Tilapias are very prolific fish that will breed even in less than ideal conditions. This can be a problem in tilapia farms as spawning can reduce the growth rate by increasing the fish density in the holding facilities. It is very hard to control breeding and prevent spawning if male and female tilapias are kept together in the same growing units. Not being able to control the rate of breeding will also affect quantity and quality of the fish, and a farm can quite rapidly be filled up with low value fish. In addition to this, the adult fish grow slower if they are allowed to breed as they use energy to spawn; energy that they would otherwise use for growing.

In short there are a lot of problems with growing out male and female tilapia together. This is the reason why more and more farmers raise mono sex tilapia. Most farmers prefer males as they grow faster and have a larger max size than females.  The largest tilapia fish are males farmed in growing units with less than 4% females in them.

The advantages of raising mono-sex tilapia has created a demand within the industry to produce mono sex tilapia to use when stocking ones ponds, tanks or cages. Several methods of breeding mono sex male tilapia have therefore been developed.

Hormones

One method is to breed mono-sex tilapia by using hormones. Tilapia fry will develop into males if they are exposed to large enough amounts of male hormones. This means that all fry will become males regardless of their “natural” sex. This method is sometimes referred to as the sex reversal-method as the females are turned into males. The most common way to achieve this is to give the fry a hormone dose through their food.  The treatment is normally fed to the young fry for 3-4 weeks. If you want to use this method in your farm to produce mono-sex tilapia there is a couple things you should know. The most important one is that the method might be illegal in your country; the use of hormones in restricted in many countries. Some countries will not allow you to use hormones in this way, while others will require you to get a license before you can breed tilapia in this fashion. There are of course also countries that have no restrictions, but since regulation can change rapidly it is always advisable to check with local authorities to be on the safe side. Another thing is to remember that the waste water from the pond can contain elevated levels of hormones and it should therefore preferably not be released into bodies of water where the hormones can affect fish, amphibians and other animals.

Hybrids

Another method to create mono-sex tilapia is to cr
ossbreed certain species of tilapia.  When certain combinations are used, “all” fry will be male. If you breed male Blue tilapia (Oreochromis aurea) or Zanzibar tilapia (Oreochromis hornorum) with female Nile tilapia (Oreochromis nilotica) or Mozambique tilapia (Oreochromis  mossambicus) all fry will be male.

This method is not 100% exact. Although most fry will be of a certain sex using this method it will not always be 100% males due to genetic differences in tilapia strains. This is due to the fact that most strains are not 100% pure. Some of the purest strains available produce about 98% males when cross bred this way.  Different strains carry different genetics and some strains are not pure bred which affects the result. One study in Israel (Oreochromis aurea mated with Oreochromis nilotica) showed that 80-90 percent of fry were male. Another study in Mexico (Oreochromis nilotica bred with Oreochromis mossambicus) showed that only 80% percent of the fry were male. This exemplifies how diverse the DNA can be in different strains. Fry created using this method is fertile and can mate successfully with both its parent species.

It is important to use pure bred fish for this method. Using non pure bred fish can dramatically increase the number of females among the fry and make this method very unreliable. This has been proven several times when none pure bred fish has been used by mistake when breeding mono-sex tilapia. The sex mix can then be even worse than in the studies above.

This result of this method can be seen in the wild as well. In the Philippines Oreochromis mossambicus has long been a problematic invasive species. Today a number of other tilapia species has also found their way into the Philippine waters and this has caused the Oreochromis mossambicus numbers to drop sharply as wide spread hybridization takes place in the wild. The hybridization creates a skewed sex ratio with a lot more males than females which over time reduce the population dramatically.

Male YY technology

Another method for creating mono-sex tilapia is through male YY technology, but this technology is still under development when this article is being written and not yet commercial viable.

Conclusion

None of the methods described above are guaranteed to give 100% males.  Since there should be no more than 4% females in a tilapia farm to achieve maximal growth none of these methods by themselves is very reliable. This is why many producers of tilapia fry and fingerlings use both methods to get as high a percentage of male fish as possible. This might however change in the future when male YY technology becomes commercial available.