This is good news, considering the potency of environmental estrogen. The synthetic estrogen ethinylestradiol is detected at potent concentrations 1,000-fold greater than that of any other mimic, affecting fish even at very low concentrations. Ethinylestradiol appears at its most toxic concentrations downstream from major sewage treatment works, where it comes from birth control pills and hormone replacement therapies.

Endocrine-disrupting chemicals such as estrogen can enter a fish through food or from the environment, where toxins in the water pass over the gills and eventually enter the blood. Depending on the nature of the chemical, it may become bioconcentrated in the fish, often reaching concentrations much higher than those in the surrounding water. From there, the chemical may biomagnify either through benthic food-chain transfer or from prey-to-predator transfer.