To achieve greater income from tilapia culture, consider the following:
- Proper water management
- Use of the right kind and amount of supplemental feeds.
- Application of the right kind and amount of fertilizer at the right time.
- Elimination and control of weeds.
- Use of appropriate fish stocking density.
- Preventing of entry and control of pests, predators and competitors
- Correct harvesting techniques
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. A simple bio-assay can be done using of these techniques:
- Place two or three fish in a pailful of water from the source to be tested for at least on-half day.
- Place “hapa” or scoop net with three or five fish 25 to 50 meters upstream of the water source.
The water is safe to use when the fish remain alive after half a day. Ocular inspection or nasal testing to determine the presence of toxic substance is generally effective.
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.
Water Quality. Water quality is one of the most important factors in ensuring a healthy fish production. Water quality in aquatic environment considerably affects the growth and reproduction of fishes. If the water quality is beyond tolerable limits, fish health is adversely affected.
Water Temperature. All fishes have upper and lower temperature tolerance limits. When the temperature goes beyond the temperature tolerance limit of the fish, the effects are:
- abnormal metabolism resulting in poor growth,
- abnormal activities or stress poor response to supplemental feeds
- very poor feed conversion ratio
An ordinary thermometer or an equivalent device can be used to measure water temperature. 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 pH of the water determines its acidity or alkalinity. If the pH is between 0-6.0, the water is acidic, and if the pH is 7.0, it is neutral. Alkaline water has a pH of 8.0 -14. The ideal pH range of freshwater culture is 6.5 – 9.0. The effect of the various pH value on fish is shown below:
|4 – below
4 – 5
5 – 6.5
6.5 – 9.0
11 – above
|Fish die because of acidity
Slow growth because of low fish food production
Fish thrive well and grow fast
Fish die because of alkalinity
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.
Hydrogen Sulfide. 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.
Ammonia. 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.
Dissolved Oxygen. All fishes, regardless of species and culture conditions, need oxygen for growth and survival. Lack of oxygen results in poor growth and outbreak of diseases or mortality. Generally, most warm water species need dissolved oxygen at a level of one part per million (ppm) for survival and about 3 ppm for comfort. Dissolved oxygen of 5 ppm is the most ideal for growth and is excellent in maintaining fish health. However, 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.
Some of the signs manifested by fish due to dissolved oxygen depletion in ponds are:
- Restlessness associated with abnormal closing or opening of the operculum.
- Gasping for air at the water surface
- The pond water becomes brownish to grayish and smells pungent
Turbidity. The presence or suspended solids in the water causes turbidity and muddiness of the water. Generally, suspended solids include sediment particles, organic matter like detritus, fecal materials and phytoplankton. 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. Turbid water has the following effects:
- suspended solids clog the gills of the fish or cause irritation on the gill filaments.
- heavy minute particles can bury eggs and larvae of organisms that live in the pond bottom.
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.
Supplemental feeds increase available food for fish and are useful for fattening. During persistent inclement weather, the natural fish feeds may become depleted. To maintain the growth of the stock, give supplemental feeds such as rice bran, coconut meal or chopped succulent vegetables singly or in mixture. Fine rice bran (D or D2) is the most common supplementary feeds. Feeding rate varies from 3 to 5% of the total body weight depending on the size of the fish. Younger fish consume more feeds at 5% of their body weight while fish weighing more than 50 grams consume 3% of their total fish weight.
To compute total fish weight in the pond, multiply the number of fingerlings by their average weight. For example: a pond stocked with 1,000 fingerlings weighing 20 gm each will have a total fish weight of 20 kg. With a feeding rate of 5%, the amount of feed to be given is one kilogram per day. Give one-half of the total ration of the day in the morning preferably between 6:30 – 7:30 and the other half between 4:30 – 5:30 pm.
Fertilize the pond to maintain the growth of plankton. Stop applying fertilizer two weeks before harvest. Do not apply fertilizer during inclement weather since fertilizers are not effective when there is no sunlight.
Aquatic Weed Control
Ponds with dense aquatic vegetation interfere with the production of fish. Weeds use up plant nutrients and occupy space intended for fish. They also make fish harvesting difficult. Examples of these weeds are “aragan”, water hyacinth, water lilies, cat tails and duck weeds. Most weed problems result from poor planning and poor pond management.
The following are the adverse effect of weeds:
- The productivity of the pond is influenced by photosynthesis. Light must penetrate sufficiently into the water to promote photosynthetic activity. Aquatic plants impede solar exposure of pond water.
- Aquatic weeds utilize soil nutrients that might otherwise support the growth of plankton.
- Weeds hinder effective grazing of stocked fish and expose them to predation by other aquatic animals. The presence of aquatic weeds also make harvesting and sampling of fish difficult.
Consider the following measures to reduce the weed population to manageable levels:
Biological control. Herbivorous fish such as the grass carp or T.Zillii can be stocked in the ponds to feed on aquatic plants. Stock grass carps at the rate of 100 to 220 fish per hectare. Proper and regular fertilization promotes and maintains the growth of plankton that shades and inhibits the growth of submerged aquatic weeds.
Mechanical control. Removing, dredging, and burning are some mechanical ways of controlling aquatic weeds. Tools that are commonly used to cut weeds are scythes and bolos. Early manual weeding is an effecting means of controlling weeds.
Consider the following preventive measures to avoid undesirable weed growth:
Pond construction. Construct ponds in a way that inhibits the formation of habitats favorable to the growth of weeds. Deepen pond edges to more than 50 cm to reduce the hazard of marginal and shallow water weed growth.
Pond fertilization. The effect of pond fertilization on weed control is indirect. Application of fertilizer induces and maintains the growth of phytoplanktons that shade the pond bottom. Hence, the growth of submerged weed is prevented.
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.
Prices of fish are largely controlled by supply and demand. Tilapia have to compete with other cultural fishes and the marine catch. Marketing trends show that tilapia commands higher price when sold during holidays, i.e., holy week, Christmas, fiestas and when supply of marine fish is low during bad weathers, full moon days and the northeast monsoon months from November to February.
To ensure the higher profits, prearrange the schedule of harvesting with fish dealers. Fish-farmers are also encouraged to form a marketing cooperative to increase their bargaining power.
source: STII and PCARRD