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Originally Posted by Xaccers
Of course many breeds of fish, unlike mamals, are able to change gender anyway.
And the fish involved live near sewerage outlets rather than in drinking water.
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true on both counts, but the gender changing that naturally occurs is usually to maximise breeding potential, for the species. There is one species, where the males collect 'harems' of females, with which they mate exclusively. As they can't do this successfully, until they get big enough to defend their harem, all newly hatched fish are female, but they change into males when they reach a certain size.
The effect of oestrogens in sewage is tending to produce sterile males, rather than 'proper' females, though.