Tilapia Raising

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Tilapia Raising

Tilapia was introduced into Thailand in the late nineties. It is raised using high quality commercial feeds and is aimed at a more consumer. Tilapia can be reared in tanks, cages or earthen ponds both in fresh and brackish water up to 25 ppt salinity. Red tilapia are more tolerant of high salinity and some strains can be raised in full strength sea water. Unlike most other fish species, tilapia have the ability to consume minute phytoplankton that they filter out of the water. For this reason, commercial pelleted feeds are not necessary for growing tilapia and nutrient-enriched water (“green water”), produced by the addition of animal manure or fertilizer, is sufficient to achieve a marketable fish of 300 to 500 grams in six months. Besides phytoplankton, tilapia will also eat zooplankton, detritus, aquatic plants, insects and even small fish fry. Commercial pellet, waste food and almost any other type of feed given, with perhaps the exception of meat, is also eagerly devoured. Very little investment is, therefore, required in their nutrition. The biggest drawback to the culture of tilapia is their highly precocious reproductive efficiency. This results in overcrowding, leading to long grow-out periods of up to a year and a harvest of small mixed sized fish with very little market value. To overcome this problem, Some Farm controls sexual development of young fry so that they all develop into male fish. This is done by feeding testosterone-impregnated feed for 21 days. After this period, the sex of the fish cannot be changed and the fish will be male for the rest of their lives. The fish are tested on a monthly basis by gonad squash method and are very close to 100% male. The benefit of this is a culture period of only six months, and a harvest of even sized, large, fat fish with high market value.
POND SIZE
There is no ideal pond size for growing tilapia and they vary from 1 to 100 rai (1 rai = 1,600 m2). Big ponds require less time in labour (fertilising, feeding, etc), but are more difficult to harvest and take longer to prepare and fill with water. Surface aeration and water flow in large ponds is generally better as the long fetch allows larger waves and currents to develop. Small ponds are more costly to excavate (per unit area), but are of advantage if selling fish directly toretailers, as a regular supply of fish in small amounts is required. The main thing to think about is the amount of production required per month and one can expect 600-800 kg per rai using fertilization only and up to a ton or more if feeds are used. Yields in excess of this are possible, but an aeration system and/or regular water exchange will be necessary to support the high biomass of fish. The bigger the production scale, the bigger the pond sizes should be. Tilapia ponds are mostly 5-20 rai in size (water area) and would typically produce 4 to 16 tons of fish per crop.

POND DEPTH
An ideal depth of pond is between 1 to 2 meters. Ponds deeper than this are not good because light, necessary for phytoplankton growth, will not penetrate to the bottom. This will lead to anaerobic conditions and poor water quality. More fertiliser will also be required to maintain a green pond. There is one exception to this rule and that concerns rain-fed ponds where a large volume of water is required to prevent the pond quickly drying out in the dry season. In this case it may be necessary to increase water depth during the rainy season to 3 meters or more. High fertilization will be necessary at this time, but during the dry season nutrients will tend to get concentrated, due the evaporating water, and so little fertilization will be necessary. Ponds shallower than 1 m are not recommended, because temperature fluctuation will be very high and production per area will be lower due to the reduced volume of water and lower overall biomass of phytoplankton.

POND PREPARATION

There are four important steps in pond preparation:

1) Eradicate all wild fish from the pond.

Eradication of any fish left from the previous culture cycle is imperative in order to ensure optimum growth of the new crop. The best way to do this is to dry the pond for 1-2 weeks. Not only will this kill any fish remaining, but it will also have a beneficial effect on the pond bottom. Application of a pisticide (such as rotenone, tea seed cake or cyanide) to any water remaining may be necessary during the rainy season when a pond cannot be dried.

2) Lime the pond bottom. After draining the pond, it is advisable to treat the pond bottom with lime. This is recommended practice in aquaculture, as it will kill disease organisms and will buffer fluctuations in pond water pH. 100 kg per rai is sufficient for most ponds, although more lime may be required in acid soil areas, particularly if the pond is new. 2) Fill the pond with filtered pond water. After drying and liming the pond it can now be filled with water. To do this, a filter bag, made from fine nylon netting, should be secured over the pump discharge pipe to ensure that no wild fish fry or eggs can get into the pond. This may not be possible for large pumps due to the high pressure. In this case, a hapa can be erected under the water inlet to catch any unwanted organisms. Unwanted breeding (due female tilapia getting into the pond) will be prevented if these procedures are followed and your all-male tilapia should grow to their full potential. 3) Add fertilizer to the pond to create green water. Farm recommends the addition of 30 kg of 16-20-0 fertilizer per rai to the newly filled pond to create a phytoplankton bloom. Animal manures can be used, but chemical fertilizer will create better water quality, thus ensuring higher survival of the newly stocked fish. A week is normally sufficient for the water to turn green, after which time fish can be stocked.

FRY TRANSPORT

The sex reversed tilapia fry you have purchased from  Farm have been starved prior to packing. This will ensure that the water in the bags stays relatively clean and the fish should survive for 18 hours without any significant mortality. Farm asks all customers to follow the following set of guidelines with respect to fry transport:

1) Please order and confirm in advance, as this will enable staff to starve the fish for the optimum period prior to packing.

2) Arrange a time to pick up the fish and arrive on time. staff will attempt to finish packing the fish at the time arranged. Not only will this minimize your wait, but it will also reduce transport time.

3) Try and avoid traveling long distances during the day in April and May due to the extreme heat.

4) If traveling during the day, then cover the bags with wet sacking to keep the temperature down. If a significant number of fish die in the bags during transport, then please inform Farm sales manager.
FRY RELEASE

There are two main considerations concerning releasing fish:

1) Avoid stocking fry when oxygen levels are low in the pond. This will be in the early morning at first light. Use of aeration is an advantage to elevate oxygen levels to give the fry the best chance of survival.

2) Care should be taken when stocking your fish that the water temperature in the bags is not very different to that in the pond. If it is, then the fish will suffer shock on contact with the water. The recommended way is to float the bags in the pond for a period of 15 minutes prior to stocking, so that temperature in and outside the bags can equilibrate. Release the fish by pulling the neck of the bag to snap off the elastic band, then hold the bag upside down and discharge the whole of the contents into the pond.

STOCKING DENSITY

Stocking density depends entirely on the size and expected yield of fish (determined by method of culture) at harvest. For example, if a farmer has a 10 rai pond and intends to use fertilization and a little supplemental feeding using cheap feedstuffs, he could expect 800 kg of fish per rai (1,600 m2). If the farmer intends to stock 2” fish and requires a target weight of 500g per fish, then he can calculate the number of fish to stock from the following equation: No. of fish to stock = ( (Y x A) / (S / 1,000) ) x M Where: Y = Yield per rai expected (kg) A = Area of pond (water only, rai) S = Size of fish required (g) M = Mortality coefficient = (100 / estimated survival). The following estimated values could be used: Mortality coefficient
Size of fish Nile tilapia Red tilapia
1″ or 0.2g 1.67 1.92
2″ or 2 g 1.39 1.52
3″ or 10g 1.27 1.32
4″ or 30g 1.18 1.20
In the case above = ( (800 x 10) / (500 / 1,000) ) x 1.39 = (8,000 / 0.5) x 1.39 = 22,240 fish recommends a grow-out period of no more than 7 months, as production decreases dramatically after a period of 6 months. At a stocking density of 2-3 fish per square meter, market-sized fish (300 – 400g) can be attained in six months using fertilization (addition of manure, fertilizer, etc) only. At stocking densities in excess of this, supplementary feeding will be necessary to get the fish to size within the recommended grow-out period. If large sized fish are required, then reduce the stocking density appropriately. Ultimately, the stocking density and method of culture used will depend on economics. As stocking density increases, more investment is required in feed, and production cost will increase. As a general rule, 3,000 x 1 inch fish (0.2g) or 1,600 x 4 inch fish (30g) per rai is an ideal stocking density for fertilized and/or supplemental fed tilapia ponds in most. Higher stocking density and investment in aeration would be an advantage in areas where tilapia prices are high (fish sales price should be at least double the cost of good quality commercial pellet per kg). Red tilapia, for example, fetch a high market price and can be raised at 2-3 fish per m2 to 700g using commercial pellet to attain 2-3 tons per rai. This is not to say it is the most profitable way to do it, as F.C.R. will be higher (more feed required per kg of tilapia produced) and there will be costs associated with aeration and water exchange. A farmer who produces 2 tons of tilapia per rai and makes per kg profit will be no better off than a farmer who produces 1 ton of tilapia per rai and makes /kg profit. The following table can be used as a guide to determining stocking density: Number of fish stocked per rai
Rearing technique
Yield (kg/rai) 1″ 2″ 3″ 4″
Fertilization only 600-800 3,200 2,700 2,400 2,250
Fertilization only 600-800 2,400 2,000 1,800 1,700
Fertilization, supplemental cheap feed 800-1,200 4,600 3,800 3,400 3,200
Fertilization, supplemental cheap feed 800-1,200 3,400 2,850 2,600 2,450
Complete feed, no aeration, low density 1,000 2,550 2,150 1,900 1,800
Complete feed, aeration, high density 2,000 5,100 4,300 3,850 3,650

POND FERTILISATION

This section is aimed mainly at farmers who aim to grow tilapia using “green water” using very
little, if any, feed. Green water is a phytoplankton bloom that provides food, removes ammonia and produces oxygen for the fish. The greener the pond, the more natural food will be available and the fish will grow fast with very little or no supplemental feeding. However, if a pond is too green, then fish mortality can result due to low early morning oxygen. In order to create a plankton bloom, it is necessary to add nitrogen and phosphorous (N & P), the two limiting nutrients to plant growth in water. Recommended levels are 1-2 kg of phosphorous and 4 kg of nitrogen per hectare (10,000 m2 or 6.25 rai) per day. The N & P can come from many sources, including chemical fertilizer, animal manure, “ami” (monosodium glutamate factory waste), compost, etc. Chemical fertilizer is more expensive, but water quality will be much better and this allows stocking of low-oxygen sensitive aquatic species, such as shrimp, prawns and carps. Fertilization should be carried out at weekly or more frequent intervals. The amount of fertilizer needed should be determined by water color. If the pond is not very green then increase the amount of fertilizer. If the pond is too green, fish are gasping for air during the morning and fish begin to die, then reduce or stop fertilizing. Generally, there will be an increase in fertilizer requirements throughout the growth period. Less fertilizer will be required in the case that commercial feeds are used, as they also contain N & P. The following table can be used as a guideline for nutrient requirements:
Type of input Amount required (kg/rai/week)
16-20-0 chemical fertilizer 30
46-0-0 (urea) + 0-46-0 (phosphate) 9 + 11
Fresh chicken manure 300
Fresh chicken manure + 46-0-0 (urea) 175 + 4
Fresh pig manure 800
Cow/buffalo manure 1,000 +
Ami 200 l/rai
Chemical fertilizer should be applied weekly (or more frequently if possible) by dissolving it in water and then broadcasting the solution over the surface of the pond. There are no strict guidelines for the application of animal manure. Most farmers either use general broadcast or they apply the manure to a few selected spots located around the edge of the pond. Frequent manuring in small amounts is advisable, but one time per week will suffice.

FEEDS & FEEDING

The relationship between feeding and stocking density was mentioned earlier. Production costs will increase with increasing stocking density. Feed will only be required once the pond biomass rises to 600-800 kg of fish per rai. However, feed can be used from day 1 to increase growth rate and so reduce culture period. For example, red tilapia will attain a size of 700g in 4 months in ponds when fed a complete diet of commercial pellet. The same fish could be raised to 700g in 6 months using much less and/or cheaper feed, and so reduce investment costs. One has to decide whether the increased feed costs would be worth the quick grow-out period. Other factors, such as land, water and pond excavation prices, should also be considered. If land rental is very expensive for example, then higher density fed systems will be more economic. Once a decision has been made to feed the fish, one should then decide what type of feed to use. This will be determined by availability and price of feedstuffs to a large extent, but the factors mentioned previously are also important. For example, if the price of tilapia is very low, then it will not be economic to feed large amounts of relatively expensive commercial pellet. Only cheap feed inputs will be cost effective in such a situation. Such feeds might include: Waste food from hospitals and schools Rice bran & broken rice – if available very cheaply Bread, wafer, yeast and other edible factory wastes Waste animal feeds The types of supplemental feeds being used by farmers are too numerous to mention and advises that farmers test out any new feed inputs before using them extensively.

Things one should consider are:

1) Will the fish eat them?

2) Are the increased returns cost effective in terms of added costs and labor?

3) Are they are safe from micro-organism toxins? In situations where the price of tilapia is twice as high as the cost of good quality commercial pellet, then cage culture and use of commercial feeds in ponds becomes economic. The advantage of using such feeds is that fish growth is quicker, they are clean and don’t produce foul odor, water quality will be better and they are easily available. Floating pellets are best, as they allow the farmer to see the feed being eaten and so reduce waste. Sinking pellets can also be used. They are generally cheaper to produce and can be made from all sorts of feedstuffs using a simple meat grinder. Monitoring feed eaten is difficult, though, and wastage is higher. They are not recommended for cage culture, as sinking feeds would be lost through the bottom of the cage. There are many types of floating commercial pellet available, but they generally only differ in crude protein level and pellet size. Price increases with increasing level of protein, but less high protein feed will be required for a certain number of fish (higher protein = lower food conversion ratio or F.C.R.) and growth rate will be faster. Protein levels under 20% are not recommended for tilapia, as they don’t find the pellet very appetizing. 30% crude protein for final grow-out may be the most cost-effective feed and a little higher for small fish.

NURSERY

Small, one-inch fry are very susceptible to predation by fish and birds. They are also less tolerant to poor water quality, as is often found in large manured grow-out ponds. Results are, therefore, a bit “hit and miss” and will depend on survival during the crucial first month. Purchasing fingerlings or nursing the fry to a large size prior to stocking can alleviate this problem. Stocking a larger fish also reduces culture period in the main grow-out ponds and so increases farm production. Nursing is best carried out in hapas or in small earthen ponds. Hapas are very useful for nursery, as the young fry can be protected from predation by birds, mammals, snakes, and carnivorous fish very easily. No special pond is required, as a canal or a pond being used for other uses will suffice. The fish can be harvested very simply, using a length of bamboo to confine the fish in the corner of the hapa. Protection of fry stocked directly in nursery ponds is more problematic, but growth is much faster. Harvesting fingerlings is also more difficult, as a fine seine net is required and the pond must be drained to catch all the fish. Whether nursery is carried out in a hapa or in a small pond, “green water” should be first created using chemical fertilizer. Animal manure is not recommended for nursery ponds as it leads to poor water quality and could affect survival. Stocking density in hapas is recommended at 500 one inch fish/m2 for the first month and 250 fish/m2 for the second month. Earthen ponds can be stocked with anything from 15-60 fish/m2 for a 2 month nursery period, but aeration and some water exchange will be necessary at high densities. Feeding should be carried out two to four times per day with floating commercial pellet (smallest pellet size) or a powdered feed, for example rice bran & fish meal with a crude protein level of at least 30%. Pond-nursed fish will have achieved a size of 10-50g in this time, whilst hapa-nursed fish will be 2-10g. Fish nursed in hapas will only grow at the same rate as fish in ponds if they are stocked at the same density per m2, but the large numbers of hapas required will make it very expensive.

POLYCULTURE
Tilapia can be raised in conjunction with other species of fish, crustaceans and mollusks. The main reason for doing this is to increase income without significantly increasing investment costs. The other advantage is reduced risk of financial loss due to high mortality or a drop in market price of one species. For example, a farmer may have high mortality of tilapia due to streptococcus, but other fish may be unaffected by this species-specific disease and the yield of these fish would increase due to the reduced density of tilapia.

POLYCULTURE OF TILAPIA WITH OTHER FISH SPECIES

Integration of tilapia with carps, gouramis, catfish and other fish species is very common. Tilapia will tend to concentrate their feeding activity on the one or two sources of food that they
prefer. This will usually be phytoplankton and/or zooplankton and/or detritus. By stocking a variety of different fish, any food not eaten by the tilapia will be eaten by another species and will not be wasted. The type of species and number of fish stocked is entirely a matter of personal preference and experience, but the following points should be considered: 1) Choose fish species that eat different types of food in different areas of the pond, so that competition between species is reduced and all available food is utilized. 2) Whatever type and number of fish is stocked, ideally they should attain market size at the same time to avoid having to restock fish. 3) In manured ponds only select fish that can tolerate poor water quality and low dissolved oxygen. These include gouramis, catfish and tilapia, and don’t include most carp species. 4) Try and match the type and number of species stocked according to the amount of food that they prefer to eat that is available. For example, if hotel food waste is used as supplemental feed, more catfish may be of advantage, as they prefer such food. If bread or wafer is used, then carps and gouramis may be able to make better use of this input. For ponds using fertilization only the following table can be used as a guide:

Type of feeder Species available

% of total fish stocked
Phytoplankton Nile tilapia, red tilapia, silver carp 90
Macrophyte Grass carp, silver barb 2
Zooplankton Roho, bighead carp, Catla, Pangasius catfish 4
Bottom/detritus Common carp,
Clarias
catfish, mrigal, snakeskin gourami

POLYCULTURE OF TILAPIA WITH SHRIMP & PRAWNS
Because tilapia are not carnivorous, they can be raised in polyculture with penaeid shrimp or giant fresh water prawns,  For this to be effective, it is important that the post larvae are either stocked earlier than the tilapia or the shrimp/prawns are pre-nursed to a size of 0.2 g or more. Although tilapia are not carnivorous, they will eat small planktonic organisms and that includes post larval shrimp or prawns.

There are two main forms of shrimp/prawn polyculture system:

1) High density shrimp culture (30-70 per m2) with monosex tilapia stocked at low density (1 per 2 m2). The tilapia improve water quality and reduced incidence of disease. Only the shrimp are fed and no doubt the tilapia will eat some of the expensive shrimp feed. Despite this, shrimp food conversion ratio (F.C.R.) is reported to be unaffected by the tilapia. Only good quality monsex tilapia should be used or the tilapia will breed and overpopulate the pond. The shrimp are usually stocked as post larvae and the tilapia stocked a month later to avoid predation of the shrimp. This system is not as suitable for giant freshwater prawns, as they don’t perform so well at high density.

2) Low density shrimp or prawn culture (4-6 per m2) with monosex tilapia stocked at high density (1-2 fish per m2). The advantage of this system over tilapia monoculture is that extra income is created for very little extra investment. Only the tilapia are fed on floating pellets and the prawns/shrimp feed on natural food at the pond bottom. Ideally, pre-nursed prawns (0.2g or more) and tilapia (10-50g) should be stocked at the same time. This way culture period is reduced to only 4 months and predation of post larvae is avoided. There will no doubt be a multitude of alternatives to the two main systems described above. It may be possible, for example, to raise both species at higher density by feeding both the shrimp and tilapia. This could take the form of a sinking feed eaten by both species, or by using floating feed for the tilapia and a sinking pellet at night for the shrimp. Other crustaceans, such as crayfish (yabbies) or crabs (brackish water), could also be used as an alternative to shrimp. Any fish species that will consume food uneaten by the tilapia and shrimp could be used to improve the system. Other aquatic species such as frogs, turtles, and mollusks provide another avenue of possibility.

HARVESTING FISH

There are two main methods used for catching tilapia in earthen ponds:

1) Seine net the fish Drop the water level of the pond by half and use a seine net (should be at least one and a half times the width of the pond in length) to catch the fish (match mesh to fish size). Repeat the procedure until few fish can be netted and then drain the pond. Any remaining fish can scooped up from the pond bottom after draining.

2) Catch in corner of pond
Set up a large pump in the corner of the pond and build a barrier with stakes (bamboo for example) fixed closely together. This barrier will stop the fish from getting to the pump intake and the fish can be netted by placing a piece of net over the barrier and lifting at intervals. The pond should be deep in this corner. The fish will naturally move to this corner with the suction of the pump. An automatic fish lifter could easily be used at this point.

CAGE CULTURE

Cage culture of tilapia is now very widespread in rivers and reservoirs. On a small scale this is fine, but on a large scale they can obstruct boat traffic and cause substantial pollution to natural waterways. Most farmers are attracted to this form of aquaculture because it is not necessary to buy or rent land, and capital investments are very low. The disadvantage is that running costs are higher than pond-based farms, as large amounts of commercial feed have to be used. Risk is also quite high, as water quality and disease cannot be controlled easily and escape of fish is always a possibility. Cage culture of tilapia is only economic if the price of tilapia is quite high and it is for this reason that red tilapia are mainly raised in cages.

Location is most important when setting up a tilapia cage farm:

1) Reservoirs Reservoirs typically have deep, clear water with excellent water quality and so are excellent sites for cage farms. Permission is necessary and sites are limited. They are most suitable for red tilapia culture, as the clear water enhances the color of the fish. Nile tilapia become very dark when raised in clear water in cages and may not be accepted by the consumer. Some reservoirs are not suitable due to off-flavor problems.

2) Rivers Cage culture of tilapia in rivers is more risky than in reservoirs, as pollution is common, and strong water flow and floating debris may cause damage. Many rivers are very muddy, particularly after heavy rain, and this can lead to fish mortality. Tilapia will tend to be lighter in color than those raised in clear water, and red tilapia will very pale. The fish get constant exercise in the flowing water and the tilapia will be fat with firm flesh. As yet there is little restriction on setting up cage farms in rivers, except that they should not hinder boat traffic. There is a huge range of cage designs and cage sizes, but they all consist of a floating structure from which nets are suspended. Changes in water level are not a problem and most cage structures can be moved around by towing with a boat. Cages are mostly constructed from galvanized steel poles to which 200 liter barrels are attached for floatation. On larger rivers they often incorporate a house of some sort for security reasons. Very large cages made of wood with a house built over the whole structure to prevent theft. In giant lakes, such as those found in Africa, European-style salmon cages, are the only option, as  cages would be destroyed by the very rough water conditions. Most cage farmers stock 20-50g fish (4 inch), as the mesh size used for the cages (2.5-3.0 cm) would allow small fry to swim through. Stocking rate is based on cage volume (width x length x depth), as one can expect a yield of tilapia from 15-25 kg per m3. For example, a farmer has a cage 5 x 5 m and 2 m deep, total volume 50 m3. He expects a yield of 1,000 kg and requires a mean size of 650g. Survival is expected to be 90%, as the farmer will stock a large size fingerling. No. of fish to stock = ( (Y x V) / (S / 1,000) ) x M Where: Y = Yield per m3 expected (kg) V = Volume of cage (m3) S = Size of fish required (g) M = Mortality coefficient = (100 / estimated survival). No. of fish to stock = ( (20 x 50) / (650 / 1,000) ) x (100 / 90) = 1,709 fish. Floating pellet is essential in cages (sinking feed would be wasteful) and most farmers feed “ad libitum” (according to fish demand and less than satiation) 2-3 times per day. A band of fine netting material should be installed inside the cage at the water surface to stop feed floating out and getting lost. Crude protein can be anything from 25-36%, and the higher the protein the faster the fish will grow. Food conversion ratio should be lower for high protein feed, but it costs more per kg. Culture period is approximately 4 months from 30g to 700g  for red tilapia raised in cages. Tilapia will grow slightly faster than this. Cages installed in farm ponds, canals and large ditches can expect slower growth, much depending on aspects of water quality, depth and water flow. Improvements can be made using paddle wheels, but growth rate is usually better in large reservoirs and rivers.

OFF-FLAVOUR

Off-flavor is a common problem associated with fish raised in freshwater. Muddy and musty off-flavors are the most common and are caused by absorption of geosmin (muddy) and 2-methylisoborneol (M.I.B.) (musty) over the gills. These chemicals can originate from certain species of phytoplankton and actinomycetes bacteria that are commonly found in freshwater ponds. Interestingly, even when off-flavor-related blue-green algal species such as Anabaena, Aphanizomenon, Oscillatoria and Microcystis are common, the fish don’t necessarily have off-flavor. Most people relate off-flavor to the use of manures, which is true to some extent, but unfertilized ponds where fish are fed commercial pellet can also suffer. Off-flavor is most common in ponds with poor water quality and very low early morning dissolved oxygen. However, it can also be found in cage-reared fish in certain reservoirs possibly due to actinomycetes bacteria in bottom mud and on decaying plants. It is also found in indoor recirculating systems and can be related to sludge beneath the biofilter.

The following factors affect the incidence off-flavor in earth ponds:

1) Pond size – Off-flavor is less common in large ponds due to improved water quality (more wave action and water flow).

2) Pond inputs – Off-flavor is less common in ponds using dry feeds and/or inorganic fertilizer.

3) Amount of fertilizer – Over-fertilization is a common reason for off-flavor, particularly when manures are used. Integrated chicken/fish or pig/fish systems are particularly bad, as there is no control over manuring rate.

4) Salinity – Off-flavor is very uncommon in ponds over 3-4 ppt salinity.

5) Aeration – aeration improves water quality and dissolved oxygen levels and so reduces incidence of off-flavor. In the event that off-flavor is a problem in fish about to be harvested,

there are one or two ways to alleviate the problem.

1) Stop fertilizing the pond and use only feed a few weeks prior to harvesting.

2) Exchange water (must be no geosmin or M.I.H. in the incoming water).

3) Purge the fish by keeping them in water free of geosmin or M.I.H. for 5 days.

FISH MARKETING

probably many other countries, there 3 main options with regards to marketing fish:

1) Sell the fish to a pond harvester. In this case the farmer negotiates a price before harvesting the fish. The price will usually be quite a bit lower than wholesale prices as the fish harvester is responsible for pumping the pond, employing staff, harvesting and selling all the fish. The farmer need only watch as the fish are weighed. This method is frequently employed by small farms, especially those involving animal integration.

2) Harvest and sale to the provincial wholesale fish market. In this case the farmer harvests his own fish and delivers them to the wholesale fish market. This enables a farmer to get a better price, but necessitates investment in a net, labour, transport, ice, etc. Many farmers find that wholesale fish markets will only quote a price once they see the fish. This leaves the farmer in a poor position because he has to harvest his fish first and is then at the mercy of the fish wholesaler who will often give a very poor price. Wholesale markets that sell by auction and take a small amount of commission are recommended, as the farmer gets paid the current retail price.

3) Harvest and sale retail This method enables a farmer to cut out the middle man and gain a control over the price of his fish by selling to shops, market stalls and the public. The disadvantage is that the fish must be sold in small volumes and it takes time to sell a whole pond. It also takes time in building up a market and it is essential that the farm always has a supply of fish to keep regular customers supplied. This method gains the highest and most stable market price.

HEALTH CONSIDERATIONS
Many people enquire about the health and safety issues regarding the consumption of sex reversed tilapia. The Asian Institute of Technology did some research many years ago. They found that blood sex hormone levels of sex reversed male tilapia and mixed sex fish were not significantly different. They concluded that adult male tilapia produced using the sex reversal process are perfectly safe to eat.

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