Pick Our Brain - December 2008

Question: Why does it matter if male fish become feminized?

Answer: Fish, like people, go through an elaborate developmental process that lets them become male or female. Which sex they become is generally prescribed by their genes, and carried out by hormones – the same process we humans go through. Unlike people, however, the sex of fish can change throughout their life – naturally – or because of an external influence. The natural sex changes appear to have evolved as an adaptation that helps certain fish populations survive. But sex changes induced by unnatural factors – like estrogens from birth control pills that pass through wastewater treatment plants into rivers, lakes, and oceans – likely imperil populations. (Other substances can also do this, including a variety of pesticides and substance in cosmetics, plastics, etc. They can all act like hormones.)

A recent study showed that a continuous exposure of small minnows to low levels of a synthetic estrogen in a lake caused the lake’s entire minnow population to crash after only two years because so many male minnows turned into females that they could not produce enough offspring anymore. If this happens in many places, or if an exposed species has a very limited range (e.g. delta smelt), this could lead to major population declines, or even, extinction.