Credits: Biovision-Infonet
Mixed-sex culture
Under mixed-sex culture of tilapia, both males and female are cultured together but harvested before or soon after they reach sexual maturity. This minimizes chances of recruitment and overcrowding. The disadvantage in this is that fish are harvested at a smaller size due to the limited growth period. In this culture practice, fish are usually stocked at low rates to reduce competition for food and promote rapid growth. One month-old, 1-gram fry are stocked at 1 to 2 per square meter and grown for about 4 to 5 months. In cold areas where the water temperatures are low and therefore growth is slow, tilapia might not reach marketable sizes in that period.
Newly-hatched fry should be used all the time because older ones will reach sexual maturity at a smaller, unmarketable size. They could also be mature fish but stunted. Supplemental feeds with 25 to 32 percent protein are generally used. The average harvest weight is about 250 g and total production about 0.25 kg/sq m for a stocking rate of 1 fish/m2. Higher stocking densities can be employed to achieve higher production but must be combined with very good management. Expected survival is about 80 percent.
Species such as Tilapia zilli, T. hornorum, or T. mossambica are not suitable for mixed-sex culture because they reach reproductive maturity at 2 to 3 months at an unmarketable size of about 30 grams. Those that are suitable for this culture are O. nilotica and O. aurea which reach reproductive maturity at 5 to 6 months. Two to three crops of fish can be produced annually in Kenya depending on the water temperatures.
Mono sex culture
To overcome the problem resulting from prolific breeding of tilapia, ponds are stocked with males only because the males grow almost twice as fast as females. Male fingerlings can be obtained by three methods:
- Hybridization
- Sex-reversal and
- Manual sexing.
None
of these methods is 100 percent effective, and a combination of methods
is recommended. Hybridization can be used to produce better results of
males only. The hybrids can then be subjected to hand sexing and/or
sex-reversal treatment. Sex-reversal requires obtaining recently hatched
fry and rearing them in tanks or hapas where they are subjected to
hormone laced feed for about three weeks.
Hormonal sex reversal needs a tank-based or hapa-based hatchery that will allow fry to be collected at the yolk sac or first feeding stages (no later than one week after they have been released from the female). The healthy fry of uniform size are transferred to the tank or hapa where you will feed them with hormone-laced diet for 21-28 days. The feed is prepared as follows:
- Mix 30 – 70 mg of hormone (methyl or ethynyl testosterone) in 700 ml of 95% neutral ethanol
- Add 700 ml of hormone solution to each kg of finely ground feed then mix thoroughly and dry. At this stage you may add any needed supplements
- This feed should be kept under refrigeration if it is not going to be used immediately
- Feed the fry at a rate of 10 – 30% of body weight per day, at least four times a day for 21 – 28 days.
- The fry must eat this feed to sex-reverse
This is not in accordance with the organic principles and therefore cannot be applied.
Manual sexing (hand sexing) involves separating males from females by visual inspection of the external urinogenital openings. Reliability of manual sexing depends on the skill of the workers, the species to be sorted and fish sizes. Experienced workers can easily sex 20-gram fingerling T. hornorum and T. mossambica, 30-gram T. nilotica, and 50-gram T. aurea. Tilapia males are preferred for culture because they grow faster than females. Females use considerable energy in reproduction and do not eat when they are incubating eggs. All-male culture permits the use of longer culture periods, higher stocking rates and fingerlings of any age. High stocking densities reduce individual growth rates, but yields per unit area are greater. If the growing season can be extended, it should be possible to produce fish of up to 500 grams. Expected survival for all-male culture is 90 percent or greater.
Females included in a population of mostly male tilapia affects the maximum attainable size of the original stock in grow-out. A stocking rate of 2/m2 is commonly used in Kenya to achieve yields of 1kg/ m2. At this stocking rate the daily weight gain will range from 1.5 to 2.0 grams. Culture periods of 6 months or more are needed to produce fish that weigh close to 500 grams. There are cases in Kenya where stocking densities of 6 juveniles/ m2 is practiced with a production of up to 3kg/m2. Higher stocking densities will require water aeration and sub-optimal feeding rates may have to be used to maintain suitable water quality.
Polyculture
In Kenya tilapia are frequently cultured with other species, mainly catfish (Clarias gariepinus) to take advantage of many natural foods available in ponds and to produce a secondary crop, or to control tilapia breeding. Polyculture uses a combination of species that have different feeding niches to increase overall production without a corresponding increase in the quantity of supplemental feed. Polyculture can improve water quality by creating a better balance among the microbial communities of the pond, resulting in enhanced production.
Other possible polyculture combinations that can be done in Kenya include:
- Tilapia and prawns (Macrobrachium rosenbergii): In this case, survival and growth of tilapia and prawns are independent. Feed is given to meet the requirements of the fish. Prawns, which are unable to compete for the feed, utilize wasted feed and natural foods that result from the breakdown of fish waste.
- Tilapia and large mouth bass (Micropterus salmoides): The bass which is carnivorous, control the breeding of tilapia in mixed sex culture. This allows the original stock to attain a larger market size. Predators must be stocked at a small size and percentage to prevent them from depleting the tilapia stock.
Growth and yields
Under proper management and optimal conditions, 1-gram fish are cultured in nursery ponds to 20 to 40 grams in 5 to 8 weeks and then stocked into grow-out ponds. In mono-sex, males can reach 200+ grams in 4 to 5 months, 400 + grams in 5 to 6 months, and 500+ grams in 8 to 9 months. Dress-out percentage on tilapia is low compared to species such as trout and catfish. Tilapias have a dress-out of 51 to 53 percent of live weight for whole-dressed fish (head-off) and 32 to 35 percent for filets.