Tilapias are warm water fishes originating from Africa and the Middle East that have been introduced to the Philippines for fish farming since I950. The main tilapia species cultured in ponds, cages and pens in the country is the Nile tilapia (Oreochromis niloticus). The Mozambique tilapia (Oreochromis mossambicus) and its hybrid with the Nile tilapia are grown in brackishwater ponds while the red tilapia (a hybrid of the Nile and Mozambique tilapias) is also produced in freshwater ponds and tanks to some extent are also commercially produced in the country.
A major drawback in the culture of tilapias is their ability to mature early at 2-3 months of age and breed frequently (every month at most). These characteristics cause the over-population of the stocked tilapia in ponds and the stunted growth of the fish. Moreover, with mixed-sex tilapia stocks, the sizes of the harvested fish vary from small to large because of the difference in the growth of the faster-growing males compared to the females.
For producing high yields of large-sized tilapia (I50 grams or more) in 3-4 months, all-male finger-lings are preferred by growers. To produce all-male tilapia, there are techniques that can be applied such as hand-sexing (separating the males and females manually), hybridization (mating of two species to produce all-male offspring) and artificial sex reversal. The most efficient and least expensive method is sex reversal.
This guide shall discuss the application of artificial sex reversal in tilapia for the production of high yields of large-sized fish.
What is artificial sex reversal?
Artificial sex reversal in fishes like tilapia is the process by which the physical sex direction (male or female) can be manipulated through the feeding of synthetic sex hormones (e.g., methyltestosterone) prior to and during the “sexless stage” of the fry. The technique was first developed in Japan in the 1950s for sex reversal of the aquarium fish (Oryzias medakd) and the common carp (Cyprinus carpio) and was first demonstrated to be commercially feasible for sex reversal of the Mozambique tilapia in the Philippines in the 1970s. Sex reversal of tilapias is now applied worldwide.
How artificial sex reversal is applied in tilapia?
There are three basic steps in the application of artificial sex reversal in tilapia:
- the production of fry for treatment,
- the sex reversal treatment of the fry, and
- the culture of the treated fish.
Production of Fry
To produce the right age of the tilapia fry for sex reversal treatment, a hatchery/nursery system is needed. For mouth-brooding species of tilapia fry (e.g., Nile and Mozambique tilapias), the breeding of the fish can be done using concrete tanks, earthen ponds and hapas (net enclosures) suspended in tanks or ponds. Breeders (50-250 grams each) are stocked at a sex ratio of three females to one male per square meter.
After 10-12 days from stocking, the schooling (swimming in batches) fry about 9-11 millimeters (mm) in total length are collected and transferred to holding units (hapas and tanks) for conditioning and sorting (removal of extra large fry) in a day or two before sex reversal treatment. It is essential that the fry be not more than 12 days old from hatching or more than 12 mm in length prior to treatment for effectiveness.