The Checkout

Transsexual Tilapia?

Ylan Mui

I know what you're thinking already. It sounds totally insane. But the tilapia you eat very likely has gone through a sex change.

Let me explain: When we began working on our story about Whole Foods' new farmed seafood standards, one bullet point caught our eye: No Methyl testosterone for sex reversal.

As it turns out, farm-raised tilapia is typically given testosterone in their food when they're very small, for the first 35-40 days of feeding, Whole Foods seafood quality standards coordinator Carrie Brownstein told me. Any fish that were on the path to a womanly fishhood change course to become strapping male tilapia instead, turning the pond into one big fishy frat party.

Before reporting this story, I had never really considered the sex of my fish. They're not like cows, which we like to imagine frolicking in a grassy field before they're turned into a burger patty. Even chickens and pigs we like to envision as capable of battling evil. But tilapia, not so much. I just think about them filleted and wrapped and sprinkled with a little of Paul Prudhomme's cajun seasoning and maybe a dash of lemon juice.

Whole Foods began banning testosterone in fish about three years ago, scouring the world to find fish farms that did not use the hormone. They even had to stop selling tilapia for a while before working out arrangements with farms in Ecuador and Costa Rica.

"We decided not to allow the treatment of any fish that are ultimately going into our seafood cases. We don't believe that the hornmone treated fish really meet the expectations that our customers have." Brownstein said.

There are some arguments for the sex change, however. Fish farmers prefer it because males grow larger than females because they do not expend energy producing eggs and trying to have fish babies. Larger fish means that farmers can harvest more for the same amount of resources; in other words, less is wasted. Here's what Jill Schwartz, a spokeswoman for the World Wildlife Fund, says about it:

"There are more environmental benefits than problems associated with this practice. For example, if tilapia are not sex reversed, females (not just males) also will be farmed. Females require more feed than males require for reproductive organ development. The more feed is used on a farm, the more waste there is likely to be in the water. Also, if we are trying to conserve wild fish used in tilapia feeds, it would be optimal to have the best conversion of feed to tilapia meat. This occurs with males."

In any case, this all makes for excellent conversation at your next dinner party. You can even volunteer to make the main course!

By Ylan Mui |  July 16, 2008; 2:03 PM ET Consumer News , Ylan Mui
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Please email us to report offensive comments.

The one time use of methyl testosterone to convert mixed sex populations of farm-raised Tilapia to nearly all male populations is a practice that has been proven to be safe. The methyl testosterone is applied in minute amounts when the fish are less than 1 gram in weight. Numerous scientific studies have proven that the substance is completely removed from the fish after a few weeks time. At the time of harvest, there is absolutely zero concentration of methyl testosterone in the fish.

Farm-raised Tilapia is an affordable, safe and nutritious source of protein for all Americans.

Do your readers a favour and report the whole story.

Respectfully Submitted,
Tom Frese
AquaSol, Inc.

Posted by: Tom Frese | July 16, 2008 4:15 PM

Grinding bovine waste and bones and feeding it to cows was also safe - until mad cow disease.

Respectfully wary,
Self-Preservation Inc.

Posted by: Buck | July 16, 2008 5:10 PM

Efficient grow out methods require a male fish. The Wholefoods' standards support this but ban the use of methyl testosterone in this sexing process. A hybrid of two species of Tilapia can achieve the all male objective without the use of such hormones. This is the method we use at HQ Sustainable Maritime Industries Inc.

Posted by: Norbert Sporns | July 16, 2008 9:01 PM


Following up on Mr. Sporn's comment above, the alternatives to sex-reversed Tilapia are to hybrids. By breeding specific species of Tilapia together the first generation offspring are 99% male. Mike Sipe, a tilapia breeder and researcher from has several lines of fish available, including specially colored hybrids that allow you to visually identify and cull the females.

The third and simplest option is to stock smaller carnivorous fish after the Tilapia reach breeding age.

The real issue isn't that the females grow more slowly, but in good conditions they can have thousands of offspring. Which can quickly overwhelm even the most elaborate oxygenation and filtration system.

Elizabeth Greene

Posted by: Elizabeth Greene | July 17, 2008 10:23 AM

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