University pioneers sex reversal in fish to boost dwindling stocks

Fish is one of Kenya’s favourite delicacies and a rich source of proteins.

However, most consumers of the popular tilapia fish are not aware that the meal on their plate is mostly from a male fish.

According to fish scientists, male tilapia are fast maturing, rich in flesh and therefore ideal for market purposes once they attain a weight of between 250 to 400 grammes.

For optimum exploitation of the sector, the experts say sex reversal can be done to ensure mass production of the popular white meat.

The University of Eldoret (UOE) is currently conducting sex reversals on 20,000 fingerlings following the recent opening of a multipurpose hatchery. The hatchery has the potential to conduct sex reversal on 500,000 fingerlings.

This, the university says, will help boost the production and supply of fingerlings across the North Rift region.

With the advent of counties, which have been promoting diversification of agriculture over the last four years, residents of the North Rift are taking up fish farming. Besides, most households are shifting from red meat to fish, which they believe is rich in nutrients and also medicinal.

According to Josiah Ani, the fish farm manager at the University of Eldoret, the institution has begun sex reversal on fingerlings using a hormone known as Methyl Testosterone which changes the female hormone to male.

“Male fish are highly marketable due to their fast growth. The male fish can achieve the market size faster than the female, which consumes a lot of food to help it in reproduction and development of eggs,” says Ani.

Before the transformation is done, brooders fed on protein have to be introduced into a fertilised pond.

“The brood should be introduced into the pond in a specific ratio, generally two against one,” says Ani.

A female tilapia is checked for fertilised eggs before it is transferred to another tank where it pours outs her eggs.

The eggs are then taken to an incubator (container) full of water that circulates through pipes and has a temperature of approximately 24-28 degrees. Here, the eggs are hatched into fingerlings.

“When they are hatched, the fingerlings have food attached to their body commonly referred to as yolk sacs. For three days to one week, the fingerlings will derive their food from these yolk sacs,” says Ani.

When the food is fully absorbed, the fingerlings will become active as they seek food. This is the most ideal moment to conduct the sex reversal.

The process, according to Ani, involves the use of methyl testosterone, a hormone, mixed with stock solution (dry matter which is food for the fish and contains high protein) and fed to the fish for 21 to 28 days.

“Feeding is done four to six times a day. It is mainly conducted during the day because tilapia, unlike catfish, is not photophobic and prefers light when feeding,” he says.

Philip Raburu, the head of fisheries and aquatic sciences at the University of Eldoret, says there is a decline in fish production due to continuous fishing.

Prof Raburu says there is need for over production to curb the continuous depletion of fish stock.

“Fish is still the safest animal for protein and highly recommendable for both the young and the old. We must strive to increase its numbers as well as conduct sex reversals to ensure more males in the market,” he explained.

He says the multi-million shilling hatchery established at the university with help of the Uasin Gishu County Government will help boost production of catfish and tilapia whose demand is high in the region.

Uasin Gishu County Fisheries Officer John Otiego says the North Rift produces 5,120 tonnes of fish annually, translating to Sh2 billion.

“Over 2,000 farmers are now engaged in fish production through establishment of ponds and dams, a number we believe can be increased through constant education,” he adds.