Chemicals flushed down household drains have caused around 20 per cent of male river fish to have female characteristics, according to research.
Males are displaying feminised traits and even producing eggs, a study by Professor Charles Tyler of the University of Exeter has found.
Some have reduced sperm quality and display less aggressive and competitive behaviour, which makes them less likely to breed successfully.
The chemicals causing these effects include ingredients in the contraceptive pill, by-products of cleaning agents, plastics and cosmetics, according to the findings.
Professor Tyler said: “We are showing that some of these chemicals can have much wider health effects on fish that we expected.
“Using specially created transgenic fish that allow us to see responses to these chemicals in the bodies of fish in real time, for example, we have shown that oestrogens found in some plastics affect the valves in the heart.”
Tests showed 20 per cent of male freshwater fish, such as roach, at 50 sites had feminine characteristics.
More than 200 chemicals from sewage plants have been identified with oestrogen-like effects and drugs such as antidepressants are also altering fish’s natural behaviour, his study found.
“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 Tyler said.
Professor Tyler also found that the offspring and grandchildren of affected fish can be more sensitised to the chemicals in subsequent exposures.
He will present his findings in the opening lecture of the 50th Anniversary Symposium of the Fisheries Society in the British Isles at Exeter University from July 3 to 7.