Text and Photos by Henrylito D. Tacio
The Philippines is now one of the world’s leading producers of tilapia. In 2013, the Philippines produced 316.536 metric tons of tilapia, making it the fourth largest producer of tilapia in the world.
The record could perhaps be credited to a sex-change operation discovered by Dr. Rafael D. Guerrero III, an Academician of the National Academy of Science and Technology and former executive director of the Philippine Council for Aquatic and Marine Research and Development.
Tilapias are very prolific. Pond-reared tilapia, which a natural ratio of 50% male and 50% female, mature as early as 60 days. They breed frequently, often every 30 days. At such growth rate, the competition for food escalates. And with reduced nutrition, the best attainable market size for the tilapia after four months (that’s the time they reach full maturity) is 150 grams.
Some studies have shown that the male tilapia grows faster and bigger than the female tilapia. The natural preference of tilapia growers, therefore, was all male tilapias. But how could male tilapias be selected and separated from the females? Was it physically possible to select the fish for stocking in the ponds?
“One of the best methods to solve the overcrowding problem of tilapia is population control,” says Dr. Guerrero, already called The Father of Tilapia in the Philippines. His solution is a process he calls “sex reversal technique” or SRT.
“SRT is the process by which genetic female tilapia fry are converted into functional males through hormone manipulation,” he explains.
Traditionally, tilapias undergo manual sexing; but this task is too laborious. In manual sexing, the male is distinguished from the female by looking at the organ called the urogenital papilla, which is found near the anus of the fish. There are two openings in the female fish, and only one in the male fish.
“Manual sexing is cumbersome and time-consuming,” says Roy C. Alimoane, the director of the Davao-based Mindanao Baptist Rural Life Center. “The technique is also only 80% accurate because of human error. Another disadvantage is that you can only sex fish when they are three months old. By then, they are almost fully grown.”
Dr. Guerrero, who has devoted most of his life to laboratory and applied science, was convinced that there was a more practical, effective and economical way of solving the problem.
A science sleuth, he first did an extensive literature quest. He studied the works of other scientists and came across the concept of using sex hormones to reverse the sex of fish developed by Dr. T.K. Yamamoto. The Japanese researcher had worked on the modaka, an aquarium fish, but the results of his study had not been applied commercially.
Dr. Guerrero became excited at the prospect of applying the concept on the Nile tilapia, the species propagated in the Philippines. So, when he went to the Auburn University in Alabama in the United States to earn his doctorate in Fisheries Management, he decided to work on the application of the artificial sex reversal concept for his dissertation.
After almost two years of research, he came out with a dissertation entitled “Use of Androgens for the Production of All Male Tilapia.” It showed that the tilapia fry could be converted to all-male populations when fed with a synthetic male hormone for a certain period during the “sexless” stage.
According to Dr. Guerrero, the the yield of large-sized tilapias is significantly increased by 30-50% because of the faster-growing males compared to females and the control of reproduction.
Since its development as a “breakthrough,” the application of tilapia sex reversal has undergone a number of innovations. While at first it was believed that the treatment of fry could only be done under a shade, it is now being done in outdoor tanks and ponds.
As much as 90% of the tilapia cultured in the country is sex-reversed. “At least 50% of the tilapia produced in the United States, Canada, Israel, the Caribbean and Asia is sex-reversed,” he points out.
Although anyone with the proper know-how and equipment can prepare the hormone-feed and apply it, there is a tilapia sex reversal feed (SRT-95) now available in the market for those who find it more convenient and practical.
“Applying the sex reversal feed method with a success rate of at least 95 percent requires the production of the right age of the fry for treatment, preparation of the hormone feed or its procurement, and the proper application of the treatment,” Dr. Guerrero says.
It is also important that the fry to be treated be not more than three days from the time they are released by the mouth brooding females in breeding ponds when the fry are about 10 days old from hatching of the eggs and 9 millimeters to 11 millimeters in total length.
“The fry to be treated also need to be stocked in tanks or ponds at the proper densities and fed at recommended feeding rates for good growth and survival, and effective sex reversal,” Dr. Guerrero says.
Because of innovation, there is now a much better commercial SRT feed and it’s called SR Premix. “From SRT-95 which was a single use feed for treating 4,000 fry with at least 95% male percentage of treated fish with proper use, we developed a sex reversal premix which can be mixed at one kilogram of the Premix with 9 kilograms of the fry feed to produce 10 kilograms of SRT-99 for treating 40,000 fry,” Dr. Guerrero says.
But the question is: are those sex reversed tilapia safe to eat?
The synthetic sex hormone used in sex reversal technology is methyltestosterone, a man-made form of testosterone. A naturally occurring sex hormone, testosterone is produced in a man’s testicles. Small amounts of testosterone are also produced in a woman’s ovaries and adrenal system.
“Methyltestosterone is used in men and boys to treat conditions caused by a lack of this hormone, such as delayed puberty or other hormonal imbalances,” notes www.drug.com. “Methyltestosterone is also used in women to treat breast cancer that has spread to other parts of the body.”
According to Dr. Guerrero, the oral treatment with methyltestosterone in tilapia is only for 3-4 weeks during the sexless stage of the fry or what he calls as “the stage of sexual differentiation.”
“After withdrawal of the treatment,” he assures, “there are no residuals left in the system of the fish after 92 hours.” He based this statement from some studies being done.
“Since the fry are grown for at least 3-4 months for market and human consumption, it is very safe,” he says. “There can, therefore, be no side effects if there is no synthetic hormone left in the systems of the fish.”
There is also the question of what happens to the metabolites (excreted compounds) of the synthetic hormone which are eliminated in the environment. Can they affect other organisms and affect people? Dr. Guerrero gives this answer: “Studies have shown that, in the tropics, the high temperature and the effect of sunlight breaks down the metabolites into simpler compounds through biodegradation and photo-oxidation that have no effect on humans.”
Aside from those done in the Philippines, there are also studies conducted in other countries that showed endocrine disruptors from plastics and insecticides in the environment can influence the direction of sex of fishes and possibly humans who consume them towards femaleness or hermaphroditism and not maleness. “Methyl testosterone, which is for maleness, is therefore not in question,” Dr. Guerrero says.
The use of methyl testosterone for tilapia sex reversal is generally accepted throughout the world except in Europe which has a strict organic (no synthetics) policy. “So far, since its application more than 30 years ago, there is no negative or harmful effect on humans reported.”
