​Gender-swapped fish key to latest commercial aquaculture industry | Phnom Penh Post

A tank teems with male red tilapia at an aquaculture farm at the Royal University of Agriculture’s (RUA) Faculty of Fisheries and Aquaculture in December last year. RUA

Tilapia is a catchall phrase for nearly a hundred species of cichlid fish. First farmed in Ancient Egypt, these fast-growing freshwater fish are one of the most commercially farmed types in the world. aquaculture design

Known to thrive in warm water – and with a preference for feeding on algae and duckweed – they are an ideal subject for commercial fisheries in Cambodia. 

Several such operations are now underway, although the practice is not yet widespread. Students at the Royal University of Agriculture (RUA) in Phnom Penh’s Dangkor district are working to change this, through the application of scientific know-how.

One of the most fascinating aspects of raising these fish is the necessity of altering the gender of the young individuals, known as fry. Male examples are more disease resistant, and grow at a much faster rate, so hormones are added to the feed while the hatchlings are young, resulting in a school of almost entirely male fish.

Vann Napun, a third-year student at the RUA’s Faculty of Fisheries and Aquaculture, shows The Post a large pond which teems with swarms of the fish, with plastic screens separating the individuals which are different stages of development.

He explains why the team is focused on producing schools of male fish, noting that an original clutch of eggs may hatch with as many as 50 or 60 per cent females.

Napun says the slower growth rate of a predominantly female school means they are not economically viable for farmers.

“In general, without intervention, there are more female fish. They grow at a similar rate for the first three months, but then reach breeding age. At this point, the females stop feeding and begin to lay eggs, meaning they stop growing,” he adds.

According to Napun, thanks to hormone treatment, farmers in neighbouring countries are able to raise individual fish to almost 1kg, making them far more competitive than an unmodified school.

He and his fellow students studied the necessary technique in Thailand, returning to Cambodia to practise their newly-acquired skills about five months ago.

As soon as the tilapia spawn, his team separate the breeding fish, and keep the eggs in a separate hatchery. Between 20,000 and 30,000 hatchlings are produced each time

The hatchlings are fed with carefully prepared fish food, which includes the hormones needed to alter their gender.

It takes around three weeks for the process to take effect, and is accomplished without the addition of extra hormones or other medication, in accordance with international food safety guidelines.

Napun explains that after the transition, each young fish weighs just 5 grammes, but is ready to begin feeding entirely on its own. With a voracious appetite – and no breeding-related distractions – the mature fish can be sold.

He warns that the fish should not be raised in the company of other fish, as tilapia are a popular prey for larger carnivorous species. 

The young aquaculture expert notes that many local fish farmers pay little heed to the particular species they raise, thus running an increased risk of poor resistance to disease, or a slow-growing school. For this reason, he and his team selectively breed a local species, perfectly suited to Cambodian conditions.

Several farmers have ordered breeding tilapia from the RUA, with prices ranging from 120 to 300 riel ($0.030 to $0.075), depending on the size of each fish.

As tilapia are the second-most prolific species grown in aquaculture worldwide, the Ministry of Agriculture, Forestry and Fisheries encourages their proliferation for purpose of domestic protein consumption. 

The ministry is currently examining the possibility of working with the WorldFish Centre in Penang, Malaysia, to introduce the Genetically Improved Farmed Tilapia (GIFT), known as the “super tilapia”, into Cambodia for incubation purposes.

GIFT is an improved strain of one species of Nile tilapia, Oreochromis niloticus, that grows faster than local varieties. It was first developed by WorldFish and their partners in 1988, through selective breeding. 

Khim Finan, undersecretary of state and spokesman for the ministry, warned that strict conditions would be imposed on the farming of the so-called “super tilapia”.

“The GIFT type tilapia is a species which has undergone several generations of carefully managed incubation, so it grows very quickly and is extremely resistant to disease. Unfortunately, the traits that make it such a valuable commercial species mean it could be a dangerous invasive species if allowed to enter Cambodia’s ecosystem,” he says.

“On principle, we do not want any new species being introduced, as we do not know what effects they may have on native species,” he adds.

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