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Farmed Fish Spread Steroids: Microbial Solution Sought
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Baseball stars, bodybuilders and cyclists have used steroids. Now it turns out that even farmed fish get “juiced”
by Craig Weatherby

Fish farmers feed infant tilapia a masculine steroid hormone called methyltestosterone (MT) to ensure that they turn out male.
The MT steroid fed to tilapia early in their life cycle is no longer present in the fishes’ flesh when people consume them.
But the male hormone is potentially harmful to other fish and to tilapia-farm workers… and to local residents who drink from or wade and swim in streams fed by waters from tilapia farms.
Now, researchers in Mexico have found some bacteria that like to make a meal of methyltestosterone.
The discovery may result in a safer environment for farm workers and nearby residents and wildlife... and it has global implications, given that tilapia is raised in more than 100 countries.
Tilapia producers add MT hormone to the powdered food they dish out to large tanks of tiny tilapias called fry every day for three to four weeks to turn them into males, which grow faster than females. Also, having only one gender prevents breeding, which makes farmers’ operations less cost-efficient.
The young tilapias swallow the steroid but then excrete it back into the water... with uncertain consequences for wildlife and people.
Sex-change steroid can harm fish-farm workers and locals
Fish biologist Wilfrido Contreras Sanchez worries that MT residue might endanger the health of workers who wade into the water to scoop up juvenile fish.
Also, many tilapia producers discharge the hormone-laced water from the tanks into streams, rivers and lagoons where it might harm other fish and amphibians, said Contreras, who heads the biological sciences division at the Autonomous Juarez University of Tabasco where the bacterial research was conducted.
Additionally, the health of local residents who swim in or wash clothes in these bodies of water might be at risk, he said.
Dr. Contreras said little is known about how the use of MT in aquaculture might affect humans or wildlife. MT is an androgen and is prescribed to stimulate puberty in slow-developing adolescent boys and to treat breast cancer.
The U.S. Food and Drug Administration says that prolonged use of high doses of androgens is associated with liver cancer, and that androgens may increase elderly men’s risk of prostate cancer.
High doses in women can lead to deeper voices, facial hair, acne and irregular menstrual cycles.
Bacteria may provide a solution
Dr. Contreras hopes that the bacteria he studied will eliminate potential hazards if added in sufficient amounts to the water filters in the tanks where the tiny tilapias dine on MT.
In lab tests, he and fellow researchers found several bacteria species that reduced the hormone by more than 95 percent in two to three weeks.
But one called P. aeruginosa multiplied rapidly in the lab, so the researchers added billions of that bacterium to filters that clean the water of three concrete fish tanks.
Each tank held 5,700 young tilapia that were fed MT daily, and the trial showed a trend toward lower levels of MT over time in the tanks where bacteria had been added.
Lab tests found no diseases in the kidneys, livers or spleens of fish raised in tanks with the bacteria. Because the bacteria are already ubiquitous and may be eaten by tilapias, Dr. Contreras doubts that they would cause any health problems in people or fish.
The Mexican team plans to conduct more experiments to fine-tune the technique, and then hopes to grow mass quantities of the best bacteria to sell to tilapia producers.
The research was funded by Oregon State University, the University of Arizona, the Autonomous Juarez University of Tabasco, and the U.S. Agency for International Development through its AquaFish Collaborative Research Support Program.
  • American Fisheries Society (AFS). Bacteria on steroids: A new way to make water at tilapia farms safer? October 8, 2010. Accessed at

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