A fifth of male fish in UK rivers are now 'trans-gender' due to chemicals in human waste, according to a new study.

Research carried out at the University of Exeter has discovered that 20% of male fish tested in English rivers are now 'trans-gender' or 'intersex', with both female and male characteristics, due to chemicals flushed down the toilet from UK households.

The contraceptive pill, cleaning agents, plastics and cosmetics all getting into the water system are having an effect on the fish population.

Professor Charles Tyler, a leading fish physiologist and eco-toxicologist from the University of Exeter, has uncovered that male fresh-water fish are displaying ‘feminised’ traits, demonstrating ‘female’ behaviour and even producing eggs. Anti-depressants, for example, flushed down the loo are reducing natural shyness in fish according to his research.

Anti-depressants in the water system are changing fish behaviour according to the research. Credit: PA Images

Professor Tyler is presenting his findings in a lecture called the Feminisation of Nature – an Unnatural History during an international conference.

The scientist will explain in his speech how 20 per cent of male fresh-water fish, such as roach, tested at 50 sites, had feminine characteristics. Over 200 chemicals from sewage plants have been identified with oestrogen-like effects through the research.

What are the effects?

  • Some male fish have reduced sperm quality

  • Some display less aggressive and competitive behaviour usually associated with attracting females, which makes them less likely to breed successfully

  • The offspring and grandchildren of transgendered fish can be more sensitive to the effects of those chemicals in when exposed to them

We are showing that some of these chemicals can have much wider health effects on fish that we expected. For example we have shown that oestrogens found in some plastics affect the valves in the heart. Other research has shown that many other chemicals that are discharged through sewage treatment works can affect fish including antidepressant drugs that reduce the natural shyness of some fish species, including the way they react to predators.

Professor Charles Tyler, University of Exeter