Saturday, May 18Food Security Matters

Sex Reversal of Tilapia – Why and How

Tilapia aquaculture has faced a longstanding challenge of precocious breeding, particularly in pond systems, where fish can reach sexual maturity as small as 8 centimeters in length. This issue is primarily due to the significant difference in growth rates between male and female tilapia. Male tilapia tend to grow faster, as females allocate a considerable amount of their energy to the production of eggs and fry. Consequently, the production of all-male tilapia has become an essential strategy for the aquaculture industry.

sex reversed tilapia have uniformed sizes
Sex-reversed tilapia have uniformed sizes

Initial attempts at producing all-male tilapia involved the use of closely related species, such as Oreochromis hornorum, O. niloticus, O. aureus, and O. mossambicus crosses. Some of these hybridizations yielded nearly 98% male fish. However, hatchery production of large numbers of fry from these hybrids proved inconsistent and unreliable. Mixed-sex culture was also explored in various tropical countries, but even high-quality stock that matured late in the wild exhibited early reproductive behaviors in aquaculture ponds, leading to commercial culture failures.

A breakthrough came when researchers discovered that the sex of Oreochromis juveniles was not determined by genetics but rather during the first three weeks of their life. In the wild, natural factors typically lead to a balanced ratio of 50:50 males and females. Through experimentation, it was found that by feeding artificial male hormones (androgens) to the fry during this crucial developmental stage, the sex of the fish could be influenced, resulting in high yields of males. For example, the use of 17 alpha-methyl testosterone (MT) in precise quantities incorporated into the feed achieved a remarkable 99.8% male production.

This method of hormonal sex reversal has become the established and preferred approach for producing high-quality tilapia fingerlings. However, it raises questions about the safety of consuming fish that have been exposed to hormones.

Safety of Hormone-Treated Tilapia

It’s essential to remember that when the fry first starts feeding, they are a mere 9mm in length. After the short 18 to 21-day hormonal treatment, they grow to a size of 15mm to 20mm, still weighing less than 0.1g. During this period, the amount of 17 alpha-methyl testosterone absorbed by the fish is infinitesimal. Scientific tests have demonstrated that no detectable “residue” of the hormone remains, even ten days after treatment, let alone when the fish reaches harvest size. The hormone merely replaces the natural cues that would have determined the sex of the fish, making it safe to eat.

Despite scientific evidence proving the safety of consuming sex-reversed tilapia, misinformation has led to some customer resistance to these products in certain cases.

Challenges and Demands of Sex Reversal

The process of feeding fry with hormones to achieve sex reversal is demanding and requires careful management. Any presence of other types of food in the culture water can lead to problems. It takes only a few females in the pond to disrupt the system with fry and juveniles, rendering the sex reversal efforts futile. For the best results, newly hatched ova or swim-up fry are ideally collected from mouth-brooding females for incubation while still carrying the yolk sac.

The success of sex reversal relies on rearing conditions that include proper water chemistry and cleanliness, as well as feeding the fry with the appropriate MT feed for 18 to 21 days. If the fry are free-swimming and more than a few days old or if green water, containing algae or zooplankton, is present in the culture tank, the sex reversal process may fail.

The Concept of “Super Males”

An alternative method for achieving sex reversal involves the production of “super male” fish. These are fish that have been selectively bred to possess YY-chromosomes, as opposed to the normal XY males (females have XX chromosomes). While this method is not as widely used and is limited to a few hatcheries worldwide, it offers the advantage of never exposing the actual fish consumed to hormonal treatment.

The Bottom Line

In the world of tilapia aquaculture, the techniques for producing all-male fingerlings have become a well-established and vital component. Mixed-sex groups are often considered a step backward and are generally viewed as doomed to failure in commercial tilapia production. While the use of hormones in sex reversal may raise concerns among some consumers, the scientific evidence overwhelmingly supports the safety of eating tilapia produced using these methods. To successfully farm tilapia on a commercial scale, obtaining high-quality all-male fingerlings from a reputable hatchery remains an essential first step in ensuring productivity and profitability.

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