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Taken from this report: How Tilapia is a More Unhealthy Food Than Bacon.

They've Had a Sex Change

Virtually all tilapia sold in American supermarkets has undergone a sex change–the result of being fed methyltestosterone during the early, sexless stage of life. Tilapia pumped full of hormones grow bigger quicker than their natural bros, because they don’t expend energy developing reproductive organs and require less food. Seafood experts consider the effects of methyltestosterone in fish to be insignificant to our health. However, there’s research to suggest the drug can be highly toxic to the liver. In fact, methyltestosterone has been taken off the market in Germany due to its high potential for liver toxicity.

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    Interesting and (as often is the case) seemingly misleading conflation of facts/claims. Germany takes methyltestosterone, itself, off the market because of concerns about liver toxicity. What does that have to do with eating meat from fully mature fish that, when they were young/juvenile, were fed methyltestosterone? Not criticizing the question, just noting the direction I anticipate fully-researched answers will go. – PoloHoleSet Jul 6 '17 at 16:32
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    @PoloHoleSet - that whole article is... fishy. Pun intended – user5341 Jul 6 '17 at 16:43
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    Wow, that leap from "sex organ growth repression" to "had a sex change" is making my neck hurt o,o – StarWeaver Jul 7 '17 at 12:38
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A quick Web Search turned up an excerpt from Testosterone: Action, Deficiency, Substitution (page 410-411). It did acknowledge the potential negative side-effects of methyltestosterone, and that the appropriate authorities in Germany had ruled that it be removed from the market (where it was being used for medical purposes). It also noted that the half-life in the blood was approximately 150 minutes, with maximal blood levels 90-120 minutes after ingestion. As such, it's pretty unlikely that clinically pertinent quantities are making their way to your dinner plate.

A bit more google searching suggests that there have been numerous books written on the subject of tilapia aquaculture, and that a fair number of those books discuss the use of methyltestosterone for the purpose. It's at least plausible, then, that most of the fish making its way to the supermarket has been so treated, and near-certain that a reasonable percentage has. There doesn't seem to be a reason to doubt that particular claim.

...and, en fin, it looks like this particular issue was a matter of some specific concern, and subject to investigation... in the 1980s. Culture of Nonsalmonid Freshwater Fishes, Second Edition, page 98. Final conclusion was that the hormone's persistence once it left the gut was minimal, and that it posed no public health concern.

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    En fin? I see what you did there. – DJClayworth Jul 7 '17 at 13:31
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A found an article from a Brazilian magazine that support this:

Para turbinar a produtividade, elas recebem uma ração especial à base de cereais (como soja e milho) e hormônios de reversão sexual durante os primeiros 20 dias de vida. Além de garantir aumento de peso, essa alimentação transforma a maior parte da população de peixes em machos – que crescem quase três vezes mais do que as fêmeas.

Translation:

To boost productivity, they receive a special cereal-based diet (such as soy and corn) and sex reversal hormones during the first 20 days of life. In addition to ensuring weight gain, this feed transforms most of the fish population into males - which grow almost three times more than females.

http://exame.abril.com.br/mundo/por-que-os-ambientalistas-odeiam-a-tilapia/

Edit: From wikipedia Indonesia created a new specie using genetic engineering:

Indonesian government research, development and introduced a new species named "genetically supermale Indonesian tilapia" (GESIT). GESIT fish are genetically engineered to hatch eggs that will produce 98% - 100% male tilapia. Monosex culture (all male) is more productive and will benefit the farmers. Now, around 14 strains of ikan nila have been developed by contributions from research institutes including MCFAD.

https://en.wikipedia.org/wiki/Aquaculture_of_tilapia

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    Something published on "Exame" isn't an article by any means. This magazine is a very poor publication that tries very hard to appear a "scientific" magazine, but more often than not fail miserably. It doesn't cite any references to its claims and it is not worth anything more than the original claim. – T. Sar Jul 6 '17 at 18:29

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