Table of Contents
List of Tables
List of Figures
1 Environmental Requirements
1.1.1 Water Temperature
1.1.3Disso lved O xygen
1.2 Nutritional Requirements
1.2.3 Essential Fatty Acids
2 Semi-Intensive System
2.1.1 Pond Fertilization
2.1.2 Periphyton Based Pond Culture
2.1.3 Supplemental Feeding
2.2 Intensive System
2.2.1 Intensive Tank Culture
2.2.2 Cage Culture
2.2.3 Greenwater Tank Culture
2.2.4 Recirculating Systems
2.2.5 Bio-Floc System
3 Integrated Tilapia Culture
3.1 Aquaponic System
3.2 Tilapia Polyculture
This literature review has been written to examine different tilapia farming practices both in semi-intensive and intensive systems. Extensive culture is not mentioned since it is not considered to be a real commercial production as the control over the system is quite limited and even semi-intensive system is being replaced by intensive system due to technological developments, high demand and increasing market prices of tilapia. In first chapter, environmental and nutritional requirements are also mentioned as they are closely correlated and play a key role in a successful production. The results of some recent studies and experiments suggest that tilapia has some superiority over other culture fish like faster growth, ability to utilize different feeds, wide tolerance for high stocking densities and environmental conditions. In addition to these advantages, tilapia do very well in integrated culture systems both with aquatic species; carp and shrimps, also crops like tomato and lettuce as well. As a result, this study is conducted to prove the advantages of commercial tilapia production covering economic values.
LIST OF TABLES
1.1 Essential amino acid requirements of tilapia as % of dietary protein
2.1 Comparison of effect of seasonal difference on fertilization process, in two different countries
2.2 Means of final individual weight, growth rates during the fertilization and feeding strategies, yield and feed conversion ratio for all-male Nile tilapia (30,000.ha-1) reared in fertilized ponds
2.3 Comparison of circular and rectangular tanks
2.4 Intensive cage culture of tilapia in some countries
2.5 Diurnal variation of some chemical parameters in the water of the greenwater fish-rearing tanks in the month of May 2002.
2.6 Water and Land Use per kg of Production of Tilapia and a Relative Comparison to an Intensive RAS Tilapia Farm ( RAS assumed to discharge 5% of system volume per day)
2.7 Technical comparison of two different tilapia culture recirculation systems
2.8 Recirculation system outputs
3.1 Fish Growth and Nitrate Removal for Fish-Only Systems and Aquaponic Systems
3.2 Yearly enterprise budgets for the tilapia production component of three model aquaponic farms having 6, 12 and 24 units
3.3 Yearly enterprise budgets for the lettuce production component of three model aquaponic farms having 6, 12 and 24 units
3.4 Enterprise budgets for three model aquaponic farms with 6, 12 or 24 tilapia and lettuce production units, and necessary infrastructure to support fingerling production, lettuce seedling production, water storage, land costs and general overhead.
3.5 Shrimp and tilapia stocking density in this experiment
3.6 The conversion rate of feed nitrogen (%) into shrimp, tilapia and waste of integrated closed recirculation system
3.7 Conversion rate of phosphorus (%) into shrimp, tilapia and waste of integrated closed recirculation system
3.8 Stocking ratios of Nile tilapia ( O. niloticus) and Carp (C. carpio)
3.9 Weight averages of Nile tilapia (O. niloticus) and Common carp (C. carpio) in monthly based weighing (g)
3.10 Length averages of Nile tilapia (O. niloticus) and Common carp (C. carpio) in monthly based measurements (cm)
3.11 At the end of trial, total yield and feed conversion ratios (FCR) of two groups
LIST OF FIGURES
2.1 Changes in fish yield and natural food supply in the pond, regarding to "critical standing crop" (CSC) and supplemental feeding
2.2 Nitrogen cycle in bio-floc ponds
3.1 Optimum arrangement of an aquaponic system
Tilapia is a freshwater fish belong to family "Chiclidae". Today, tilapia is a general name used for three genera; Tilapia, Sarotherodon and Oreochromis (Dikel, 2009). Tilapias are naturally distributed in many different areas include African lakes and rivers, Nile River, Palestine, Israel and Syria. Then they were introduced into many tropical, subtropical or temperate regions of the world due to their fast growth, distinct resistance to diseases, ease to breeding and high tolerance to even some severe conditions which cannot be tolerated by other culture species. Main reason of these introductions was production of cheap protein source by tilapia farming in rural areas to fight against poverty. With time, tilapia has become a popular fish in market with white flesh, good taste. Therefore, formerly used extensive culture system which was mainly depending on primary productivity has been replaced by semi-intensive and intensive culture systems.
Today, day by day increasing demand for tilapia, higher market prices and technological developments have encouraged producers for bigger investments. Tilapias' low levels in food chain and ability to utilize different feed sources, reasonable growth rate and great adaptation for culture environment have been the driving force for the expansion of the industry. Moreover, their tolerance for crowding stress and suitability for integrated culture systems are the other advantages.
Integrated systems serve to improve feeding efficiency and water quality due to complementary feeding behaviors of culture species and produce a secondary product to be offered for market as an additional value.
1 ENVIRONMENTAL REQUIREMENTS
Although tilapia is known to be one of the most tolerant culture species for unfavorable environmental conditions, they have some limits as all the other aquatic species do. In commercial tilapia production, due to economical concerns, maximized growth and feeding efficiency is desired. Hence, a great attention should be paid for all the environmental parameters, as they are closely correlated and highly affecting production yield. These parameters and their effects are explained below;
1.1.1 Water Temperature
Water Temperature: Intolerance of tilapia to low water temperatures is the most serious constraint for commercial tilapia culture. Even if water temperature is above the lethal limits and does not lead to direct mortality, this situation induces susceptibility for the fungus and infections occurrence. Tilapia cannot grow well below 16 C° and they cannot survive more than a few days below 10 C° (Tekelioglu, 2005). Preferred temperature values are between 20 and 35 C°, reproduction takes place at 25 C° to 36 C° and feeding activity ceases when water temperature is down to 16-17 C° (Lim and Webster, 2006).
Salinity: Although tilapias are well known examples of fresh water, some strains are euryhaline and able to tolerate high salinity values. It has been suggested that tilapia have marine origins and undergone an evolution (Beveridge and Mc Andrew, 2000).
However, there are some serious limitations for commercial tilapia production in saline waters. For instance, Oreochromis spilirus has been reported to have low fecundity (Al-Ahmed 2001). In addition, Oreochromis niltoicus x Oreochromis mossambicus hybrid has failed to adapt at 35%o (Alfredo and Hector, 2002).
1.1.3 Dissolved Oxygen
Dissolved Oxygen: It is a well-known fact that increasing water temperatures lead to reduction of dissolved oxygen rate in the water (El-Sayed, 2006).
However, tilapias are known with their high tolerance at low ambient oxygen levels (reviewed by Kutty, 1996). A test reported by Tsadik & Kutty (1987) suggested that specific growth rates (SGR) were closely correlated with dissolved oxygen levels and following specific growth rates (SGR) were found with varying oxygen levels;
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In this trial it was also indicated that feed conversion efficiency increased with increased dissolved oxygen saturation up to 90% (Bergheim, 2007).
pH: Tilapia show best growth in water that is close to neutral or slightly alkaline (Lim and Webster, 2006). It is well known that pH level in freshwater species rearing ponds ranges between pH6.5 - pH8.5. This level can be kept under control with carbonate-bicarbonate buffer system. During daytime, as a result of photosynthesis activity, CO2 level decreases and pH increases. In the nighttime, shift from photosynthesis to respiration, CO2 is released into water in form of carbonic acid and pH drops. Since tilapia are mainly found in the areas where the primary productivity is quite intense, they have adapted to withstand wide ranges of pH, between pH5-pH11 (Tekelioglu, 2005). Tilapia are able tolerate a wide range of pH from 3.7 to 11, but best growth is achieved between pH 7-9 (Ross, 2000) and growth is negatively affected in acidic waters (Lim and Webster, 2006).
Ammonia: It is the main form of the metabolic wastes excreted via gills and kidney of the fish. Excreted ammonia might be found in two different forms; unionized NH3 form (UIA-N) , which is toxic to fish and ionized NH4+ form, which is far less toxic (El-Sayed, 2006). Toxicity of ammonia is closely correlated with pH level and to some extent, water temperature and dissolved oxygen concentration (Lim and Webster, 2006). Low levels of dissolved oxygen (DO) elevates ammonia toxicity (Lim and Webster, 2006) and when pH level exceeds neutral value, an increasing portion of total ammonia is converted from the ionic form (NH4+) to the toxic un-ionized (NH3) form; toxicity tends to increase with the higher temperature (Soderberg, 1997). Tilapia mass mortality occurs in a few days just after their direct transfer to water that has ammonia concentrations higher than 2 mg. L-1 (Lim and Webster, 2006). On the other hand, extended (up to several weeks) exposure to un-ionized ammonia concentration above 1-mg. L-1 causes losses, particularly among fry and juveniles when the dissolved oxygen (DO) is low (Lim and Webster, 2006). Beside of mortality problems, un-ionized ammonia, even as low as 0.08 mg.L-1 may lead to poor appetite of tilapia (Popma and Masser, 1999).
Nitrite: It is toxic for fish since it immobilizes haemoglobin to carry more oxygen (£agiltay, 2006). First, ammonia is oxidized into nitrite (NO2) and then into nitrate (NO3) through the activities of nitrifying bacterias, which are grown on organic matters (El-Sayed, 2006). Fish size is effective on tolerance of the tilapia to nitrite. It was found that smaller tilapia (4.4 g) were more tolerant compared to larger ones (90.7 g) (Atwood et al 2001). However, chloride is reported to reduce the toxicity effect of NO2 (Yanbo et al., 2006). Therefore, chloride (Cl) level should be maintained in earthen ponds at a ratio of 10:1 (Cl: NO2) (Durborow et al., 1997). On the other hand, final product of ammonia oxidization, nitrate is relatively non-toxic to tilapia; however, long terms of exposure to high levels of nitrate may affect immunity and increase mortality rate (Plumb, 1997).
1.2 NUTRITIONAL REQUIREMENTS
Quite similarly to the environmental parameters, feeding has also great importance. Feeds comprise the most expensive input of a commercial tilapia farm. If the given feeds are far from meeting the nutritional demands of tilapia, this situation will result in reduced growth and yield, which is the worst scenario in commercial production. On the other hand, if an excess amount of feed is given, it will be quite costly and in addition, uneaten feeds will negatively affect the water quality and indirectly will lead to the same results.
Proteins are made of amino acids. Fish cannot synthesize some of these amino acids, thus they must be readily available in the diet. Tilapias require the same 10 essential amino acids as other fish species, terrestrial animals and humans as well. These amino acids are valine, arginine, histidine, threonine, lysine, isoleucine, methionine, phenylalanine, leucine and tryptophan (Lim and Webster, 2006).
Table 1.1: Essential amino acid requirements of tilapia as % of dietary protein: (Modified from Fagbenro, 2000)
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Although several other factors like salinity, water quality and temperature are affecting tilapia protein requirements, tilapia' protein requirements for protein in their diet tend to decrease with the increasing size, as many other fish species. While 20-30 % dietary protein is required for adult tilapias for optimum performance, for juvenile tilapias this value ranges between 30-40% (Gunasekera et al., 1996a, b; Siddiqui et al., 1998a, b; El-Sayed et al., 2003).
Lipids are known to have protein-sparing effect. It was showed that the level of protein in the diet of Nile tilapia (Oreochromis niloticus) can be reduced from 33.2 to 25.7 percent by increasing dietary lipid from 5.7 to 9.4 percent and carbohydrate from 31.9 to 36.9 percent (Li et al. 1991).
However, it has been reported that the dietary lipid level in excess of 12 percent depressed the growth of juvenile O aureus x O. niloticus hybrids and increased the accumulation of carcass lipid (Jauncey, 2000). In addition, excess levels of lipid may cause difficulties with feed pelleting process. However, extruded feed where fat is added after the pelleting process has eliminated this problem. Typical oil content of commercial tilapia feed is usually around 4-5%. (Orachunwon, Thammasarat, & Lohawatanakul, 2001)
1.2.3 ESSENTIAL FATTY ACIDS (EFA)
"More recently, reports have suggested that hybrid tilapia require both n-3 (omega-3) and n-6 omega-6) fatty acids and it has been proposed that diets for farmed tilapia should contain 0.5-1.0 % of both n-3 and n-6 PUFA". (Lim, Yildirim-Aksoy, & Klesius, 2011; Ng, 2005). Not only for meeting the nutritional demand of tilapias to support maximum growth, essential fatty acids are also important for final fatty acid content of tilapia fillets. Farmed tilapia, with enriched n-3 PUFA content, may present some significant health benefits to consumers such as; effects on cardiovascular system ( Lecerf, 2009; Russo 2009), autoimmune (Ruxton, Reed, Simpson, & Millington, 2007) and inflammatory disorders ( Calder, 2006).
Roughly, it is estimated that aqua feeds comprise 90% of the global supply of fish oil (FO) and due to expanding aquaculture industry; supply will imminently not meet the demand (Tacon and Metian, 2008; Turchini et al., 2009). Considering the high demand, shortage in supply and tremendously increasing prices of fish oil (FO), much research is conducted on finding suitable lipid sources as an alternative for fish oil (Turchini et al., 2009)
Although the vegetable oils are more cost effective compared to fish oil and always readily available, not much is known about their effects on tilapia production. However, several authors have reported some promising results. In a recent study, red hybrid tilapia was fed the crude palm oil (CPO) based diets from stocking to marketable size, they have figured out that the gonado -somatic index of both the female and male fish was much bigger compared to fish fed the fish oil based diet (Bahurmiz and Ng, 2007)
Fish do not have a specific requirement for carbohydrates, as they need lipids and proteins due to their several functions other than being energy sources. However, carbohydrates are added in fish diets because they have protein sparing effect, functional as pellet binders and serve as precursor for the formation of various metabolic intermediates required for growth (NRC 1993).
"It was reported that the protein sparing effect of carbohydrates (dextrin or starch) in hybrid tilapia (Oreochromis niloticus x Oreochromis aureus) only occured when the dietary protein level was suboptimal" (Shiau and Peng, 1993). It has been reported that feeding frequency affected the utilization of dietary carbohydrates by O. niloticus x O. aureus hybrids. "As feeding frequency increased from 2-6 times per day, so did carbohydrate utilization -especially of glucose although this was still much lower than for fish feed either starch or dextrin" (Beveridge and Mc Andrew, 2000). It is also demonstrated for O. niloticus x O. aureus hybrids, that larger fish utilized carbohydrate better than smaller ones (Tung and Shiau 1992, 1993).
Carbohydrates could have anti-nutritional factors in content, which may result in reduced utilization by fish. It was found that wheat bran, which contains protease inhibitor, might negatively affect food digestibility (El-Sayed et al., 2000).
In fertilized earthen ponds, tilapias are stocked from small to moderate densities to obtain required vitamins depending on natural food organisms (Shiau and Lin, 2006). Since natural food organism are limited or totally absent in intensive systems, required vitamins must be readily available in the formulated diets of tilapias (Shiau and Lin, 2006).
Deficiencies of vitamins are resulted in some specific problems, which are listed below;
- Vitamin B i (Thiamin): Thiamin level of 2.5 mg/kg of diet was reported to meet the demands for maximized growth (Lim et al., 1991). Vitamin B1 deficiency in red hybrid tilapia (Oreochromis mossambicus x Oreochromis niloticus) fingerlings, which are cultured in seawater showed, reduced growth, lower feed efficiency and low haematocrit (Shiau and Lin, 2006).
- Vitamin B 2: For juvenile Nile tilapia (Oreochromis aureus), vitamin B2 requirement was reported as 6 mg/kg of diet (Soliman and Wilson, 1992a). Reported deficiency signs were; anorexia, reduced growth, high mortality, fin erosion, abnormal body color, dwarfism and cataract (Shiau and Lin, 2006).
- Vitamin B 3 (Niacin): Two different optimum values have been reported depending on the diet used. These are 26-mg/kg for fish fed a glucose diet and 121-mg/kg for fish fed on a dextrin diet (Shiau and Lin, 2006). Deficiency symptoms of vitamin B3 were; hemorrhage, deformed snout, gill and skin oedema, fin and mouth lesions (Shiau and Suen, 1992).
- Vitamin B 5 (Pantothenic acid): 10 mg of vitamin B5/ kg of diet has been reported to be sufficient to maintain healthy status of Nile tilapia (Oreochromis niloticus) (Soliman and Wilson, 1992 b). Reported deficiency symptoms were; poor growth, hemorrhage, sluggishness, anemia, hyperplasia of cells of gill lamellae and increased mortality (Soliman and Wilson, 1992 b).
- Vitamin B 6 (Pyridoxine): For juvenile hybrid tilapia (O. niloticus x O. aureus) reared in freshwater, optimal dietary requirements were 1.7-9.5 mg/kg of diet containing 28% crude protein and 15.0-16.5 mg/kg of diet containing 36% protein (Shiau and Hsieh, 1997). Reported clinical deficiency signs were; low growth, high mortality, abnormal neurological signs, caudal fin erosion, mouth lesion and convulsions (Shiau and Lin, 2006).
- Vitamin B 7 (Biotin): For hybrid tilapia (O. niloticus x O. aureus) required vitamin B7 amount has been determined to be 0.06 mg/kg of the diet (Shiau and Chin, 1999). Deficiency symptoms include; poor growth, reduced hepatic pyruvate carboxylase and acetyl CoA carboxylase activities (Shiau and Chin, 1999).
- Vitamin B 9: Reported vitamin B9 requirement for Nile tilapia (Oreochromis niloticus) is 0.5 mg/kg of the diet (Lim and Klesius, 2001). Deficiency symptoms are; reduced feed efficiency and feed intake, poor growth (Lim and Klesius, 2001).
- Choline: Dietary requirement for hybrid tilapia (O. niloticus x O. aureus) was estimated to be 1,000 mg/kg of diet (Shiau and Lo, 2000). Specific symptoms for choline deficiency are; poor growth, reduced survival, reduced blood triglyceride and phospholipids concentrations (Shiau and Lo, 2000).
- Vitamin B 12: There is no reported specific requirement for vitamin B12 as it is produced in gastrointestinal tract of tilapia via bacterial synthesis (Shiau and Lung, 1993).
- Vitamin C: Reported requirement for hybrid tilapia (O. niloticus x O. aureus) is 19 mg/kg of the diet (Shiau and Hsu, 1999). Specific deficiency symptoms are; poor growth, lordosis, scoliosis, reduced feed efficiency, anemia, exopthalmia, hemorrhage, gill and opercular deformities (Shiau and Hsu, 1999).
- Vitamin A: For hybrid tilapia (O. niloticus x O. aureus), requirement is reported to be 5,850-6,970 IU /kg of the diet (Hu et al., 2006). Deficiency symptoms are; low growth, abnormal movements, restlessness, exopthalmia,pot belly syndrome, reduced mucous secretion, high mortality, haemorrhage (Shiau and Hwang, 1993).
- Vitamin D: It was reported that vitamin D is not essential for Oreochromis aureus (O' Connel and Gatlin, 1994). On the other hand, for hybrid tilapia (O.niloticus x O. aureus), suggested amount is 374.8 IU/kg of the diet (Shiau and Hwang, 1993).
- Vitamin E: For hybrid tilapia (O. niloticus x O. aureus), determined requirement is 42-44 mg/kg of the diet with 5% lipid content and 60-66 mg/kg of the diet with 12% lipid content (Shiau and Shiau, 2001). Specific deficiency symptoms are; anorexia, reduction in weight gain and feed efficiency, muscle degeneration, skin hemorrhage,ceroid in liver and spleen, and abnormally colored skin (Roem et al., 1990).
- Vitamin K: Estimated dietary requirement for hybrid tilapia (O.niloticus x O.aureus) is 5.2 mg/kg of the diet (Lee, 2003). Poor growth and low plasma prothrombin have been observed when tilapia was fed vitamin K free-diet during 8 weeks (Lee, 2003).
- Magnesium (Mg): For Nile tilapia dietary magnesium levels of 0.59-0.77 (Dabrowski et al., 1989) and for blue tilapia 0.5-0.65 (Reigh et al., 1991) have been reported to be sufficient. On the other hand, dietary magnesium deficient diets resulted in reduced growth, low tissue magnesium concentrations and abnormal tissue mineralization (Lim and Webster, 2006). In addition, excess amounts of magnesium (3.2 g /kg) when the dietary protein was suboptimal (24%) resulted in low hematocrit, hemoglobin and sluggishness, and depressed growth as well (Dabrowski et al., 1989).
- Manganese (Mn): 12 mg/kg of manganese is the recommended value for Nile tilapia (Watanabe et al., 1988). Lim and Webster (2006) reported that deficiency of manganese leads to specific problems like; reduced growth, anorexia, equilibrium loss and increased mortality.
- Zinc (Zn): Required level of dietary zinc for Nile tilapia has been reported as 30 mg/kg of the diet (Elhamid Eid and Ghonim, 1994).
- Potassium (K): Specific dietary requirement of K for optimized growth, gills Na+-K+ ATPase activity and K retention of hybrid tilapia (O.niloticus x O. aureus) was determined as 0.2-0.3 g / kg (Shiau and Hsieh, 2001).
- Calcium (Ca): O'Connell and Gatlin (1994) obtained best growth and high concentrations of minerals in bone and scale of blue tilapia that were reared in water with < 0.1 g Ca. L-1 and fed purified diets supplemented with 7.5 g (0.75%) Ca.kg-1.
- Iron (Fe): It has also been considered to be an important mineral in tilapia diet. It has been suggested that 150-160 mg/kg of diet from iron citrate meets the Fe demand of hybrid tilapia (O. niloticus x O. aureus) (Shiau and Su, 2003).
2 SEMI-INTENSIVE SYSTEM
Semi-intensive culture can be described as producing fish depending on either pond fertilization or supplemental feeding additional to the fertilization process. As a result of low inputs and low stocking densities in the system, low-cost fish is produced. Hence, a successful pond fertilization is a prerequisite in order to delay commercial feeding or totally eliminate it. Semi-intensive culture method is quite common for small scale producers in developing countries.
2.1.1 Pond Fertilization
Fertilizers can be defined as substances that are used in ponds to promote the primary productivity. These substances are divided into two groups; organic and inorganic fertilizers. Whereas organic fertilizers are natural and comprise various nutrients, inorganic fertilizers are man-made and comprise high amounts of one specific nutrient.
"The main objective of pond fertilization is to stimulate the primary productivity in fish ponds and enhance autotrophic and heterotrophic microbiological food production" (El-Sayed, 2006).
Nitrogen (N), phosphorus (P) and carbon (C ) are considered to be the major inputs of fertilization process (El-Sayed, 2006). In a fish pond, average nutrient composition of phytoplankton comprises 45-50 % C, 8-10 % N and 1% P, which gives a roughly ratio of 50:10:1 (Edwards et al., 2000).
Liming is also an important procedure that may serve to several improvements on water quality and productivity. These include; stabilization of pH at 7-8 , increase of phosphorus availability and CO2 amount in order to enhance photosynthetic activity. Most prominent liming materials are; quick lime (CaO), sloked lime (Ca (OH)2) and ( CaCO3).
Important criteria for a successful fertilization process can be summarized as below;
Characteristics of the pond: Pond structure should be known for a sustainable and efficient fertilization. As an example the more mud the bottom contains, it tends to absorb more phosphorus (P) (Shrestha and Lin, 1996 a, b). Hence, exact phosphorus (P) requirement for pond fertilization is determined by type of the bottom soils and their phosphorus (P) saturation (Knud-Hansen, 1992).
Type of manure used: Different animal manures like cow manure and chicken litter have been successfully used but their availability might be the limiting factor for use. However, for instance buffalo manure is not recommended for pond fertilization, since it causes drop in dissolved oxygen (DO) due to respiratory demands of bacterias (Edwards et al., 1994a). Also it was reported that only 6% of buffalo manure nitrogen was released as soluble, reactive phosphorus (P) (Shevgoor et al., 1994)
Season of the year: A study was conducted in Panama and Honduras. 10,000 Nile tilapia per hectare were stocked into the ponds and weekly fertilized with 1,000 kg.ha-1 chicken litter. 141 to 150 days production cycle was applied during the dry and rainy seasons in each country. Layer chicken litter was used in Honduras and it was composed of 88.9 % dry matter. In Panama, broiler chicken litter was used, which was composed of manure, rice hulls, feathers and waste feed. It was averaged 89.8% dry matter. As a result, although no seasonal significant differences were observed in Honduras, in Panama the yields for dry season were considerably greater. Better results of dry season might be linked to the decreased light penetration into the pond and high turbidity as a result of heavy rains (Green et al.1990).
Next table shows the yields obtained during similar culture periods in two different countries, both in rainy and dry seasons. Same fertilization procedure with chicken manure were applied and densities/ hectar were same for all the treatments
Table 2.1: Comparison of effect of seasonal difference on fertilization process, in two different countries (modified from El-Sayed, 2006)
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2.1.2 Periphyton-based Pond Culture
Periphyton is referred to organisms living attached on submerged materials or substrates (Van Dam et al., 2002). In a periphyton-based culture system, different rigid materials like bamboo poles or woody branches are fixed in shallow waters such as ponds or lagoons to enhance the growth of sessile aquatic biota known as "periphyton". "The periphyton community comprises of bacteria, fungi, protozoa, phytoplankton, zooplankton, benthic organisms and a wide range of invertebrates" (Milstein, 1997; Azim et al., 2001). Therefore, with the enrichment of natural productivity, such a system serves to provide natural food for fish have omnivorous or herbivorus feeding habits. Most of the tilapias have the ability to use phytoplankton and as well as periphyton (Dempster et al., 1993, 1995). It has been indicated that substrate type used and manuring process have a significant effect on periphyton productivity and fish production as well (Azim et al 2001 b). Recent studies showed that periphyton-based system is very applicable for tilapia culture. "It has been reported that 10 bamboo poles/m2 resulted in increase of fish yield 20% to 100% (Azim et al.,2001; Keshavanth et al; 2004 ; Milstein and Lev, 2004)". In another study it was found out that bamboo poles produced more and better periphyton compared to jute stick and branches of hizol tree (Azim et al., 2001). It has been found that rearing of blue tilapia on natural periphyton showed quite similar values in growth, survival and yield compared to fish fed on pelleted diets. Therefore, it is resulted in a significant reduction in feed costs (Milstein and Lev 2004).
2.1.3 Supplemental Feeding
After a proper fertilization, natural food supply can meet the demands in semiintensive systems, however, supplemental feeding is a necessity when larger fish cannot obtain enough nutrients from plankton alone and growth begins to slow down. This critical point is defined as "critical standing crop" (CSC) and there are several factors determine the time of "critical standing crop", such as stocking density of fish and fertilization.
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Figure 2.1: Changes in fish yield and natural food supply in the pond, regarding to "critical standing crop" (CSC) and supplemental feeding (Modified from De Silva, 1995)
Semi-intensive system is mainly practiced in developing countries and local market prices of tilapias are quite low. Therefore, the use of high-quality commercial feeds are not recommended in such systems due to economical concerns (Yakupitiyage, 1995)
In a study, formulated feed was replaced by chicken litter fertilization during the first 60 days periods of tilapia rearing without significant impacts in frame of yield or production economy. Sex-reversed Nile tilapia were stocked into ponds as 20,000 ha-1 with an average weight of 18.6 g. Tested pond management strategies were feed only (which includes 23 % crude protein), layer chicken litter only (1,000 kg. ha-1/week, on dry matter basis) for the first 60 days which followed by feed only (3% of fish biomass, daily basis), or layer chicken litter (500 kg.ha-1/ week, on dry matter basis) plus feed (1.5% of fish biomass on daily basis). Mechanical aeration and water exchange were not used during the 151-days trial period. Only insignificant differences were observed among treatments and the values were; 4,470,4,522 and 4,021 kg.ha-1 for feed-only, chicken litter and feed afterwards and chicken litter+ feed treatments , respectively (Green 1992). As a result, fish growth had slowed down in the chicken litter then feed treatment by day 61, which was an indicator of the exceeded critical standing crop (CSC), that means natural pond productivity was not sufficient to maintain rapid growth of tilapia (Green 1992; Green et al. 2002). It was found out that delayed provision of formulated feed in fertilized ponds until individual fish weights reach from 100 g up to 150 g for each one, may obtain an improved input utilization efficiency (Diana et al.,1996). In this study, sex-reversed Nile tilapia with an average weight of 15 grams were stocked into ponds at 30,000 ha-1. All the ponds were treated with urea (60 kg.ha-1) + triple superphosphate (34 kg.ha-1) combination as fertilizer every week. Formulated feed with 30% crude protein content was offered daily at 50% of the ad libitum rate once fish reached individual weights of 50, 100, 150, 200, 250 g. When each fish was weighed 500-600 g, ponds were harvested. Mechanical aeration and water exchange processes were not used. First feed offer was after 38, 80, 153, 178, or 234 days, respectively, for the 50, 100, 150, 200 or 250 g treatments. In all treatments, during the fertilizer only stage, tilapia growth rate average was 1.17 g.day-1, which was quite lower than 3.10 g.day-1 average growth rate reached during the feeding stage. By day 38, critical standing crop was reached and considerable growth rate increase was observed with the given formulated feed. Delayed initiation of feeding until fish reach 50 to 100 g, did not show any effect on growth, final size, and yield grow-out duration in compare to the other treatments.