Many of the most commonly farmed tilapia species breed easily and rapidly even in farm conditions. This means that if a farmer keeps male and female tilapias together, it will be hard to control breeding rate, quantity and quality. Soon, the growing units will be filled with fry of low economic value. The adult fish will also spend energy breeding instead of growing. It is therefore easy to understand why so many farmers prefer to raise only one sex. Most farmers choose to raise male tilapia only, since male tilapias grow bigger than the females. This has placed a demand on the fish breeding industry to produce mono-sex tilapia, i.e. males only groups that can be sold to farmers.  

Breeding mono-sex tilapia by using hormones
If newly hatched tilapia fry are subjected to sufficient amounts of male hormones, they will turn into reproductively functional males. This method is also known as the sex reversal-method. It is normally carried out by feeding newly hatched tilapia fry with special hormone treated food for 3-4 weeks. The size of tilapia fry is normally determined 2-6 weeks after hatching. It is important to keep in mind that the use of hormones is restricted in many parts of the world. You might for instance require a special license to produce hormone-food and/or feed it to your tilapia.

Breeding mono-sex tilapia through hybridization
When certain species of tilapia mate, the offspring will be all-male. This will for instance occur when Blue tilapia (Oreochromis aurea) or Zanzibar tilapia (Oreochromis hornorum) males mate with Nile tilapia (Oreochromis nilotica) or Mozambique tilapia (Oreochromis mossambicus) females. In Singapore, the problem with invasive tilapia in the form of Oreochromis mossambicus might actually be on its way of resolving due to the introduction of other exotic tilapia species and hybrids, since the sex ratio of the resulting hybrids are so strongly skewed in favour of males.

The genetics of tilapia hybrids is complicated and many crosses will not produce 100 percent males. Oreochromis aurea females that mated with Oreochromis nilotica males in a study in Israel did for instance produce 80-90 percent males, while Mexican try-out where Oreochromis nilotica females were bred with Oreochromis mossambicus males produced no more than 80 percent males.

Females in a batch can also be the result of non-pure breeding lines. Nile tilapia (Oreochromis nilotica) imported to Israel from Uganda did for instance yield a surprisingly high amount of females and the problem turned out to be non-pure fish.

The F1 generation of tilapia crosses is normally fertile and may backcross with a parent. This can in turn result in offspring of both sexes.   

Breeding mono-sex tilapia by using YY male technology
YY male technology for tilapia breeding is still under development and is not yet a commercially available alternative.