The tilapias are members of the family Cichlidae.
- characterisitic body shape
- long dorsal fin
- fins have sharp spines
- pectoral and pelvic fins are close to the head
Tilapias are native to Africa and the Middle East. They have spread mainly through introductions for fish farming and are now found in all tropical and semi-tropical continents. Today, tilapias are one of the most popular fish for culture in the world, being reared in Africa, Europe, throughout the Pacific, China, Israel, USA and the Caribbean.
This is an "all-male" hybrid O.niloticus x O.aureus grown in intensive tanks in Zambia.
Until the late 1970's the tilapias, were all classified into a single genus, Tilapia, however most taxonomists now classify them into three genera, Tilapia, Saratherodon and Oreochromis according to their breeding behaviour.
Tilapia species construct nests on the pond bottom and guard the eggs and young in the nest. They have coarse teeth and few gill rakers, are generally herbivorous and macrophytophagous. e.g. Tilapia zilli, Tilapia rendalli
Sarotherodon species brood the eggs and the larvae in the mouth of the male. They have fine teeth and more gill rakers, and are microphagous and omnivorous. e.g. Sarotherodon galilaeus.
The genus Oreochromis contains all the best species for aquaculture. They have similar feeding habits to Sarotherodon spp., however eggs and fry are incubated in the mouth of the female. e.g. Oreochromis niloticus, Oreochromis mossambicus.
Not all taxonomists accept this classification and it is still common to see the Nile Tilapia, Oreochromis niloticus referred to as Tilapia nilotica, particularly in American literature.
The following factors all contribute to the ease of cultivation of tilapias; a) Resistance to poor water quality and disease b) Tolerance to a wide range of environmental conditions c) Ability to convert efficiently organic, domestic and agricultural wastes, into high quality protein d) Good growth rates e) Easy to grow in intensive culture
The life cycle of Nile Tilapia is as follows:
1. The male makes a nest (or defends an area on the bottom of a tank or even a fine meshed cage) and attracts ripe females to the nest with courtship displays.
2. The female lays eggs in the nest, where they are fertilised by the male, and immediately picked up in the mouth of the female.
3. The male will continue to court other females, while the female which has just spawned retreats away from the nest to incubate the eggs.
4. The eggs hatch in the mouth of the female after about five days (depending on temperature) and the hatchlings remain in the mouth while they absorb their yolk sac.
5. Gradually the fry start swimming out of the mouth to feed, but return to the mouth at any sign of danger
6. Once the fry have become too large to fit in the females mouth, they become totally independent and swim to warm, sheltered water such as the edge of a pond
7. The eggs of a female are stimulated to develop once the previous batch of offspring are released, so after a period of recovery, she will return to spawn within 4 weeks.
There are a number of points to note:
- for most species of Oreochromis, water temperatures need to be above 22oC for spawning to take place. If temperatures are above 22oC all year, spawning will be continuous, if it falls below 22oC, spawning will be seasonal.
- the male plays no part in parental care and can mate with many females at a time, therefore sex ratios in breeding ponds can be as high as 7 females : 1 male
- the mouth size of the female determines how many fry are in a brood, bigger females have bigger broods, however usual brood sizes would be 100-500 fry
- the eggs are relatively large, producing large fry which do not need live feed at first feeding
- removing the eggs or fry prematurely from a brooding female, will increase the frequency of spawning
- in poor conditions, species such as Oreochromis mossambicus can become sexually mature at a small size (from 10g) within 4-6 months of hatching; this can lead to ponds becoming overpopulated with small, unsaleable fish.
- in actively breeding populations of tilapia, much of the resources of the females are tied up with reproduction, either producing eggs, or being unable to feed during mouthbrooding; this means that the growth rates of males are much higher than females.
Tilapia were introduced to many countries in the 1950's and 1960's as a "wonder fish" however bad experiences where over breeding in ponds led to the production of small, low value fish at harvest meant that many discouraged tilapia culture, calling it a "weed fish". This image still persists despite the fact that husbandry techniques developed over the last 20 years have revolutionised the expectations of tilapia culture, making it one of the most productive forms of finfish aquaculture in the world.
Four hatchery and stocking practices have contributed most to this change:
- stocking good species such as Oreochromis niloticus instead of Oreochromis mossambicus
- hand sexing of juveniles
- hybridization
- hormonal sex reversal
Often the easiest option to improve performance is the selection of a better tilapia species. O. mossambicus was generally the first tilapia to be spread round the world. It is hardy, but displays some of the most undesireable characteristics. It breeds at a very small size, is difficult to grow to a reasonable market size (100g+) with huge differences in growth between the sexes. O. niloticus is much faster growing, tends to breed later at a larger size, and given the right conditions can easily grow to 300g or even to over 1kg. The switch from O. mossambicus to O. niloticus has been one of the major reasons for the impressive production levels now seen in countries such as Taiwan and the Philippines. The main problem with changing from one species to another has been maintaining stocks of the pure new species. The species will often interbreed to produce breeding hybrids, which rapidly lose their original growth performance particularly after the infiltration of O. mossambicus into the strain.
There are clear differences between the sexes in tilapia species, particularly in the form of the urinogenital opening, fin morphology and adult colouration. Sexing large fish is easier than small fish as differences in the urinogenital openings are more obvious. Skilled hatchery workers can achieve over 95 % male populations on 5-7 cm fish. It helps to use aids such as magnifying glasses and coloured dyes (halcyon blue, ink, malachite green) to accentuate the genitalia.
Hand sexing requires extensive nursery facilities for ongrowing of fry to produce advanced fingerlings for sexing at around 20 g. Fish of this size are also more able to resist handling, stress and predation. It is, however a relatively inefficient means of producing an all male population; half of the fish are rejected. Culled females can be sold or used as feed.
Tilapia genetics is complicated; hybridisation between species sometimes produces offspring with a skewed sex ratio, more males than females or more females than males; over 25 different hybrid combinations of tilapias have been shown to produce 80 % male (this is desirable because males grow faster than females).
Commercial use of hybridisation is not widespread because little is known about the characteristics of these hybrids other than their sex ratio.
Hybrid performance will be intermediate to the characteristics of the parents; there is little evidence for hybrid vigour between any interspecific hybrids, but can show positive traits from both parents.
O. hornorum is the only known species which consistently produces all male fry when crossed with O. niloticus or O. mossambicus.
O. niloticus female and O. aureus male, produces 80-90 % males, with the growth vigour of O. niloticus and the cold tolerance of O. aureus.
The main problem is the maintenance of the pure lines.
This is a cost effective way of producing a 100 % male population. You need a tank or hapa (small cage net) based hatchery so that fry can be collected at the yolk sac or first feeding stages, no later than one week after the fry have been released from the female. Fry must eat the feed containing the sex reversal hormone, not natural food, the feed must be eaten readily by the fry therefore they have to be in good condition.
The method for preparing sex reversal feed is as follows:
a) Mix 30-70 mg of hormone (methyl or ethynyl testosterone in 700 ml of 95 % ethanol
b) Add 700 ml of hormone solution to each kg of finely ground feed
c) Mix thoroughly and dry
d) Add any supplements
e) Refrigerate (if the feed is not to be used immediately)
f) Feed at a rate of 10-30 % of body weight/day, at least 4 times a day for 21-28 days.
Although tilapias breed prolifically in unmanaged ponds, their low fecundity means that fish farms need to invest in systems for fry and fingerling production. The systems used are ponds, net enclosures (hapas) and tanks. Breeding systems depend on the natural behaviour of the fish. Unlike catfish or carp, each female produces relatively few eggs and it is not easy to strip eggs for artificial fertilisation and incubation.
a) Unmanaged ponds are the most widely used tilapia hatchery system. It requires low input management but also results in low production; 1-2 fry/m2/month or 5 fry/female/month
b) Managed ponds generally have improved pond design and management. The pond is built so that it can be fully drained from its deepest point. The farmer carries out effective water quality management, manuring, fertilisation and possibly supplementary feeding. Fish of 100-200g weight can be stocked at a rate of 1-3 fish/m2 and at a sex ratio of 1 male:3 females for pure species or 1:1 for hybrid production. They should produce 6-15 fry/m2/month or 35-100 fry/female/month. Production can be increased further by spawning a greater number of larger fish in larger ponds, increasing the harvesting frequency and using different methods for fry collection. If broodstock of 1-1.5 kg are used and fry are harvested every 17-19 days, seed production can be increased to 45 fry/m2/month or 380 fry/female/month. It is also useful to collect the fry with hand nets from along the edges of the pond on a daily basis to avoid disturbance of spawners and damage to fry.
c) Net enclosures (hapas) are usually made of fine nylon, plastic mosquito netting or cotton mesh. Hapas are very easy to manage; because fry cannot escape harvesting is much easier. Hapas can be any manageable size from 1-40 m2 with a depth of 1-2 m and suspended on poles. They can be simple squares or rectangles. There are also more complex designs, some with a series of nets separating broodstock from fry. A hapa can be routinely cleared of all fry by simply sliding a pole under the net and by moving it across beneath the hapa. Two persons can easily harvest a 40 m2 hapa. Usual broodstock densities are 2-7 fish/m2, male:female sex ratio of 1:2 to 1:7. Production rates range from 150 fry/m2/month or 50 fry/female/month to over 880 fry/m2/month or 300-400 fry/female/month. The main ways to improve productivity are to clean nets and harvest fry regularly. As with all fry production systems it may also be worthwhile to rest fish regularly rather than trying to breed from them continuously.
d) Tank based hatcheries are easily managed but are relatively expensive to build. A hatchery is usually subdivided into spawning units and fry ongrowing units. Spawning units usually circular tanks of 1-6 m diameter containing 0.5-0.7 m of water; no substrate is required. Broodfish between 100-200 g are usually stocked at 1-5m2 at a sex ratio of 1 male:2-7 females and fed a 30-40 % protein diet at a rate of 1-2 % body weight/day. Fry production will be maximised if there is frequent manual harvesting of seed by dip nets and inspection of females. Seed yields of up to 400-3,000 fry/m2/month or 200-1,500 fry/female/month can be achieved if eggs are collected every 5 days or eggs and fry are collected every 10-14 days, however this requires that unhatched eggs and sac-fry are held in an artificial incubation system until they are ready to feed. Incubation systems are designed to keep the eggs in gentle motion; violent agitation will lead to mechanical damage of eggs. Three main types of systems are used; an upwelling zuger jar, a downwelling round-bottomed jar and shaking tables.
Many different types of ponds are used for tilapia culture. The most widespread, but most unproductive are low input ponds with uncontrolled breeding and irregular harvesting; yields are typically 500-2000 kg/ha/yr of uneven sized fish. The next step up is manured ponds with uncontrolled breeding and regular harvesting; yields are typically 3000-5000 kg/ha/yr of uneven sized fish. To achieve higher yields requires much greater investment in terms of management and stocking. If monosex fish are stocked, and regular manuring and supplementary feeding is practiced yields can be up to 8000 kg/ha/yr of even sized fish. Very high yields can be achieved by using monosex fish, complete feeds and changing the water regularly. This can result in yields of 15,000 kg/ha/yr of even sized fish. It is quite common for tilapia to be grown in polyculture ponds with carps, catfish or freshwater prawn (Macrobrachium).
The main advantage of ponds is that fish can be grown very cheaply through fertilisation. There are several disadvantages: they are expensive to build; they take up a large area; they can only be built in areas with suitable soils; they require a lot of water to compensate for seepage and evapouration; unless mono-sex fish are stocked, there will be a wide range of fish sizes at harvest; pond reared tilapia can have a "muddy" taint; fish in ponds are very susceptible to theft and predation.
Cage culture of tilapia avoids problems with over breeding because eggs fall through the cage meshes. The other main advantage is that the farmer does not necessarily need to own the water body where the cages are placed. Two main types are practiced. Small, low intensity cages are placed in a nutrient rich lake or canal. The cages can be made of netting or are sometimes made from bamboo or other locally available materials. The fish derive most of their nutrition from the surrounding water, however they may also be fed supplementary feeds. Typical stocking rates at harvest are 10 kg/m3 maximum.
Intensive cage culture of tilapia is becoming more common. Several very large commercial projects have started recently in Africa and the Carribean. Fish are stocked at higher rates and fed complete feeds. Stock rates at harvest are typically around 25 kg/m3. The maximum density is limited by the availability of oxygen.
Cage culture typically requires much lower capital investment than pond or tank culture. It also does not need a running or pumped water supply. The main disadvantage is susceptibility to theft or malicious damage.
Intensive tank culture avoids problems with overbreeding because there is no space for males to set up territories. It requires a constant supply of water, either gravity fed or pumped. This makes tank culture difficult in areas without a very substantial supply of running water or good power supplies for pumps. Water usage depends on the management system but it can be considerably less than that for a similar amount of production in ponds and uses a much smaller area. Usual maximum stocking rates in tanks where the water is changed every 1-2 hours would around at 25-50 kg/m3. With aeration this could be raised to 70 kg/m3 and with oxygenation this could be as high as 150kg/m3. The fish are fed complete feed; usually 30-35% Crude Protein for fish over 50g average weight. Security is much easier with a tank system because production is concentrated on a small site which can completely fenced in.