Several oestrogen-like substances are found in sewage and are only partially removed at treatment works: natural oestrogens from human waste, synthetic ethinyl oestradiol (EE2) from contraceptive pills, and hormone-mimicking substances like alkyl phenols and bisphenol A (ENDS Report 386, p 29 
).
Their impacts have been documented in laboratory studies and in wild populations downstream of sewage outfalls. High exposures may cause male fish to grow eggs in their testes and impair reproduction, but the chronic effects of low doses are difficult to quantify.
In an attempt to determine their impacts, Karen Kidd from the Canadian Freshwater Institute and her research team used three small lakes in Ontario. One 34-hectare lake was dosed with EE2 three times a week for three years to give oestrogen levels of 5-6 nanograms per litre. The other two were left untreated.
Male fathead minnows caught seven weeks after the oestrogen was first added were producing elevated levels of vitellogenin, a protein linked to egg production which is an indicator of feminisation. The next spring they showed delayed sperm production and the following year some produced eggs. So-called ‘intersex’ males have been shown to have reduced fertility (ENDS Report 327, pp 24-28 ).
The EE2-dosed lake initially held thousands of minnows - an important food source for other fish - but they became virtually extinct after the trial’s second year. Numbers had not recovered two years after dosing ceased.
Another minnow species found in the same lake, the pearl dace, was unaffected. The researchers believe this is because it has a longer life cycle than the fathead minnow which generally lives for only two years.