The discharge or seepage of sewage effluent into fresh water rivers has been found to contain man-made chemicals which act as female oestrogens which cause male fish to develop smaller testes and become "feminised" in that they produce the female yolk protein found in eggs. In recent years more and more man-made chemicals have been shown to be weak oestrogens, but because they are used in large amounts there are large amounts of them making their way into the aquatic environment. There are estimated to be some 60,000 made-made chemicals which get discharged into the environment through sewage; to date some 30 of these have been identified to have oestrogenic properties. Male fish exposed to oestrogens behave physiologically as females, producing extraordinary amounts of yolk, with some fish developing both male and female sexual characteristics.
A five-year study of ten rivers in England conducted by the Environment Agency reported that 50% of male fish had developed eggs in their testes or female reproductive ducts, 10% of the fish were sterile and 25% were producing damaged sperm.