As concerns over sex reversal grow, mixed-sex tilapia farming gets a second look

Unique traits of developing tilapia put greater focus on sex reversal techniques for producers

Nile tilapia (Oreochromis niloticus) is a key aquaculture species that supports food security and economic development in tropical regions. The fish is valued for its omnivorous diet, rapid growth, tolerance to a wide range of salinity, dissolved oxygen and temperatures, and its ease of reproduction.

Under natural conditions, male tilapia grow more quickly and larger than females, often reaching market size up to twice as fast because they do not expend energy on egg production. The females, meanwhile, mature early and reproduce during grow-out, resulting in stunted, low-value fish.

To prevent this, tilapia fry are commonly fed a male hormone before gonadal differentiation to produce all-male stocks. Once the testes develop and natural hormone production is established, hormone treatment is stopped. Some of the advantages of monosex tilapia stocks include better feed conversion ratios (FCRs), high survival rates and higher net returns.

“Some tilapia can begin spawning at around 50 grams unless selectively bred, although this depends highly on species and culture conditions,” John Bostock, senior research assistant at the Institute of Aquaculture, University of Stirling, told the Advocate. “Once females start spawning, their growth slows because energy is diverted to reproducing, while the males grow faster. This leads to ponds dominated by smaller females and aggressive, territorial males that reduces overall growth and production, prompting a greater use of sex reversal techniques.”

“One of the early problems with tilapia was that they bred before reaching market size,” added Professor Dave Little, also of the Institute of Aquaculture. “This was considered a major problem, especially in areas where tilapia were introduced for food production. Hormonal sex reversal proved effective as hatchery production intensified. Monosex tilapia stocks offer harvests of more uniform, larger fish and, consequently, better returns in many contexts.

“A key breakthrough, achieved almost four decades ago, was refining methods that could produce enough monosex seed to make production economically viable, alongside ongoing improvements in strain quality and genetics,” Little continued. “Overall success is still dependent on the strain of tilapia, and how different male and female genotypes grow, but in general, monosex populations make for more efficient animals.”

However, although sex reversal procedures have some encouraging effects, and the hormone doses used to produce monosex stocks are low and leave no detectable increase in hormone levels in harvested fish, regulatory barriers and concerns about welfare, worker safety and environmental impacts remain. Furthermore, achieving successful sex reversal requires careful control of environmental conditions, including temperature, hormone dosages and water quality. Maintaining these factors demands adequate resources as well as skilled and experienced personnel.

“Effluent from hatcheries containing hormones is flagged as a potential environmental risk, but the quantities are very small,” said Bostock. “Compared to hormone inputs from human populations entering river systems, for example, it is likely negligible. A greater concern may be the safety of people handling the hormones.”

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To assess whether monosex stocks remain the best option for tilapia farming, Little, Bostock and their colleagues partnered with Thailand-based Nam Sai Farms to evaluate mixed-sex production. They conducted a field trial comparing mono and mixed-sex grow out for Nile tilapia of the Big Nin strain, known for its high growth rate.

“Mixed-sex tilapia farming can be a lower-cost alternative, especially for small-scale, rural producers, and offer nutritional and economic advantages in developing countries,” said Little. “If large tilapia are processed into fillets, you get high quality nutrition, but in poorer countries people often eat smaller, cheaper fish whole, which actually provides more nutrition per kilo – especially important where micronutrients are deficient. A second advantage of mixed-sex stocks is that proximity to fry suppliers isn’t essential. Mixed-sex stocks can self-recruit and be maintained year-round, making them particularly useful in areas with limited infrastructure.”

The trial in Thailand found that although females grew more slowly – reducing harvest biomass in the mixed cages – doubling stocking densities in the mixed-sex cages increased total yield. The team also created a financial model, which indicated that monosex farming remains more profitable in Thailand at current market prices. However, the choice between mono and mixed-sex populations depends on a host of factors, such as production method (ponds or cages), costs, market conditions and the relative value of small versus large fish.

“Previous research showed that monosex populations are better suited to large-scale producers with capital for hatcheries,” said Little. “We therefore examined how feasible mixed-sex populations are for small-scale farmers. In Southeast Asia and Bangladesh, we found strong results when tilapia are stocked in rice fields with spring crops, producing large fish just when farmers in rural areas wanted to buy them. This supports local trade without the need to rely on external suppliers.”

Although mixed-sex tilapia farming may not suit producers in Thailand, the team concluded that it could work in other regions, such as parts of Africa, where smaller female tilapia have ready markets and can sometimes sell for up to twice the price per kilo seen in Asia.

Sex reversal in tilapia has transformed aquaculture methods globally, while mixed-sex tilapia production remains key to developing sustainable, cage-based, or non-hormone-treated farming systems, stemming from concerns over hormone-treated fish and the need for alternative, cost-effective management strategies.

Looking ahead, Little, Bostock and their colleagues hope that future research will address knowledge gaps in areas such as fish welfare to improve both monosex and mixed-sex tilapia production.

“It can be hard to obtain reliable financial data on costs and production conditions, so more detailed economic modeling would be useful,” said Bostock. “Questions also remain about the welfare impacts of hormonal sex reversal and its long-term implications on fish health and behavior. Our trials have consistently shown better survival in monosex populations, but fish welfare definitely warrants further study.”

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