Mixed-Sex Tilapia Farming: A Promising Alternative to Standard Production

Recent trials in Thailand suggest that mixed-sex tilapia farming could be a viable alternative to the cultivation of hormone-induced all-male fish, especially in cage farms in Africa. Tilapia is the second most farmed genus of fish, after carps. The annual global production of tilapia is almost 7 million tonnes and is increasing. Much of this industry growth is occurring across Asia and Africa, where tilapia provides an important food source and a stable income for producers.

Why Mono-Sex Tilapia Farming is Common

Mono-sex male tilapia stocks are commonly used because of their higher growth rates, better feed conversion ratios (FCRs), and higher survival rates compared to females, leading to greater yields and improved financial returns.

Mono-sex stocks are created through:

• Manual selection of males
• Artificially induced sex reversal of female fry

The most efficient and widely practiced technique is the artificially induced sex reversal of female fry. As tilapia fry do not possess developed gonads when recently hatched, the application of hormones can be used to achieve what would appear to be a mono-sex population. Androgenous steroids like methyltestosterone are the hormones most commonly used to induce the development of secondary sex characteristics. The hormone treatment does not alter genotypic sex, but treated fish will express physical characteristics of the desired sex, such as the higher growth rates of male tilapia.

The Mixed-Sex Tilapia Farming Trial

Researchers from the University of Stirling's Institute of Aquaculture researched the efficacy and feasibility of mixed-sex cultures in comparison to all-male stocks. Concerns surrounding regulatory barriers, welfare, staff health and safety, and environmental effects motivated the study. The researchers collaborated with Nam Sai Farms in Thailand to conduct a field trial comparing mono- and mixed-sex grow-out for Nile tilapia of the Big Nin strain. The cages were placed in the Bambopong River to replicate common culture conditions. Different stocking densities were applied to the mono- and mixed-sex cages because of the differing growth rates between males and females.

Key findings of the trial:

• While female tilapia exhibited slower growth, doubling the stocking densities in mixed-sex cages resulted in a greater total yield.
• Grading out the females at four and eight weeks showed similar results in terms of weight at harvest, survival, and food conversion rate when compared to hormone-treated fish.
• There were no differences at harvest in fillet yield and nutritional composition.

Financial Implications

A financial model revealed that, given the current market prices for tilapia in Thailand, mono-sex farming remains the most profitable practice. However, if producers could achieve an 8% premium for non-hormone-treated fish, the financial returns would be comparable. Mixed-sex rearing may be more interesting for larger commercial producers targeting more sophisticated urban or international markets sensitive to messaging on production methods. Selling the female tilapia for broodstock is another option, though the market demand is unreliable.

Regional Applications

Mixed-sex tilapia farming may be applicable in some areas of Africa where there is a ready market for smaller female tilapia, which can fetch prices per kg up to double those in Asian markets. Follow-up studies are already taking place in Kenya.