The precocious breeding of tilapia, especially in ponds, with fish maturing as small as 8cm in length, bedevilled their culture for many years. It was soon realised that male fish grew significantly faster than females as the latter turned most of their energy to the production of eggs and fry. Production of all-male fish therefore became essential.
Initial attempts used closely related species such as Oreochromis hornorum, O. niloticus, O. aureus and O. mossambicus crosses. Some of these hybrids produced nearly 98% male fish, but hatchery production of large numbers of fry was inconsistent and unreliable. Mixed-sex culture was tried in many tropical countries, but even good quality stock that matured late in the wild started reproducing at an early age in aquaculture ponds, and commercial culture failed time and again.
It was then discovered that the sex of Oreochromis juveniles was only fixed during the first three weeks of life. In the wild, various factors would normally result in a balanced ratio of 50:50 males and females. It was further found that if artificial male hormones (androgens) were fed to the fry for a fixed period of time during this stage, the sex of the fish could be ‘determined’ and high yields of males achieved.
For example, 17 alpha-methyl testosterone (MT)incorporated into the feed at precise levels produced about 99,8% males.
This is now the established method for producing quality tilapia fingerlings – but does the use of a hormone mean the fish are safe to eat?
Remember, the fry are only 9mm long when they first feed, and after the short 18-day to 21-day treatment can be from 15mm to 20mm – still a minute fish, weighing less than 0,1g. The amount of MT absorbed is infinitesimal, and scientific tests have proven that no ‘residue’ is detectable even 10 days after treatment, never mind when the fish is at harvest-size.
All that’s happened is that the MT has replaced those slight natural ‘cues’ that would have turned half the fish male, and half females. Thus they are safe to eat. Unfortunately, though, it has to be noted that misinformation has, in some cases, resulted in customer resistance to the use of sex-reversed tilapia, irrespective of scientific proof of their safety.
The methods of feeding the fry are quite demanding. If any other type of food is present in the culture water, you’ll have problems. It only takes a few females in the pond to flood the system with fry and juveniles, and the whole sex reversal exercise is wasted. Newly hatched ova or swim-up fry are best collected from mouth-brooding females for incubation while still carrying the yolk sac.
If then reared under the correct water chemistry and cleanliness conditions, and fed on the MT feed for 18 to 21 days, then a high success rate can be achieved. If, on the other hand, the fry are free-swimming and more than a few days old, or green water (containing algae or zooplankton, on which the fry will feed) is present in the culture tank, then the sex reversal fails.
One other sex reversal method is the production of ‘super male’ fish. These are fish that have been bred to have YY-chromosomes, as opposed to the normal XY males (females are XX). It is only used in a few hatcheries worldwide, but the advantage is that the actual fish eaten are never hormone-treated. For those who want to farm tilapia commercially and successfully, obtaining good quality all-male fingerlings from a competent hatchery is an essential first step.
Nicholas James is an ichthyologist and hatchery owner. Contact him at [email protected]. Please state ‘Aquaculture’ in the subject line of your email.