A third of the male fish in British rivers are turning into females because of "gender-bending" pollutants being discharged from sewage outflows.
The first national survey of 42 rivers has found that more than one in three male fish are growing female reproductive tissues and organs.
Researchers said there was unequivocal evidence that human female sex hormones released into rivers from sewage farms are responsible.
The hormones are excreted naturally and act as endocrine disrupters in fish. They include synthetic hormones from the contraceptive pill which stay highly potent for several weeks in rivers.
Nick Cartwright, the chemicals policy adviser for the Environment Agency, said the effects have now been observed in coarse fish, and young fish are particularly susceptible, making them less fertile, which has serious implications for future fish populations."Fish populations in general are better than they were but what we're seeing now are more subtle effects."
The appearance of "intersex" fish first came to notice more than 10 years ago after a study on roach in the river Lea in Hertfordshire, a tributary of the Thames. A study of fish in eight rivers near large sewage-treatment works linked the intersex phenomenon with pollution, which was confirmed by laboratory studies.
Dr Cartwright said the natural hormones oestradiol and oestrone could affect development of male fish, but the synthetic hormone ethinyloestradiol was about 1,000 times more potent. Ethinyloestradiol, a component of the Pill, can persist in the environment for up to 90 days, 20 times longer than natural hormones.
Andrew Skinner, head of environmental protection at the Environment Agency, said the survey reinforced the need to find ways of removing endocrine disrupters from sewage outflows. "We have duties for the protection of fish," he said. The agency has proposed two pilot plants to test ways of removing endocrine disrupters from sewage effluent.Reuse content