|Manure Loading rate per unit area in terms of:|
In Pond 1, the major nutrient don't appear to be limiting while in Pond 2 carbon and nitrogen appear to be limiting factors. The theoretical algal yields based on total solar radiation were calculated. Results show that the actual algal yields in Ponds 1 (11.39 mg/l) and 2 (43.55 mg/l) were far below the theoretical values (1674.9 mg/l for Pond 1 and 1036.9 mg/l for Pond 2). Inspite of this, light may be a limiting factor in algal production because of light attenuation by the high total solids in the ponds and through mutual shading by the algal cells (Phang 1990).
Algae form a very important component in a livestock-fish integrated system. Besides providing a major source of natural food, algae convert the polluting organic manure into useful biomass resulting in adequate dissolved oxygen and acceptable water quality for fish culture.
From the research work on livestock-fish integrated systems at the IAS, University of Malaya, the role and importance of algae in integrated farming system include:
Duck manure with a C:N ratio of 7.2 offers a better balanced substrate for algal growth than goat manure with a C:N ratio of 14.8. Goat manure is better anaerobically digested prior to discharge into the ponds.
Algal productivities in the ponds (0.14 – 0.70 g C/m2/hr for Pond 1 and 0.12 – 0.90 g C /m2/hr for Pond 2) were comparable to reported values, although they are low.
Algal biomass is positively correlated to dissolved oxygen levels and fish yields.
Increase in algal biomass (11 to 44 mg/l) in the ponds may result in increase in fish production, without causing water quality problems. However the optimum algal productivity in the system cannot be determined without further studies.
The algal flora was dominated by members of the Euglenophyta (Euglena, Phacus and Trachelomonas spp.) and Chlorophyta (Chlorella, Scenedesmus and Ankistrodesmus spp.). These genera are tolerant of organic conditions, many of them are heterotrophic. The effect of algal species composition on fish production is not fully studied.
Table 5. Average water quality of Ponds 1 and 2.
|Parameter||Pond 1||Pond 2|
|Semi-diurnal surface DO, mg/l||12 - 11.5||0.5 - 20.0|
Table 6. Theoretical and actual algal productivity in Ponds 1 and 2.
|Pond||Growth potential based on||Actual PGB|
The livestock-fish integrated system has great potential for production of protein-rich food source, especially in agricultural countries where it would be most beneficial to farmers already engaged in livestock farming. To fully exploit the integrated farming system a better understanding of the pond dynamics (including productivities of the various components in the pond like the bacteria and algae, and their contribution to fish nutrition; the role of algae in nutrient dynamics, especially with regards to removal of carbon, nitrogen and phosphorus and dissolved oxygen concentrations) should be a priority. Studies to elucidate the relationship between organic loading rate and algal production and pond water quality, with differing fish stocking densities are also recommended.
Alabaster, J.S. & R.Lloyd (1980). Water quality criteria for freshwater fish. FAO Publication, 297 pp.
Balarin, J.D. & R.D. Haller (1982). The intensive culture of tilapia in tanks, raceways and cages. In: Recent Advances in Aquaculture. Muir, J.F. & .J. Roberts (eds), Croom Helm, London & Canberra, pp. 266–355.
Becker, E.W. (1986). Nutritional Properties of Microalgae: Potentials and Constraints. In: CRC Handbook of Microalgal Mass Culture, Ed. by Richmond, A. pp 339–420.
Chattopadhyay, G.N., P.K. Saha, A. Ghosh & H.C. Karmakar (1988). A study on optimum BOD levels for fish culture in wastewater ponds. Biological Wastes 25: 79–85.
Colman, J.A. & P. Edwards (1987). Feeding pathways and environmental constraints in waste-fed aquaculture: balance and optimization. In: Detritus and microbial ecology in aquaculture, Moriarty, D.J.W. & R.S.V. Pullin (eds), ICLARM Conference Proceedings 14, pp. 240–281.
Edwards, P., R.S.V. Pullin & J.A. Gartner (1988). Research and Education for the Development of Integrated Crop-Livestock-Fish Farming Systems in the Tropics. ICLARM. Studies and Reviews 16, ICLARM, Manila, Philippines, 53 pp.
Geeta, S., T.K. Mukherjee & S.M. Phang (1988). Some aspects of goat-fish and duck-fish farming. In: Proc. 11th Ann. Conf. M'sian Soc. Animal Prod, pp. 123–127.
Geeta, S., T.K. Mukherjee & S.M. Phang (1991). Productivity and water quality of experimental livestock-fish integrated ponds. In: Management and Utilization of Agricultural and Industrial Wastes, Goh S.H. et al. (eds).
Goldman, J.C. (1979). Outdoor algal mass cultures. II Photosynthetic yield limitations. Water Res. 13: 119–136.
Jauncey, K. (1982). Carp (Cyprinus carpio L.) Nutrition - A Review. In: Recent Advances in Aquaculture Muir, J.F. & R.J. Roberts. (eds), Croon Helm, London & Canberra, pp. 217–263.
Phang S.M. (1990). Algal Production from AgroIndustrial and Agricultural Wastes in Malaysia. Ambio 19: 415–418.
Pullin, R.S.V. (1987). General Discussion on detritus and microbial ecology in aquaculture. In: Detritus and microbial ecology in aquaculture, Moriarty, D.J.W. & R.S.V. Pullin (eds), ICLARM Conference Proceedings 14, pp. 368–381.
Rohani Ahmad (1991). A study of two integrated livestock-fish farming systems. Unpublished Master of Philosophy Thesis, Institute for Advanced Studies, University of Malaya, 179 pp.