Tilapia are sometimes known as "aquatic chicken", due to their high growth rates, adaptability to a wide range of environmental conditions, ability to grow and reproduce in captivity and feed on low trophic levels. As a result, these fishes have become excellent candidates for aquaculture, especially in tropical and subtropical regions. Indeed, tilapia culture has been expanding rapidly, and is now practiced in more than one hundred countries worldwide.
(Source: Department of Agriculture – Bureau of Agricultural Research, Date accessed 24 March 2014)
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Tilapia cage culture is growing tilapia in cages made of nylon nettings and bamboo frames that are floated, submerged or fixed at the bottom. It utilizes bodies of water such as dams, rivers, lakes, bays, reservoirs and coves. This is one of the effective technologies used in raising tilapia. It started out in 1974 in Sampaloc Lake and Laguna Bay and being practiced now in different regions like in Magat Dam Reservoir in Region II.
The following are the advantages of tilapia cage culture:
- easier handling, inventory and harvesting of fish
- better control of fish population
- efficient control of fish competitors and predators
- effective use of fish feeds
- reduced mortality
- high stocking rate
- total harvesting and swift or immediate return of investment
- less manpower requirement
- minimum supervision
The Magat Dam has water current circulation throughout the area that gives a continuous flushing of water inside the cages, making dissolved oxygen highly available to fish and wash out metabolites. Wind direction from northeast to southeast or vice versa prevails in the months of March to August. The prevailing winds augment the distribution of natural fish food within the dam.
Protection from winds and waves
Locate the site in waters protected from strong wind action and water currents caused by flush flood or heavy runoff. In the Magat Dam, there are few floating debris; the quantity increases as the wind changes its direction and force. However, this could be checked by providing floating bamboo barricades or wave breakers facing the direction of the wind.
Dissolved oxygen concentration
The ideal range of dissolved oxygen concentration on the water must be at least 3 ppm (parts per million). For tilapia, a lesser ppm is not considered lethal. However, growth and reproduction is greatly affected.
This is one factor that plays a major role in the growth of the fish stock. The suggested range is from 20°C to 30°C. The lethal temperature levels are 12°C and 42°C.
The fish-farmer should know the effect of thermal, biological and chemical pollutants to the fish stock which may come from domestic, industrial and agricultural sources. pH Level. To enhance a better growth, the recommended pH range is 6.8 to 8.0.
The site must be accessible to land and water transportation to facilitate bringing in of inputs and marketing of produce.
One social problem existing in any fishery establishment is poaching. Poachers get into the project at night, bore hole on nettings to let the stock escape, then set gill net on surroundings of the project. This gives a bountiful catch overnight. This problem can be remedied by the management by establishing good public relation with the people in the vicinity. Another consideration is the source of fingerlings for periodic stocking.
Structural Design and Construction of Cages
There are two types of cage design — fixed and floating. The fixed cage is suitable with a water depth of 1 to 5 meters and the usual size is 50 to 200 square meters.The floating fish cage, on the other hand is from 5 meters deep and about 50 x 25 x 3 cubic meters depending on the area where the fish cage is placed. It is supported at the bottom with a stone weighing 40 to 100 kilos and covered with a net to prevent the fishes to escape.
The success of the project depends on the quality of breeds or species of fish as well as the production capability of the selected site in enhancing the maximum growth of the fish. Tilapia species is widely used as fish stock because it grows fast. It takes only four months for fingerlings to reach an average weight of 100 grams.
The design of fish cages is determined by the behavior of the culture species. For Tilapia nilotica, which is less active and sometimes territorial in habitat, the shape of the cage does not affect its mobility. In this case, design rectangular cages for easy assemblage and management. The arrangement of the cages is not a problem if there are only few of these. However, 8 or more should be arranged depending upon the direction of the wind.
There are many kinds of nets that could be used for cage fabrication. The most common are the B-net (1/4? mesh), DD-net (3/8? mesh) and CC-net (1/2# mesh). However, the most popular is the B-net because smaller fingerlings do not need a nursery cage. It is cheaper per unit area because it is wider (108 inches) than other nets, hence, labor cost in fabricating cages is much lower, and tearing of one or two meshes do not easily provide an escape route for bigger fish.
Generally, floating net fish cages are made of nylon nettings supported on all sides and corners with polyethelyne rope fixed by a nylon twine. Each is hung within a rectangular area, the top is supported by bamboo braces and the bottom is provided with lead sinkers. The size of net cages used in Magat Dam for commercial production of tilapia is 6 m deep, 6 m wide and 12 m long. This size makes possible the full utilization of bamboo poles and nets. To do it, hang the net cages in bamboo raft type frame which also serve as catwalk for workers allowing 1 m of the net above the water level and fix the synthetic ropes to four corners of the poles to prevent the fish from escaping by jumping out. The longest side of the cages is oriented perpendicular to the direction of the wind.
Construct the net fish cages in the following manner:
- Cut the net according to desired specification
- Double-lace every mesh of the four corners using nylon twine 210 d/6, double-twine beginning at the second mesh row using rolling hitch or clove hitch with a single hitch as lock at intervals of 7.62 - 10.16 cm.
- Double-lace the nylon salvage net to the top edges of the cage with a nylon twine, using either a rolling hitch or clove hitch with single hitch as lock. Start the second half from the second mesh row.
- Rig all sinkers (No. 7) to the rib lines of the bottom side and centers. Attach the rib lines on all sides using rolling or rib hitch with an interval of 7.62 - 10.16 cm.
- Make splices on the four corners of the hanging lines (top portion of net cage) for the attachment of four stretching ropes with weight. Continue with the other units following the same procedure.
Fish Stocking and Stocking Rate
Stock fish either early in the morning or late in the afternoon when the water is relatively cool. Acclimatize the fingerlings before stocking them. To do this, float bags of fingerlings in water where the fish is to be stocked, thereby allowing the water on the bags to float on the pond for 30 minutes. Determine the water temperature of both bag and pond waters with a thermometer. A difference of more than 30°C may cause eventual death of stock. Open the bags and introduce water gradually from the pond to the plastic bags until the temperature is almost the same. Let the fingerlings get out freely from the bags.
The number of fingerlings to be stocked in a cage varies from 10 - 15 pieces per cu m to reach a size of 100 g each in 150 days without supplementary feeding during summer months. Use the same density at the start of the rainy months up to early part of summer. However, the growth rate is lower during rainy months because the water is cooler and there is little presence of natural food in the water.
Management of Cages
Tilapia in cages requires minimal care and maintenance. Aside from occasional intrusion of predators, mechanical damage to the net screen and poaching, no serious problem can be expected.
Feeds and Feeding
Feeding of tilapia in cages is necessary for higher yield especially if productivity of the surrounding water is poor. Feed ingredients are locally available and economical. Supplementary feeds given to the fish using 3-5% body weight as basis for food ration per day. Divide the ration into 4 portions at an interval of 2 hours adjusted directly after sampling. Place feeds in small plastic bags. Weigh and label carefully according to the number of cages to avoid error when feeding.
Before the onset of predictable bad weather, loosen anchor ropes and using a banca, tow all the units towards a sheltered area. Put additional anchors to strengthen the whole project units.
Maintenance of Support Facilities
Support facilities refer to the service "banca" raft, caretakers hut and other facilities in the project. The "banca" should always be in tip-top condition since this is the only available means of transporting the produce and for management personnel to supervise the project.
Protect plastic and pandan bags, styrofoam boxes, spare nylon net cages, nylon twine and other equipment from rodents. Repair leaking roof of huts to give ease and comfort to the project personnel.
This is a scheduled monthly activity of grading the fish stock into different size groups to reduce the adverse effect of uneven growth and association of "size hierarchies" within the fish population. Smaller fish are harvested later when they reach the desired size and weight.
Most cage operators buy tilapia fingerlings of size 22 when high quality fingerlings are now readily available. These are not directly stocked in grow-out cages; instead, they are reared for about a month in cages with a mesh size of no. 24. After a month, grade the fingerlings and transfer the bigger ones in cages with mesh size no. 17. Repeat the process until they are stocked in cages with mesh size no .14 where they are reared up to marketable size. This technique of grading the fish, called modular method, enables the cage operator to stock and harvest the fish continuously.
Care and Maintenance of Cages
For daily and routine work, check loose twine and torn meshes of the nets and repair immediately by mending or patching. Remove by brushing bio-fouling organisms such as freshwater algae, sponges and debris that set on nylon net as soon as they detected. Replace the whole cage with spare net cage, when may fouling organisms had accumulated obstructing water exchange.
Check the net screen everyday for wear and tear as there might be possible damage that leads to the escape of the fish stock. Dive occasionally and inspect the condition of nettings and other materials submerged under water.
Before harvesting, prepare all the materials and equipment needed. Two to three persons can manage one fish cage.Untie the top corners of the net and allow the bamboo raft to enter the rectangular area. Two harvesters lift the net and push the raft toward an end of the cage. Scoop out the confined fish at one end of the net and place these in pails. Sort according to size. Place sorted fish in transport containers.
The need for proper handling and processing of tilapia is important both for the fishing industry and for the consumers.Improvement of the processing and handling of tilapia in terms of quality, product range and volume results in increased economic activity and employment. It is also one way of stabilizing fish marketing by providing an outlet for surplus and peak catch even during emergency harvest, thereby ensuring high fishing activities and stable prices. It can also contribute to the efforts related to nutritional goals.
The quality of fish depends on how it is handled from the time it is taken out from the water until it reaches the kitchen. Fish landed is usually subjected to rough handling treatments. Consider these 3 cardinal rules in handling fresh fish:
1. Cleanliness. Observe cleanliness throughout the fish handling chain. Clean the deck, checkboards and containers thoroughly before the fish haul comes aboard. Clear one haul away before the next ones come aboard. If you have to leave some fish on deck, move them at one side of the deck so that fresh fish are not placed on top of these.
2. Care. The fish you are handling is food, treat it as such.
- Work on fish as quickly and as promptly as possible.
- Sort fish properly before packing
- When fish have to wait on deck or on the fish landing for some time before working on these, cover these to protect them from heat and other elements.
- Drain fish before icing
- Avoid brushing the fish
- Don't throw, trample or kick the fish
3. Cooling. Temperature is the most important single factor affecting the quality of fish.
- Use plenty of ice. Put additional layer of ice on top, bottom and side of fish in boxes or shelves
- Don't over-fill a box or shelf. The next box or shelf on top will smash the fish below.
- Lay the fish belly downward - this prevent entry of dirt water into the fish.
- Don't pack fish so tightly that melted ice cannot flow.
Fish is cooled more quickly when ice cold water is poured on them. Fish spoils easily when allowed to stay in stagnant water, blood or slime. If the vessels is provide with fish room, store fish in ice as quickly as possible. Make sure the fish room is always kept clean.
Fresh fish transported to far distances must be packed with ice to ensure freshness when they reach the consumers. Proper packing of fresh fish with ice means arranging the fish and ice alternately in the container to maintain 5°C chilling temperature is attained with the ratio of 1 kg of ice to 2 kg of fish. Fresh fish may be transported in styrofoam boxes, conical tubes (bañera) basket ("kaing"/"bayong") and plastic containers.
The more sophisticated method is the refrigerated truck.
When transporting tilapia within the region, wholesalers pack them in ice. Upon reaching their destination, fish are repacked with ice and sold to retailers and eventually to consumers.
The success of freshwater fishpond farming depends on the selection of ideal fishpond site, proper planning and layout design, proper construction and appropriate pond management.
Considering the expenses involved in pond construction, freshwater fishponds smaller than half a hectare are not commercially viable. This technoguide is designed for freshwater fishponds with an area of one-half hectare or more.
Water supply is the foremost factor to consider in selecting a fishpond site. The site must be accessible to adequate water supply throughout the year and free from pesticide contamination and pollution. Sources of water can be a surface runoff, stream, creek or irrigation.
Clay, clay loam, and sandy loam soils with deposits of organic matter of about 16% are best for fishponds. Hard mud of the above types are preferable to the soft and very loose kind. Avoid sandy, rocky or stony soils because these do not retain water in the ponds. Choose flat terrains for easier excavation and leveling. If the topography is to undulating, the construction costs increase greatly and excavation work removes the fertile portion of the pond bottom. Avoid sites that are frequently flooded.
Other factors to consider
Availability of quality fingerlings and cheap skilled labor, accessibility to market and peace and order condition.
Design and Layout
In designing and planning the layout of freshwater fishponds, give careful consideration to the following:
- Pond compartments. There are three compartments in a complete freshwater fishpond system namely: nursery pond, brood pond and production or rearing pond. The nursery and brood ponds may comprise 10% of the total area, and 90% for the production pond.
- Water supply. Provide each compartment with an individual water supply system and drainage outlet. Provide also a mechanical emergency spillway for the flow of excess water from ordinary rain and to maintain desired water level in the pond.
- Drainage. Construct the pond to facilitate easy drainage when harvesting fish stock and proper cleaning of the pond bottom.
- Elevation. Construct the pond one meter or more lower than the source of water supply but slightly higher than the drainage area to obtain at least an average water depth of one meter for maximum production.
- Wind direction. Wind plays a role in fishpond design. Strong wind generates wave action that destroys the sides of the dikes. To minimize this, position the longer pond dimensions parallel to the direction of the prevailing wind to lessen the side length of the dike exposed to wave action.
- Protection from flood. If the fish pond site is prone to flooding, construct a diversion canal along the perimeter dike to divert runoff water during heavy downpour. Construct a larger and higher perimeter dike to prevent inflow of water.
Construct dikes with trapezoidal cross section with the top width, the side slopes and the height proportionally designed according to the soil material used. The following are guidelines in designing the dikes:
- Height above water line. Extend the top of the dike sufficiently above the water line to give a safe margin against overtopping during flood. Include margin for wave action caused by exposure to winds. Perimeter dike should have, after shrinkage, a freeboard height of 0.60 - 1.0 m above the maximum level observed in the locality. Freeboard for secondary dikes is 50 cm. The allowance for settlement and shrinkage depends on the characteristics of soil fill, soil foundation, and on the method of construction. On the average, an allowance for settlement and shrinkage is 25%. Provide a settlement allowance of not less than 40% for soils high in organic matter while dikes compacted by construction equipment is 5% less than the filled height.
- Top width. The minimum top width or crown is 1 m for dikes less than 3 m high. The top width of dikes used as access road is 4 m. Provide a 0.60 m wide berm or shoulder on each side of a roadway dike to prevent rovelling.
- Side Slope. The side slope or steepness of the dike is the ratio of the horizontal length to the vertical rise. Fishpond dikes lower than 3 m should have a slope of 1:1. Dikes above 3 m should adopt a 2:1 slope. Refer to the table below for relationship among the top width, bottom width and height of dikes.
Relationship among the top width, bottom width and height of dikes with a given side slope:
Construction of Pond System
Plan fishpond construction carefully and systematically. The system of pond construction is based on the prepared program and schedule of development.
Clear the dike site of vegetation, slumps and debris. Clear the strip 2-4 m wider than the base of the dike. For sites with decaying matters, construct a puddle trench at the center of the path of the dike. Excavate 0.5 m wide by 0.5 m deep trench filled with clay soil to prevent excessive seepage on the finished dike. Dig blocks of mud for construction of dike at least one meter from its base. Allow each layer to settle firmly before adding another layer until the desired height is attained. Construct dikes either manually, mechanically or both.
It is very important to have a uniform dike height. To do this, get a 50 m long transparent plastic hose. Fill the hose with water. Hold one end of the hose at the first station and the other end at the next 40 m away. If the water level at both ends are the same, the dike is level. Repeat the process until the last station has been marked.
Construct the canals simultaneously as the adjacent dikes. Stake markers to serve as guide during the excavation of canals. Slope the canal gently towards the drainage gate of pipe to keep the flow of water sluggish and to avoid excessive erosion.
Construction and Installation of Water Control Structures.
Water inlet or outlet structures are usually made of wood or concrete gates, galvanized iron sheets or reinforced concrete pipes.
Place 3 pairs of grooves on each side of wooden or concrete gates extending to the top of the dike where they are installed. The middle pair of grooves allows the removable slabs to regulate the flow of water. The first and third pairs enable the screens to prevent the escape of cultured fish. These screens may either be of bamboo splits or nylon attached to a wooden frame.
In freshwater fishponds, galvanized iron pipes or reinforced concrete pipes are often used instead of concrete wooden gates. The following is a guide in determining the proper pipe diameter to be installed.
Size of drain pipe in inches – Condition:
- 4 - Can drain 1 ha. pond with average depth of 1 m in 6 days
- 6 - Can drain the same in three days
- 12 - Can drain the same in one day
With proper scheduling of draining time, it is adequate to use 4 to 6 inch pipe for one hectare pond and 6 to 11 inch pipe for larger ponds. Construct water supply and drainage system simultaneously with the dikes.
Pond Bottom Leveling
Mechanical leveling is cheaper and faster than manual leveling if the pond bottom can support the equipment used. Use farm tractors or tillers with a back blade. The carabao and the harrow may be used in small ponds. The pond bottom should slope gently towards the drainage gate to facilitate complete drainage.
After leveling the pond, plant creeping grasses at the dikes to prevent erosion. Plant bananas at the outside slope of the perimeter dike to serve as wind breakers. Do not plant trees along the dikes because the roots will cause leakage and seepage.
Prepare the ponds a month before stocking fish in the following manner:
Draining and drying.
Drain and dry the pond completely. Dry for about a week or more, depending upon the weather, until the bottom cracks or harden sufficiently to support a man on his feet without sinking more than 1 cm. Make sure the pond soil is dried every time the pond is harvested. Periodic drying stabilize soil colloids and oxidizes organic matters that encourage the growth of natural fish foods. Draining and drying eradicate competitor fishes and predators, and kill disease-causing organisms.
Cultivation of pond bottom.
Till or cultivate the pond bottom as soon as it is drained. Do this by stirring or cultivating with a shovel or a rake for small ponds. For large ponds, use a rotavator. Cultivation makes sub-surface nutrients available at the surface for the growth of fish food in the pond, eradicate burrowing predators like mudfish and eliminate undesirable pond weeds like "aragan."
Level the pond bottom after this is cultivated. Leveling makes the pond bottom slope gradually from its farthest end down towards the drainage structures - the deepest portion of the pond.
Repairing gates and screens.
Check all gates and pipes for broken slabs and other parts. Repair screens to prevent predators and pests from entering the pond system. Clean to remove debris which may cause clogging.
Check all dikes for leakages and seepages. All dikes must be water-tight. Put a puddle trench excavated about 30 cm wide and 50 cm deep or more along the dike. Build this at the center of dike towards one side, or dig two puddle trenches at both sides of puddled trench long enough to cover the entire seepage and sufficiently deep to go beyond the general level of the pond floor. Fill the trench with new mud or soil. Allow the soil to settle well to give a firm line of earth.
Pests, Competition and Predator Control
Fish production in ponds is commonly affected by some pests and predators. Predators are organisms which prey on the cultured fish. Animals that compete for food or space are called competitors.
- Piscivorous or predatory fish and other competitors: Catfish (hito), mudfish (dalag) and gurami may enter ponds during floods or when accidentally stocked with the cultured fish. These predators devour fry and fingerlings during or after stocking. To avoid them, drain the pond totally after harvest or before stocking. Mudfish which tends to burrow into the mud, can be totally eliminated by using tobacco dust at the rate of 500 kg/ha. Screen water gates and outlets properly to prevent entry of unwanted fishes. Check fingerlings properly for any possible contamination by predatory fish prior to stocking. Competitors are associated with predators. Both compete with the stocked fish for space and food.
- Birds: Herons, kingfishers and other birds must be prevented from frequenting the ponds. They devour fish and fingerlings and are also carriers of parasites. Ponds constructed without shallow areas are not attractive to birds.
- Snakes: Snakes prey on small fish. Always keep banks and dikes clean to prevent snakes from harboring n the ponds.
- Frogs: Frogs eat fry and fingerlings. Tadpoles also compete with the fish for space and oxygen. Frogs are seldom found in well-fertilized and well-stocked ponds. Their population can be controlled by removing their egg sacks from the pond water.
Soil acidity limits the production of natural fish food by decreasing the amount of plant nutrients and, in some extreme cases, kill fish. In cases where soil pH is below 7.0, it is important to control acidity to ensure high fish production.
Analyze pond soil at least once a year to determine its exact pH value. Soil analysis is especially recommended for newly constructed fishponds as basis for proper soil conditioning. Refer to Appendix D for proper collection of soil sample.
Methods of controlling and correcting acidity
- Leaching. Wash or flush the pond bottom to reduce acidity. This process is effective in slightly acidic soil.
- Liming. Apply lime in fishponds primarily as a soil conditioner. Liming corrects soil acidity, promotes the release of soil nutrients, precipitates suspended materials which hamper light penetration and reduces incidence of fish diseases.
Agricultural lime (CaCO3) is the most common time used in fishponds. Unslaked lime or quicklime (CaO) and slaked lime (Ca COH)2 or hydrated lime may also be applied. These are available on arrangement with agricultural input dealers.
Broadcast or spread the needed lime over the drained but moist pond bottom. Mix the lime, thoroughly with the soil to attain maximum effectiveness. Allow one week to lapse before applying phosphate fertilizer.
Applying fertilizer in ponds to supply the nutrients needed for plant growth is a fundamental part of fishpond management. Fish production per unit area can be increased as much as five-fold by proper application of fertilizer. Fertilizers are classified into two groups:
1. Organic Fertilizer: The nutrients and organic matter content of manure increase the water holding capacity of the soil, decrease the rate of evaporation and increase enzymatic activity, all of which increase fertility and yield. Animal manures contain the major nutrient components such as nitrogen (N), phosphorous (P), and potassium (K), in addition to such trace elements as calcium (Ca), copper Cu) iron (Fe) and magnesium (Mg). Phosphorous comes mainly from feces except from swine manure which has more nitrogen and potassium. Animals fed with roughage ration excrete more potassium than those fed with high concentrate ratios.
The chemical composition of manure also varies depending upon the animals, nature and amount of manure and the handling and storage of the manure before use. The most common organic fertilizer used in fishponds are chicken dung, cattle manure and swine manure. Chicken manure may be utilized as fish feeds and at the same time helps create a soft mulch bottom to make a habitat for other food organisms. Compost, rice bran, and sewage may also be used.
2. Inorganic Fertilizer: These are chemical fertilizers containing concentrated amount of at least one of the three major plant nutrients: nitrogen, phosphorous, and potassium. The common fertilizers used in fishponds are Super phosphate (0-20-0), Monoammonium phosphate (16-20-0), and Diammonium phosphate (18-45-0).
Tilapia consumes plankton as food. Plankton is responsible for producing greater fish weight than any other type of natural food raised in ponds.
Stocking of Fingerlings
Sources of Fingerling
High quality fingerlings ensure better profit in tilapia culture. Inferior tilapia fingerlings grow slowly and may not reach the desired marketable size of 85-100 grams in 4-5 months culture period. Fish-farmers should secure their initial stocks from reliable sources.
Time and Method of Stocking
The best time for stocking fingerlings in the pond is late afternoon or early morning. Before the fingerlings are released, ensure even temperature between the water in the plastic bags and the pond where these are to be stocked. If the difference in the temperature is more than 3°C, introduce water gradually from the pond to the plastic bags until the temperature is almost the same.
To stock, bring the plastic bag of fingerlings to the pond preferably near the gate which is supposed to be the deepest part of the pond. Bring down each bag to the pond and tilt towards one side to allow the pond water to flow gently into the bag. Allow the fish to swim out of the container voluntarily. One oxygenated plastic bag contains 300 fingerlings.
Fish Stocking Density
Fish stocking ratio is one of the several factors that affects fish growth. At low stocking density, the amount of natural food in the pond is higher for each individual fish and the excess food is not utilized. As long as other factors are not limiting, the growth of fish is not utilized. As long as other factors are not limiting, the growth of fish will be better. The maximum physiological growth of tilapia is attained at low stocking density.
A one-hectare pond with plankton can accommodate 10,000 to 20,000 fingerlings of nile tilapia measuring 3-4 cm. Supplemental feeding is necessary to have better produce.
The stocking density influence the inputs needed and yield of the fishpond. Failure to select the most appropriate stocking density results in poor growth and low market value of fish.
The water should be free from toxic chemical contamination and unwanted predatory or wild fishes and must be available when needed. Employ precautionary measures when using water from rivers, streams and communal irrigation systems.
Maintain water depth from 70 to 100 cm to satisfy fish requirement for space and oxygen and to prevent over-heating of water during hot weather. Early breeding of tilapia results when water temperature highly fluctuates, as in the case of rice-fish paddies where tilapia are observed to breed earlier. To discourage early reproduction and to increase the growth rate of tilapia, maintain water depth at one meter in grow-out pond.
Employ some management modifications when water supply is seasonal. When using rainwater or irrigation water with limited flow, it is necessary to increase the volume of pond water by increasing depth if possible, store enough water in the pond during the rainy days.
Tilapia nilotica can tolerate water temperature range of 14° - 42°C. However, for culture purposes, the ideal water temperature should range from 25°C to 30°C.
Hydrogen ion (pH) Concentration
The ideal pH range of freshwater culture is 6.5 - 9.0. The effect of the various pH value on fish is shown below:
Measure the pH with the use of litmus paper, pH comparator, portable pH meters or the Hach kit. In the absence of any of these equipment, tasting the water is a practical way to determine the pH. The water is acidic if it tastes sour and it tastes bitter, if it is alkaline.
Acidic water usually comes from swamps, bogs or water in stagnant areas. Liming and correct water management corrects pH in pond water.
This is a poisonous gas which evolves from the pond bottom as a result of decaying and decomposing organic matter. Its presence can be detected by a smell similar to that of a rotten hard boiled egg. Hydrogen sulfide in the pond causes mass mortality and small patches or hemorrhage in the gill region of the fish.
Eliminate hydrogen sulfide before stocking by draining and drying the pond for 1-2 weeks until the bottom cracks. If this is not possible, agitate the water with any gadget or by running pump-boats or introduce freshwater into the pond. Do not apply organic fertilizer until the smell disappears.
This is highly toxic to fish. the symptoms of ammonia toxicity in the fish are:
- spongy appearance of gill filaments
- presence of bloody gills
- excessive production of slime
- poor growth of fish
One of the most common causes of high ammonia level in ponds is the heavy application of manure. Organic matters increase the ammonia level during decomposition and overgrowth of plankton.
Tilapia species can grow well at dissolved oxygen level of 1 - 3 ppm.
Some causes of oxygen deficiency in ponds are plankton bloom, decaying or dead fish, and decomposed organic matter. Most prominent of these, however, is heavy application of organic fertilizer in ponds since decaying organic matter absorb oxygen and give off carbon dioxide.
Good water management prevents the occurrence of dissolved oxygen depletion. In order to maintain high dissolved oxygen level in the pond, do the following:
- Prevent the growth of unnecessary aquatic vegetation, such as "kangkong" over the pond surface.
- These plants shield the pond from sunlight and slow down photosynthetic activities of phytoplankton to produce oxygen.
- Follow the recommended stocking rate of the pond. Over-stocking leads to high oxygen consumption and possible oxygen depletion especially at night.
- Avoid giving excess feeds to the fish since unconsumed feeds pollute the pond water when they sink to the bottom and decay.
- Apply only the recommended fertilizer rates. Putting more than enough organic fertilizers result in dissolved oxygen depletion as a consequence of decomposing organic matter.
Turbidity can be either an advantage or a disadvantage in fish culture. It is advantageous if it is caused by plankton. However, if the water is turbid due to minute solid particles, then this becomes a disadvantage because the sediment particles prevent photosynthesis.
To solve turbidity problem, spread about 2-3 ton/ha or 200 grams/sq m of rice stalks or chopped hay on the pond bottom.
The simplest way to measure water transparency is by using a Secchi disc or one's hand. A Secchi disc is a white and black disc (about 30 cm in diameter) suspended from a calibrated rope (usually in centimeters) into the water. If the disc appears at the depth of 30-35 cm, the water is not turbid, but if it disappears within a depth less than 30 cm, the water is turbid. With the right arm stretch forward, slowly dip your hand into the water until the palm becomes invisible. Water transparency is expressed by the distance (cm) from the wet wrist to the end of the water mark on the arm.
For better regulation of fish density in ponds, employ harvesting methods that can efficiently remove most of the fish. A small number of fish left in the pond after harvest may be caught during the next harvest to allow the fish to grow larger. However, too many fish left in the pond may affect the growth rate of the fish stocked in the next production cycle. Harvest only sufficient number of stocks while allowing enough space for the remaining fish to grow.
Methods of Harvesting
- Thinning. Start harvesting partially in the later part of the growing season. Wild spawning normally occurs in this part of the culture period. When the fish reach maturity, thin the bigger fish in the pond to allow growth of the remaining fish stock. If thinning is done for marketing only, use a net that can catch the desired size of fish. In tilapia culture, thin only once. Harvest the fish totally one to two months after thinning.
- Seining. Although seining is often recommended in harvesting fish in the pond, it is not very effective in ensuring total harvesting of the stock. Tilapia often burrow themselves into the mud to escape from the net.
- Draining. Drain the pond to the half-level mark the night before harvesting. Catch larger fish with a 1? mesh sieve and place in a drum, suspension net or "hapa" or large buckets filled with clean water to wash away mud. To keep tilapia alive indefinitely, place these in cages or net enclosures in a pond with clear water. In case the fish has an earthy smell or taste, hold them for about two days in separate pond with flowing water to improve their taste.
- Catch the remaining fish by lowering the water level using a fine mesh sieve to collect the fingerlings. Transfer and keep the fingerlings alive in suspended net enclosures (hapa) installed in another pond. Do not overstock the fingerlings in the holding units to prevent heavy mortality. Sell or use these fingerlings for future stocking.
- Harvest only the exact amount of stocks that can be absorbed by the consumers at the specific time. There is risk in harvesting the stock in bulk without any formal or closed arrangement in the market.
- To eliminate undesirable predatory species and competitors, drain the pond completely. If needed, spray pesticides for total elimination of predators left. Expose the pond bottom to sunlight to increase its fertility.
Source: Department of Agriculture – Bureau of Agricultural Research, Date accessed 24 March 2014