The Transsexual Fish Mystery

An Increasing Number of Male Fish are Developing Female Traits
Around the world, it is more and more common to find male fishes with female traits. Some even produce eggs! This has been blamed on chemicals that find their way into water and mimic the female hormone estrogen. But a new study puts some of the blame on an entirely different class of chemicals... Read on for more details.

Tyler, along with Susan Jobling at Brunel University in London and other colleagues, looked at chemical run-off in 51 rivers throughout the United Kingdom. By combining concentrated water samples with cultures of yeast genetically engineered to have androgen receptors, the scientists were able to measure the amount of anti-androgen activity in each sample.
The researchers' results, published in the journal Environmental Health Perspectives, revealed a significant amount of anti-androgenic activity in nearly all of the samples tested.
The researchers also collected fish from each site. With statistical models, they were able to show that anti-androgens were just as responsible for the feminization of fish as estrogenic compounds were -- if not more so.

Where do these chemicals come from? Mostly the usual suspects: pesticides and pharmaceuticals.