Today's Punto
Today's Punto
Feature Article
Sex reversal makes tilapia grow bigger
By Anselmo Roque

Jan 11, 2018

SCIENCE CITY OF MUÑOZ – Commercial culture of mixedsex tilapia in ponds is not profitable as it should be. For one, its female kind will reproduce as early as 60 days and as often as 30 days and thus become less attractive, sizewise, in the market.

Its nature as a mouthbrooder tends it to be less competitive in size than its male counterpart. It builds its nest and when the eggs are fertilized, it picks them up and keep them in the mouth for incubation and hatching for several days.

Even as the fries are released, she looks after them and takes them again in her mouth for refuge at the first sign of danger. Naturally, this motherly attention to its offspring disturbs it from its regular feed meals.

“Studies had shown that the male tilapia grows bigger and faster than the female tilapia,” Dr. Rafael Guerrero III, then dean of the College of Inland Fisheries of the Central Luzon State University (CLSU) here, said.

His comments were made in the mid-70s. He was then one of the lead figures in the intense research and development efforts on the tilapia mounted by CLSU. The university later launched the Freshwater Aquaculture Center which, in tandem with the college (renamed as College of Fisheries), scored many breakthroughs in tilapia culture and production.

“Manual sexing of the tilapia is too laborious and not one hundred sure as the practice is prone to mistakes. A more practical and easier way is to do sex reversal of the fries,” he said.

He then perfected the SRT technology. That’s sex reversal technique in tilapia, a breakthrough in tilapia culture and production in the country that spawned SRT-based technology that captured world attention in this fish species’ improved commercial production. That technology, the YY-male technology which involved in the first stage of the SRT, produced the “superfish” which was likened when it was presented in a world conference in Wales, United Kingdom, to the Biblical “miracle fish” that can feed millions in Third World countries.

Actually, the SRT was a product of Guerrero’s works as part of his requirements for his doctoral degree in Fish Management at the Auburn University in Alabama, USA. His dissertation, “Use of Androgens for the Production of All Male Tilapia”, indicated that the tilapia fry could be converted to all-male.

Taking a cue from Japanese scientists who had worked on the technique on an aquarium fi sh, but was not applied commercially, he used sex hormone to reverse the sex of the Nile tilapia.

The synthetic sex hormone is identified as methyltestosterone.

“The fries are treated with the hormone starting not more than three days from the time they are released from the mouth of the brooding females. They are stocked in tanks or ponds at the proper densities and fed at recommended feeding rates for good growth and survival, and effective sex reversal,” Guerrero explained.

In simple term, he then labelled the SRT as “the process by which genetic female tilapia fry are converted into functional males through hormone manipulation.”

95 percent

Guerrero, with the help of wife Luzviminda, who was also a part of the CLSU team that conducted R&D on the tilapia, later on produced a hormone-treated feed called SRT-95. The “95” was meant to insinuate that it is almost certain of producing 95 percent success rate for the sex reversal technique.

He hastened to add, though, that anyone with the proper know-how and equipment can prepare the hormone-feed and apply it commercially.

“Let’s say it’s a population control method in the commercial culture of the tilapia,” Guerrero said then.

He said that with the faster growing male tilapia, and the population control, the yield, kilogram-wise, can be increased dramatically and, consequently, profitably.

Guerrero, because of his successful works in the tilapia R&D that also included the now widely used cage culture of tilapia, won several ac- claims. He had been cited as one of the “Ten Outstanding Young Men” (TOYM) awardee and “Presidential Rizal Pro-Patria Awardee” and was also given the “Manuel Roxas Leadership Award”, “Outstanding Scientist Award”, and the “IBM Award for Science and Technology”, among others. He became director of the Philippine Council for Aquatic and Marine Research and Development (PCAMARD) and an Academician of the National Academy of Science and Technology.

Many fish growers in the country now prefer obtaining sex-reversed tilapia fingerlings for their commercial production endeavors.

But, as the SRT is preferred for culture and production, the question that lingers in the mind of many individuals is about its possible adverse effects to humans as well as the environment.

Is the sex-reversed tilapia safe to eat? How about its excreted compounds (metabolites), are they safe for other organisms?

Guerrero readily answers it in very understandable terms. “There are no residual effects left in the system of the fish after 92 hours of the withdrawal of treatment,” he said. “Therefore, there could be no side effects of the synthetic hormone after consuming the fish at its marketable size,” he added.

He said this assertion was based from the many studies done about this concern on the sex-reversed tilapia.

“Studies have shown that, in the tropics, the high temperature and the effect of sunlight breaks down the metabolites into simpler compounds, thus, there is no effects on human as well as to other organisms,” Guerrero said.

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